Category Archives: Linux

General Linux and Open Source related blog posts

Webauthn in Linux with a TPM via the HID gadget

Account security on the modern web is a bit of a nightmare. Everyone understands the need for strong passwords which are different for each account, but managing them is problematic because the human mind just can’t remember hundreds of complete gibberish words so everyone uses a password manager (which, lets admit it, for a lot of people is to write it down). A solution to this problem has long been something called two factor authentication (2FA) which authenticates you by something you know (like a password) and something you posses (like a TPM or a USB token). The problem has always been that you ideally need a different 2FA for each website, so that a compromise of one website doesn’t lead to the compromise of all your accounts.

Enter webauthn. This is designed as a 2FA protocol that uses public key cryptography instead of shared secrets and also uses a different public/private key pair for each website. Thus aspiring to be a passwordless secure scalable 2FA system for the web. However, the webauthn standard only specifies how the protocol works when the browser communicates with the remote website, there’s a different standard called FIDO or U2F that specifies how the browser communicates with the second factor (called an authenticator in FIDO speak) and how that second factor works.

It turns out that the FIDO standards do specify a TPM as one possible backend, so what, you might ask does this have to do with the Linux Gadget subsystem? The answer, it turns out, is that although the standards do recommend a TPM as the second factor, they don’t specify how to connect to one. The only connection protocols in the Client To Authenticator Protocol (CTAP) specifications are USB, BLE and NFC. And, in fact, the only one that’s really widely implemented in browsers is USB, so if you want to connect your laptop’s TPM to a browser it’s going to have to go over USB meaning you need a Linux USB gadget. Conspiracy theorists will obviously notice that if the main current connector is USB and FIDO requires new USB tokens because it’s a new standard then webauthn is a boon to token manufacturers.

How does Webauthn Work?

The protocol comes in two flavours, version 1 and version 2. Version 1 is fixed cryptography and version 2 is agile cryptography. However, version1 is simpler so that’s the one I’ll explain.

Webauthn essentially consists of two phases: a registration phase where the authenticator is tied to the account, which often happens when the remote account is created, and authentication where the authenticator is used to log in again to the website after registration. Obviously accounts often outlive the second factor, especially if it’s tied to a machine like the TPM, so the standard contemplates a single account having multiple registered authenticators.

The registration request consists of a random challenge supplied by the remote website to prevent replay and an application id which is constructed by the browser from the website supplied ID and the web origin of the site. The design is that the application ID should be unique for each remote account and not subject to being faked by the remote site to trick you into giving up some other application’s credentials.

The authenticator’s response consists of a unique public key, an opaque key handle, an attestation X.509 certificate containing a public key and a signature over the challenge, the application ID, the public key and the key handle using the private key of the certificate. The remote website can verify this signature against the certificate to verify registration. Additionally, Google recommends that the website also verifies the attestation certificate against a list of know device master certificates to prove it is talking to a genuine U2F authenticator. Since no-one is currently maintaining a database of “genuine” second factor master certificates, this last step mostly isn’t done today.

In version 1, the only key scheme allowed is Elliptic Curve over the NIST P-256 curve. This means that the public key is always 65 bytes long and an encrypted (or wrapped) form of the private key can be stashed inside the opaque key handle, which may be a maximum of 255 bytes. Since the key handle must be presented for each authentication attempt, it relieves the second factor from having to remember an ever increasing list of public/private key pairs because all it needs to do is unwrap the private key from the opaque handle and perform the signature and then forget the unwrapped private key. Note that this means per user account authenticator, the remote website must store the public key and the key handle, meaning about 300 bytes extra, but that’s peanuts compared to the amount of information remote websites usually store per registered account.

To perform an authentication the remote website presents a unique challenge, the raw ID from which the browser should construct the same application ID and the key handle. Ideally the authenticator should verify that the application ID matches the one used for registration (so it should be part of the wrapped key handle) and then perform a signature over the application ID, the challenge and a unique monotonically increasing counter number which is sent back in the response. To validly authenticate, the remote website verifies the signature is genuine and that the count has increased from the last time authentication has done (so it has to store the per authenticator 4 byte count as well). Any increase is fine, so each second factor only needs to maintain a single monotonically increasing counter to use for every registered site.

Problems with Webauthn and the TPM

The primary problem is the attestation certificate, which is actually an issue for the whole protocol. TPMs are actually designed to do attestation correctly, which means providing proof of being a genuine TPM without compromising the user’s privacy. The way they do this is via a somewhat complex attestation protocol involving a privacy CA. The problem they’re seeking to avoid is that if you present the same certificate every time you use the device for registration you can be tracked via that certificate and your privacy is compromised. The way the TPM gets around this is that you can use a privacy CA to produce an arbitrary number of different certificates for the same TPM and you could present a new one every time, thus leaving nothing to be tracked by.

The ability to track users by certificate has been a criticism levelled at FIDO and the best the alliance can come up with is the idea that perhaps you batch the attestation certificates, so the same certificate is used in hundreds of new keys.

The problem for TPMs though is that until FIDO devices use proper privacy CA based attestation, the best you can do is generate a separate self signed attestation certificate. The reason is that the TPM does contain its own certificate, but it’s encryption only, not signing because of the way the TPM privacy CA based attestation works. Thus, even if you were willing to give up your privacy you can’t use the TPM EK certificate as the FIDO attestation certificate. Plus, if Google carries out its threat to verify attestation certificates, this scheme is no longer going to work.

Aside about Browsers and CTAP

The crypto aware among you will recognise that there is already a library based standard that can be used to talk to a variety of USB tokens and even the TPM called PKCS#11. Mozilla Firefox, for instance, already supports using this as I demonstrated in a previous blog post. One might think, based on what I said about the one token per key problem in the introduction, that PKCS#11 can’t support the new key wrapping based aspect of FIDO but, in fact, it can via the C_WrapKey/C_UnwrapKey API. The only thing PKCS#11 can’t do is the new monotonic counter.

Even if PKCS#11 can’t perform all the necessary functions, what about a new or extended library based protocol? This is a good question to which I’ve been unable to get a satisfactory answer. Certainly doing CTAP correctly requires that your browser be able to speak directly to the USB, Bluetooth and NFC subsystems. Perhaps not too hard for a single platform browser like Internet Explorer, but fraught with platform complexity for generic browsers like FireFox where the only solution is to have a rust based accessor for every supported platform.

Certainly the lack of a library interface are where the TPM issues come from, because without that we have to plug the TPM based FIDO layer into a browser over an existing CTAP protocol it supports, i.e. USB. Fortunately Linux has the USB Gadget subsystem which fits the bill precisely.

Building Synthetic HID Devices with USB Gadget

Before you try this at home, I should point out that the Linux HID Gadget has a longstanding bug that will cause your machine to hang unless you have this patch applied. You have been warned!

The HID subsystem is for driving Human Interaction Devices meaning keyboard and mice. However, it has a simple packet (called report in USB speak) based protocol which is easy for most things to use. In order to facilitate this, Linux actually provides hidraw devices which allow you to send and receive these reports using read and write system calls (which, in fact, is how Firefox on Linux speaks CTAP). What the hid gadget does when set up is provide all the static emulation of HID device protocols (like discovery pages) while allowing you to send and receive the hidraw packets over the /dev/hidgX device tap, also via read and write (essentially operating like a tty/pty pair1). To get the whole thing running, the final piece of the puzzle is that the browser (most likely running as you) needs to be able to speak to the hidraw device, so you need a udev rule to make it accessible because by default they’re 0600. Since the same goes for every other USB security token, you’ll find the template in the same rpm that installs the PKCS#11 library for the token.

The way CTAP works is that every transaction is split into 64 byte reports and sent over the hidraw interface. All you need to do to get this setup is initialise a report descriptor for this type of device. Since it’s somewhat cumbersome to do, I’ve created this script to do it (run it as root). Once you have this, the hidraw and hidg devices will appear (make them both user accessible with chmod 666) and then all you need is a programme to drive the hidg device and you’re done.

A TPM Based Hid Gadget Driver

Note: this section is written describing TPM 2.0.

The first thing we need out of the TPM is a monotonic counter, but all TPMs have NV counter indexes which can be created (all TPM counters are 8 byte, whereas the CTAP protocol requires 4 bytes, but we simply chop off the top 4 bytes). By convention I create the counter at NV index 01000101. Once created, this counter will be persistent and monotonic for the lifetime of the TPM.

The next thing you need is an attestation certificate and key. These must be NIST P-256 based, but it’s easy to get openssl to create them

openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:prime256v1 -pkeyopt ec_param_enc:named_curve -out reg_key.key

openssl req -new -x509 -subj '/CN=My Fido Token/' -key reg_key.key -out reg_key.der -outform DER

This creates a self signed certificate, but you could also create a certificate chain this way.

Finally, we need the TPM to generate one NIST P-256 key pair per registration. Here we use the TPM2_Create() call which gets the TPM to create a random asymmetric key pair and return the public and wrapped private pieces. We can simply bundle these up and return them as the key handle (fortunately, what the TPM spits back for a NIST P-256 key is about 190 bytes when properly marshalled). When the remote end requests an authentication, we extract the TPM key from the key handle and use a TPM2_Load to place it in the TPM and sign the hash and then unload it from the TPM. Putting this all together this project (which is highly experimental) provides the script to create the devices and a hidg driver that interfaces to the TPM. All you need to do is run it as

hidgd /dev/hidg0 reg_key.der reg_key.key

And you’re good to go. If you want to test it there are plenty of public domain webauthn test sites, webauthn.org and webauthn.io2 are two I’ve tested as working.

TODO Items

The webauthn standard specifies the USB authenticator should ask for permission before performing either registration or authentication. Currently the TPM hid gadget doesn’t have any external verification, but in future I’ll add a configurable pinentry to add confirmation and possibly also a single password for verification.

The current code also does nothing to verify the application ID on a per authorization basis. This is a security problem because you are currently vulnerable to being spoofed by malicious websites who could hand you a snooped key handle and then use the signature to fake your login to a different site. To avoid this, I’m planning to use the policy area of the TPM key to hold the application ID. This should work because the generated keys have no authorization, either policy or password, so the policy area is effectively redundant. It is in the unwrapped public key, but if any part of the public key is tampered with the TPM will detect this via a hash in the wrapped private error and give a binding error on load.

The current code really only does version 1 of the FIDO protocol. Ideally it needs upgrading to version 2. However, there’s not really much point because for all the crypto agility, most TPMs on the market today can only do NIST P-256 curves, so you wouldn’t gain that much.

Conclusions

Using this scheme you’re ready to play with FIDO/U2F as long as you have a laptop with a functional TPM 2.0 and a working USB gadget subsystem. If you want to play, please remember to make sure you have the gadget patch applied.

Using TPM Based Client Certificates on Firefox and Apache

One of the useful features of Apache (or indeed any competent web server) is the ability to use client side certificates. All this means is that a certificate from each end of the TLS transaction is verified: the browser verifies the website certificate, but the website requires the client also to present one and verifies it. Using client certificates, when linked to your own client certificate CA gives web transactions the strength of two factor authentication if you do it on the login page. I use this feature quite a lot for all the admin features my own website does. With apache it’s really simple to turn on with the

SSLCACertificateFile

Directive which allows you to specify the CA for the accepted certificates. In my own setup I have my own self signed certificate as CA and then all the authority certificates use it as the issuer. You can turn Client Certificate verification on per location basis simply by doing

<Location /some/web/location>
SSLVerifyClient require
</Location

And Apache will take care of requesting the client certificate and verifying it against the CA. The only caveat here is that TLSv1.3 currently fails to work for this, so you have to disable it with

SSLProtocol -TLSv1.3

Client Certificates in Firefox

Firefox is somewhat hard to handle for SSL because it includes its own hand written mozilla secure sockets code, which has a toolkit quite unlike any other ssl toolkit1. In order to import a client certificate and key into firefox, you need to create a pkcs12 file containing them and import that into the “Your Certificates” box which is under Preferences > Privacy & Security > View Certificates

Obviously, simply supplying a key file to firefox presents security issues because you’d like to prevent a clever hacker from gaining access to it and thus running off with your client certificate. Firefox achieves a modicum of security by doing all key operations over the PKCS#11 API via a software token, which should mean that even malicious javascript cannot gain access to your key but merely the signing API

However, assuming you don’t quite trust this software separation, you need to store your client signing key in a secure vault like a TPM to make sure no web hacker can gain access to it. Various crypto system connectors, like the OpenSSL TPM2 and TPM2 engine, already exist but because Firefox uses its own crytographic code it can’t take advantage of them. In fact, the only external object the Firefox crypto code can use is a PKCS#11 module.

Aside about TPM2 and PKCS#11

The design of PKCS#11 is that it is a loadable library which can find and enumerate keys and certificates in some type of hardware device like a USB Key or a PCI attached HSM. However, since the connector is simply a library, nothing requires it connect to something physical and the OpenDNSSEC project actually produces a purely software based cryptographic token. In theory, then, it should be easy

The problems come with the PKCS#11 expectation of key residency: The library allows the consuming program to enumerate a list of slots each of which may, or may not, be occupied by a single token. Each token may contain one or more keys and certificates. Now the TPM does have a concept of a key resident in NV memory, which is directly analagous to the PKCS#11 concept of a token based key. The problems start with the TPM2 PC Client Profile which recommends this NV area be about 512 bytes, which is big enough for all of one key and thus not very scalable. In fact, the imagined use case of the TPM is with volatile keys which are demand loaded.

Demand loaded keys map very nicely to the OpenSSL idea of a key file, which is why OpenSSL TPM engines are very easy to understand and use, but they don’t map at all into the concept of token resident keys. The closest interface PKCS#11 has for handling key files is the provisioning calls, but even there they’re designed for placing keys inside tokens and, once provisioned, the keys are expected to be non-volatile. Worse still, very few PKCS#11 module consumers actually do provisioning, they mostly leave it up to a separate binary they expect the token producer to supply.

Even if the demand loading problem could be solved, the PKCS#11 API requires quite a bit of additional information about keys, like ids, serial numbers and labels that aren’t present in the standard OpenSSL key files and have to be supplied somehow.

Solving the Key File to PKCS#11 Mismatch

The solution seems reasonably simple: build a standard PKCS#11 library that is driven by a known configuration file. This configuration file can map keys to slots, as required by PKCS#11, and also supply all the missing information. the C_Login() operation is expected to supply a passphrase (or PIN in PKCS#11 speak) so that would be the point at which the private key could be loaded.

One of the interesting features of the above is that, while it could be implemented for the TPM engine only, it can also be implemented as a generic OpenSSL key exporter to PKCS#11 that happens also to take engine keys. That would mean it would work for non-engine keys as well as any engine that exists for OpenSSL … a nice little win.

Building an OpenSSL PKCS#11 Key Exporter

A Token can be built from a very simple ini like configuration file, with the global section setting global properties, like manufacurer id and library description and each individual section being used to instantiate a slot containing one key. We can make the slot name, the id and the label the same if not overridden and use key file directives to load the public and private keys. The serial number seems best constructed from a hash of the public key parameters (again, if not overridden). In order to support engine keys, the token library needs to know which engine to invoke, so I added an engine keyword to tell it.

With that, the mechanics of making the token library work with any OpenSSL key are set, the only thing is to plumb in the PKCS#11 glue API. At this point, I should add that the goal is simply to get keys and tokens working, not to replicate a full featured PKCS#11 API, so you shouldn’t use this as something to test against for a reference implementation (the softhsm2 token is much better for that). However, it should be functional enough to use for storing keys in Firefox (as well as other things, see below).

The current reasonably full featured source code is here, with a reference build using the OpenSUSE Build Service here. I should add that some of the build failures are due to problems with p11-kit and others due to the way Debian gets the wrong engine path for libp11.

At Last: Getting TPM Keys working with Firefox

A final problem with Firefox is that there seems to be no way to import a certificate file for which the private key is located on a token. The only way Firefox seems to support this is if the token contains both the private key and the certificate. At least this is my own project, so some coding later, the token now supports certificates as well.

The next problem is more mundane: generating the certificate and key. Obviously, the safest key is one which has never left the TPM, which means the certificate request needs to be built from it. I chose a CSR type that also includes my name and my machine name for later easy discrimination (and revocation if I ever lose my laptop). This is the sequence of commands for my machine called jarvis.

create_tpm2_key -a key.tpm
openssl req -subj "/CN=James Bottomley/UID=jarivs/" -new -engine tpm2 -keyform engine -key key.tpm -nodes -out jarvis.csr
openssl x509 -in jarvis.csr -req -CA my-ca.crt -engine tpm2 -CAkeyform engine -CAkey my-ca.key -days 3650 -out jarvis.crt

As you can see from the above, the key is first created by the TPM, then that key is used to create a certificate request where the common name is my name and the UID is the machine name (this is just my convention, feel free to use your own) and then finally it’s signed by my own CA, which you’ll notice is also based on a TPM key. Once I have this, I’m free to create an ini file to export it as a token to Firefox

manufacturer id = Firefox Client Cert
library description = Cert for hansen partnership
[mozilla-key]
certificate = /home/jejb/jarvis.crt
private key = /home/jejb/key.tpm
engine = tpm2

All I now need to do is load the PKCS#11 shared object library into Firefox using Settings > Privacy & Security > Security Devices > Load and I have a TPM based client certificate ready for use.

Additional Uses

It turns out once you have a generic PKCS#11 exporter for engine keys, there’s no end of uses for them. One of the most convenient has been using TPM2 keys with gnutls. Although gnutls was quick to adopt TPM 1.2 based keys, it’s been much slower with TPM2 but because gnutls already has a PKCS#11 interface using the p11 kit URI format, you can easily build a config file of all the TPM2 keys you want it to use and simply use them by URI in gnutls.

Unfortunately, this has also lead to some problems, the biggest one being Firefox: Firefox assumes, once you load a PKCS#11 module library, that you want it to use every single key it can find, which is fine until it pops up 10 dialogue boxes each time you start it, one for each key password, particularly if there’s only one key you actually care about it using. This problem doesn’t seem solvable in the Firefox token interface, so the eventual way I did it was to add the ability to specify the config file in the environment (variable OPENSSL_PKCS11_CONF) and modify my xfce Firefox action to set this in the environment pointing at a special configuration file with only Firefox’s key in it.

Conclusions and Future Work

Hopefully I’ve demonstrated this simple PKCS#11 converter can be useful both to keeping Firefox keys safe as well as uses in other things like gnutls. Unfortunately, it turns out that the world wide web is turning against PKCS#11 tokens as having usability problems and is moving on to something called FIDO2 tokens which have the web browser talking directly to the USB token. In my next technical post I hope to explain how you can use the Linux Kernel USB gadget system to connect a TPM up easily as a FIDO2 token so you can use the new passwordless webauthn protocol seamlessly.

Why Microsoft is a good steward for GitHub

There seems to be a lot of hysteria going on in various communities that depend on GitHub for their project hosting around the Microsoft acquisition (just look in the comments here and here).  Obviously a lot of social media ink will be expended on this, so I’d just like to explain why as a committed open source developer, I think this will actually be a good thing.

Firstly, it’s very important to remember that git may be open source, but GitHub isn’t: none of the scripts that run the service have much published source code at all.  It may be a closed source hosting infrastructure that a lot of open source projects rely on but that doesn’t make it open source itself.  So why is GitHub not open source?  Well, it all goes back to the business model.  Notwithstanding fantastic market valuations there are lots of companies that play in the open source ecosystem, like GitHub, which struggle to find a sustainable business model (or even revenue).  This leads to a lot of open closed/open type models like GitHub (the reason GitHub keeps the code closed is so they can sell it to other companies for internal source management) or Docker Enterprise.

Secondly, even if GitHub were fully open source, as I’ve argued in my essays about the GPL, to trust a corporate player in the ecosystem, you need to be able to understand fully its business motivation for being there and verify the business goals align with the community ones.  As long as the business motivation is transparent and aligned with the community, you know you can trust it.  However, most of the new supposedly “open source” companies don’t have clear business models at all, which means their business motivation is anything but transparent.  Paradoxically this means that most of the new corporate idols in the open source ecosystem are remarkably untrustworthy because their business model changes from week to week as they struggle to please their venture capitalist overlords.  There’s no way you can get the transparency necessary for open source trust if the company itself doesn’t know what its business model will be next week.

Finally, this means that companies with well established open source business models and motivations that don’t depend on the whims of VCs are much more trustworthy in open source in the long term.  Although it’s a fairly recent convert, Microsoft is now among these because it’s clearly visible how its conversion from desktop to cloud both requires open source and requires Microsoft to play nicely with open source.  The fact that it has a trust deficit from past actions is a bonus because from the corporate point of view it has to be extra vigilant in maintaining its open source credentials.  The clinching factor is that GitHub is now ancillary to Microsoft’s open source strategy, not its sole means of revenue, so lots of previous less community oriented decisions, like keeping the GitHub code closed source, can be revisited in time as Microsoft seeks to gain community trust.

For the record, I should point out that although I have a github account, I host all my code on kernel.org mostly because the GitHub workflow really annoys me, having spent a lot of time trying to deduce commit motivations in a sparse git commit messages which then require delving into github issues and pull requests only to work out that most of the necessary details are in some private slack back channel well away from public view.  Regardless of who owns GitHub, I don’t see this workflow problem changing any time soon, so I’ll be sticking to my current hosting setup.

Using Elliptic Curve Cryptography with TPM2

One of the most significant advances going from TPM1.2 to TPM2 was the addition of algorithm agility: The ability of TPM2 to work with arbitrary symmetric and asymmetric encryption schemes.  In practice, in spite of this much vaunted agile encryption capability, most actual TPM2 chips I’ve seen only support a small number of asymmetric encryption schemes, usually RSA2048 and a couple of Elliptic Curves.  However, the ability to support any Elliptic Curve at all is a step up from TPM1.2.  This blog post will detail how elliptic curve schemes can be integrated into existing cryptographic systems using TPM2.  However, before we start on the practice, we need at least a tiny swing through the theory of Elliptic Curves.

What is an Elliptic Curve?

An Elliptic Curve (EC) is simply the set of points that lie on the curve in the two dimensional plane (x,y) defined by the equation

y2 = x3 + ax + b

which means that every elliptic curve can be parametrised by two constants a and b.  The set of all points lying on the curve plus a point at infinity is combined with an addition operation to produce an abelian (commutative) group.  The addition property is defined by drawing straight lines between two points and seeing where they intersect the curve (or picking the infinity point if they don’t intersect).  Wikipedia has a nice diagrammatic description of this here.  The infinity point acts as the identity of the addition rule and the whole group is denoted E.

The utility for cryptography is that you can define an integer multiplier operation which is simply the element added to itself n times, so for P ∈ E, you can always find Q ∈ E such that

Q = P + P + P … = n × P

And, since it’s a simple multiplication like operation, it’s very easy to compute Q.  However, given P and Q it is computationally very difficult to get back to n.  In fact, it can be demonstrated mathematically that trying to compute n is equivalent to the discrete logarithm problem which is the mathematical basis for the cryptographic security of RSA.  This also means that EC keys suffer the same (actually more so) problems as RSA keys: they’re not Quantum Computing secure (vulnerable to the Quantum Shor’s algorithm) and they would be instantly compromised if the discrete logarithm problem were ever solved.

Therefore, for any elliptic curve, E, you can choose a known point G ∈ E, select a large integer d and you can compute a point P = d × G.  You can then publish (P, G, E) as your public key knowing it’s computationally infeasible for anyone to derive your private key d.

For instance, Diffie-Hellman key exchange can be done by agreeing (E, G) and getting Alice and Bob to select private keys dA, dB.  Then knowing Bob’s public key PB, Alice can select a random integer r, which she publishes, and compute a key agreement as a secret point on the Elliptic Curve (r dA) × PB.  Bob can derive the same Elliptic Curve point because

(r dA) × PB = (r dA)dB × G = (r dB) dA × G = (r dB) × PA

The agreement is a point on the curve, but you can use an agreed hashing or other mechanism to get from the point to a symmetric key.

Seems simple, but the problem for computing is that we really want to use integers and right at the moment the elliptic curve is defined over all the real numbers, meaning E is of infinite size and involves floating point computations (of rather large precision).

Elliptic Curves over Finite Fields

Fortunately there is a mathematical theory of finite fields, called Galois Theory, which allows us to take the Galois Field over prime number p, which is denoted GF(p), and compute Elliptic Curve points over this field.  This derivation, which is mathematically rather complicated, is denoted E(GF(p)), where every point (x,y) is represented by a pair of integers between 0 and p-1.  There is another theory that says the number of elements in E(GF(p))

n = |E(GF(p))|

is roughly the same size as p, meaning if you choose a 32 bit prime p, you likely have a field over roughly 2^32 elements.  For every point P in E(GF(p)) it is also mathematically proveable that n × P = 0. where 0 is the zero point (which was the infinity point in the real elliptic curve).

This means that you can take any point, G,  in E(GF(p)) and compute a subgroup based on it:

EG = { ∀m ∈ Zn : m × G }

If you’re lucky |EG| = |E(GF(p))| and G is the generator of the entire group.  However, G may only generate a subgroup and you will find |EG| = h|E(GF(p))| where integer h is called the cofactor.  In general you want the cofactor to be small (preferably less than four) for EG to be cryptographically useful.

For a computer’s purposes, EG is the elliptic curve group used for integer arithmetic in the cryptographic algorithms.  The Curve and Generator is then defined by (p, a, b, Gx, Gy, n, h) which are the published parameters of the key (Gx, Gy represent the x and y elements of point G).  You select a random number d as your private key and your public key P = d × G exactly as above, except now P is easy to compute with integer operations.

Problems with Elliptic Curves

Although I stated above that solving P = d × G is equivalent in difficulty to the discrete logarithm problem, that’s not generally true.  If the discrete logarithm problem were solved, then we’d easily be able to compute d for every generator and curve, but it is possible to pick curves for which d can be easily computed without solving the discrete logarithm problem. This is the reason why you should never pick your own curve parameters (even if you think you know what you’re doing) because it’s very easy to choose a compromised curve.  As a demonstration of the difficulty of the problem: each of the major nation state actors, Russia, China and the US, publishes their own curve parameters for use in their own cryptographic EC implementations and each of them thinks the parameters published by the others is compromised in a way that allows the respective national security agencies to derive private keys.  So if nation state actors can’t tell if a curve is compromised or not, you surely won’t be able to either.

Therefore, to be secure in EC cryptography, you pick and existing curve which has been vetted and select some random Generator Point on it.  Of course, if you’re paranoid, that means you won’t be using any of the nation state supplied curves …

Using the TPM2 with Elliptic Curves in Cryptosystems

The initial target for this work was the openssl cryptosystem whose libraries are widely used for deriving other uses (like https in apache or openssh). Originally, when I did the initial TPM2 enabling of openssl as described in this blog post, I added TPM2 as a patch to the existing TPM 1.2 openssl_tpm_engine.  Unfortunately, openssl_tpm_engine seems to be pretty much defunct at this point, so I started my own openssl_tpm2_engine as a separate git tree to begin experimenting with Elliptic Curve keys (if you don’t use git, you can download the tar file here). One of the benefits to running my own source tree is that I can now add a testing infrastructure that makes use of the IBM TPM emulator to make sure that the basic cryptographic operations all work which means that make check functions even when a TPM2 isn’t available.  The current key creation and import algorithms use secured connections to the TPM (to avoid eavesdropping) which means it’s only really possible to construct them using the IBM TSS. To make all of this easier, I’ve set up an openSUSE Build Service repository which is building for all major architectures and the openSUSE and Fedora distributions (ignore the failures, they’re currently induced because the TPM emulator only currently works on 64 bit little endian systems, so make check is failing, but the TPM people at IBM are working on this, so eventually the builds should be complete).

TPM2 itself also has some annoying restrictions.  The biggest of which is that it doesn’t allow you to pass in arbitrary elliptic curve parameters; you may only use elliptic curves which the TPM itself knows.  This will be annoying if you have an existing EC key you’re trying to import because the TPM may reject it as an unknown algorithm.  For instance, openssl can actually compute with arbitrary EC parameters, but has 39 current elliptic curves parametrised by name. By contrast, my Nuvoton TPM2 inside my Dell XPS 13 knows precisely two curves:

jejb@jarvis:~> create_tpm2_key --list-curves
prime256v1
bnp256

However, assuming you’ve picked a compatible curve for your EC private key (and you’ve defined a parent key for the storage hierarchy) you can simply import it to a TPM bound key:

create_tpm2_key -p 81000001 -w key.priv key.tpm

The tool will report an error if it can’t convert the curve parameters to a named elliptic curve known to the TPM

jejb@jarvis:~> openssl genpkey -algorithm EC -pkeyopt ec_paramgen_curve:brainpoolP256r1 > key.priv
jejb@jarvis:~> create_tpm2_key -p 81000001 -w key.priv key.tpm
TPM does not support the curve in this EC key
openssl_to_tpm_public failed with 166
TPM_RC_CURVE - curve not supported Handle number unspecified

You can also create TPM resident private keys simply by specifying the algorithm

create_tpm2_key -p 81000001 --ecc bnp256 key.tpm

Once you have your TPM based EC keys, you can use them to create public keys and certificates.  For instance, you create a self-signed X509 certificate based on the tpm key by

openssl req -new -x509 -sha256  -key key.tpm -engine tpm2 -keyform engine -out my.crt

Why you should use EC keys with the TPM

The initial attraction is the same as for RSA keys: making it impossible to extract your private key from the system.  However, the mathematical calculations for EC keys are much simpler than for RSA keys and don’t involve finding strong primes, so it’s much simpler for the TPM (being a fairly weak calculation machine) to derive private and public EC keys.  For instance the times taken to derive a RSA key from the primary seed and an EC key differ dramatically

jejb@jarvis:~> time tsscreateprimary -hi o -ecc bnp256 -st
Handle 80ffffff

real 0m0.111s
user 0m0.000s
sys 0m0.014s

jejb@jarvis:~> time tsscreateprimary -hi o -rsa -st
Handle 80ffffff

real 0m20.473s
user 0m0.015s
sys 0m0.084s

so for a slow system like the TPM, using EC keys is a significant speed advantage.  Additionally, there are other advantages.  The standard EC Key signature algorithm is a modification of the NIST Digital Signature Algorithm called ECDSA.  However DSA and ECDSA require a cryptographically strong (and secret) random number as Sony found out to their cost in the EC Key compromise of Playstation 3.  The TPM is a good source of cryptographically strong random numbers and if it generates the signature internally, you can be absolutely sure of keeping the input random number secret.

Why you might want to avoid EC keys altogether

In spite of the many advantages described above, EC keys suffer one additional disadvantage over RSA keys in that Elliptic Curves in general are very hot fields of mathematical research so even if the curve you use today is genuinely not compromised, it’s not impossible that a mathematical advance tomorrow will make the curve you chose (and thus all the private keys you generated) vulnerable.  Of course, the same goes for RSA if anyone ever cracks the discrete logarithm problem, but solving that problem would likely be fully published to world acclaim and recognition as a significant contribution to the advancement of number theory.  Discovering an attack on a currently used elliptic curve on the other hand might be better remunerated by offering to sell it privately to one of the national security agencies …

TPM2 and Linux

Recently Microsoft started mandating TPM2 as a hardware requirement for all platforms running recent versions of windows.  This means that eventually all shipping systems (starting with laptops first) will have a TPM2 chip.  The reason this impacts Linux is that TPM2 is radically different from its predecessor TPM1.2; so different, in fact, that none of the existing TPM1.2 software on Linux (trousers, the libtpm.so plug in for openssl, even my gnome keyring enhancements) will work with TPM2.  The purpose of this blog is to explore the differences and how we can make ready for the transition.

What are the Main 1.2 vs 2.0 Differences?

The big one is termed Algorithm Agility.  TPM1.2 had SHA1 and RSA2048 only.  TPM2 is designed to have many possible algorithms, including support for elliptic curve and a host of government mandated (Russian and Chinese) crypto systems.  There’s no requirement for any shipping TPM2 to support any particular algorithms, so you actually have to ask your TPM what it supports.  The bedrock for TPM2 in the West seems to be RSA1024-2048, ECC and AES for crypto and SHA1 and SHA256 for hashes1.

What algorithm agility means is that you can no longer have root keys (EK and SRK see here for details) like TPM1.2 did, because a key requires a specific crypto algorithm.  Instead TPM2 has primary “seeds” and a Key Derivation Function (KDF).  The way this works is that a seed is simply a long string of random numbers, but it is used as input to the KDF along with the key parameters and the algorithm and out pops a real key based on the seed.  The KDF is deterministic, so if you input the same algorithm and the same parameters you get the same key again.  There are four primary seeds in the TPM2: Three permanent ones which only change when the TPM2 is cleared: endorsement (EPS), Platform (PPS) and Storage (SPS).  There’s also a Null seed, which is used for ephemeral keys and changes every reboot.  A key derived from the SPS can be regarded as the SRK and a key derived from the EPS can be regarded as the EK. Objects descending from these keys are called members of hierarchies2. One of the interesting aspects of the TPM is that the root of a hierarchy is a key not a seed (because you need to exchange secret information with the TPM), and that there can be multiple of these roots with different key algorithms and parameters.

Additionally, the mechanism for making use of keys has changed slightly.  In TPM 1.2 to import a secret key you wrapped it asymmetrically to the SRK and then called LoadKeyByBlob to get a use handle.  In TPM2 this is a two stage operation, firstly you import a wrapped (or otherwise protected) private key with TPM2_Import, but that returns a private key structure encrypted with the parent key’s internal symmetric key.  This symmetrically encrypted key is then loaded (using TPM2_Load) to obtain a use handle whenever needed.  The philosophical change is from online keys in TPM 1.2 (keys which were resident inside the TPM) to offline keys in TPM2 (keys which now can be loaded when needed).  This philosophy has been reinforced by reducing the space available to keep keys loaded in TPM2 (see later).

Playing with TPM2

If you have a recent laptop, chances are you either have or can software upgrade to a TPM2.  I have a dell XPS13 the skylake version which comes with a software upgradeable Nuvoton TPM.  Dell kindly provides a 1.2->2 switching program here, which seems to work under Freedos (odin boot) so I have a physical TPM2 based system.  For those of you who aren’t so lucky, you can still play along, but you need a TPM2 emulator.  The best one is here; simply download and untar it then type make in the src directory and run it as ./tpm_server.  It listens on two TCP ports, 2321 and 2322, for TPM commands so there’s no need to install it anywhere, it can be run directly from the source directory.

After that, you need the interface software called tss2.  The source is here, but Fedora 25 and recent Ubuntu already package it.  I’ve also built openSUSE packages here.  The configuration of tss2 is controlled by environment variables.  The most important one is TPM_INTERFACE_TYPE which tells it how to connect to the TPM2.  If you’re using a simulator, you set this to “socsim” and if you have a real TPM2 device you set it to “dev”.  One final thing about direct device connection: in tss2 there’s no daemon like trousers had to broker the connection, all your users connect directly to the TPM2 device /dev/tpm0.  To do this, the device has to support read and write by arbitrary users, so its permissions need to be 0666.  I’ve got a udev script to achieve this

# tpm 2 devices need to be world readable
SUBSYSTEM=="tpm", ACTION=="add", MODE="0666"

Which goes in /etc/udev/rules.d/80-tpm-2.rules on openSUSE.  The next thing you need to do, if you’re running the simulator, is power it on and start it up (for a real device, this is done by the bios):

tsspowerup
tssstartup

The simulator will now create a NVChip file wherever you started it to store NV ram based objects, which it will read on next start up.  The first thing you need to do is create an SRK and store it in NV memory.  Microsoft uses the well known key handle 81000001 for this, so we’ll do the same.  The reason for doing this is that a real TPM takes ages to run the KDF for RSA keys because it has to look for prime numbers:

jejb@jarvis:~> TPM_INTERFACE_TYPE=socsim time tsscreateprimary -hi o -st -rsa
Handle 80000000
0.03 user 0.00 system 0:00.06 elapsed

jejb@jarvis:~> TPM_INTERFACE_TYPE=dev time tsscreateprimary -hi o -st -rsa
Handle 80000000
0.04 user 0.00 system 0:20.51 elapsed

As you can see: the simulator created a primary storage key (the SRK) in a few milliseconds, but it took my real TPM2 20 seconds to do it3 … not something you want to wait for, hence the need to store this permanently under a well known key handle and get rid of the temporary copy

tssevictcontrol -hi o -ho 80000000 -hp 81000001
tssflushcontext -ha 80000000

tssevictcontrol tells the TPM to copy the key at transient handle 800000004  to permanent NV handle 81000001 and tssflushcontext erases the transient key.  Flushing transient objects is very important, because TPM2 has a lot less transient storage space than TPM1.2 did; usually only about three handles worth.  You can tell how much you have by doing

tssgetcapability -cap 6|grep -i transient
TPM_PT 0000010e value 00000003 TPM_PT_HR_TRANSIENT_MIN - the minimum number of transient objects that can be held in TPM RAM

Where the value (00000003) tells me that the TPM can store at least 3 transient objects.  After that you’ll start getting out of space errors from it.

The final step in taking ownership of a TPM2 is to set the authorization passwords.  Each of the four hierarchies (Null, Owner, Endorsement, Platform) and the Lockout has a possible authority password.  The Platform authority is cleared on startup, so there’s not much point setting it (it’s used by the BIOS or Firmware to perform TPM functions).  Of the other four, you really only need to set Owner, Endorsement and Lockout (I use the same password for all of them).

tsshierarchychangeauth -hi l -pwdn <your password>
tsshierarchychangeauth -hi e -pwdn <your password>
tsshierarchychangeauth -hi o -pwdn <your password>

After this is done, you’re all set.  Note that as well as these authorizations, each object can have its own authorization (or even policy), so the SRK you created earlier still has no password, allowing it to be used by anyone.  Note also that the owner authorization controls access to the NV memory, so you’ll need to supply it now to make other objects persistent.

An Aside about Real TPM2 devices and the Resource Manager

Although I’m using the code below to store my keys in the TPM2, there’s a couple of practical limitations which means it won’t work for you if you have multiple TPM2 using applications without a kernel update.  The two problems are

  1. The Linux Kernel TPM2 device /dev/tpm0 only allows one user at once.  If a second application tries to open the device it will get an EBUSY which causes TSS_Create() to fail.
  2. Because most applications make use of transient key slots and most TPM2s have only a couple of these, simultaneous users can end up running out of these and getting unexpected out of space errors.

The solution to both of these is something called a Resource Manager (RM).  What the RM does is effectively swap transient objects in and out of the TPM as needed to prevent it from running out of space.  Linux has an issue in that both the kernel and userspace are potential users of TPM keys so the resource manager has to live inside the kernel.  Jarkko Sakkinen has preliminary resource manager patches here, and they will likely make it into kernel 4.11 or 4.12.  I’m currently running my laptop with the RM patches applied, so multiple applications work for me, but since these are preliminary patches, I wouldn’t currently advise others to do this.  The way these patches work is that once you declare to the kernel via an ioctl that you want to use the RM, every time you send a command to the TPM, your context gets swapped in, the command is executed, the context is swapped out and the response sent meaning that no other user of the TPM sees your transient objects.  The moment you send the ioctl, the TPM device allows another user to open it as well.

Using TPM2 as a keystore

Once the implementation is sorted out, openssl and gnome-keyring patches can be produced for TPM2.  The only slight wrinkle is that for create_tpm2_key you require a parent key to exist in the NV storage (at the 81000001 handle we talked about previously).  So to convert from a password protected openssh RSA key to a TPM2 based one, you do

create_tpm2_key -a -p 81000001 -w id_rsa id_rsa.tpm
mv id_rsa.tpm id_rsa

And then gnome keyring manager will work nicely (make sure you keep a copy of your original private key in case you want to move to a new laptop or reseed the TPM2).  If you use the same TPM2 password as your original key password, you won’t even need to update the gnome loginkeyring for the new password.

Conclusions

Because of the lack of an in-kernel Resource Manager, TPM2 is ready for experimentation in Linux but definitely not ready for prime time yet (unless you’re willing to patch your kernel).  Hopefully this will change in the 4.11 or 4.12 kernel when the Resource Manager finally goes upstream5.

Looking forwards to the new stack, the lack of a central daemon is really a nice feature: tcsd crashing used to kill all of my TPM key based applications, but with tss2 having no central daemon, everything has just worked(tm) so far.  A kernel based RM also means that the kernel can happily use the TPM (for its trusted keys and disk encryption) without interfering with whatever userspace is doing.

TPM enabling gnome-keyring

One of the questions about the previous post on using your TPM as a secure key store was “could the TPM be used to protect ssh keys?”  The answer is yes, because openssh uses openssl (so you can simply convert an openssh private key to a TPM private key) but the ssh-agent wouldn’t work because ssh-add passes in the private keys by their component primes, which is not possible when the private key is guarded by the TPM.  However,  that made me actually look at gnome-keyring to figure out how it worked and whether it could be used with the TPM.  The answer is yes, but there are also some interesting side effects of TPM enabling gnome-keyring.

Gnome-keyring Architecture

Gnome-keyring consists of essentially three components: a pluggable store backend, a secure passphrase holder (which is implemented as a backend store) and an agent frontend.  The frontend and backend talk to each other using the pkcs11 protocol.  This also means that the backend can serve anything that also speaks pkcs11, which most encryption systems do.  The stores consist of a variety of file or directory backed keys (for instance the ssh-store simply loads all the ssh keys in your $HOME/.ssh; the secret-store uses the gnome default keyring store, $HOME/.local/shared/keyring/,  to store collections of passwords)  The frontends act as bridges between a variety of external protocols and the keyring daemon.  They take whatever external protocol they’re speaking in, convert the request to pkcs11 and query the backends for the information.  The most important frontend is the login one which is called by gnome-keyring-daemon at start of day to unlock the secret-store which contains all your other key passwords using your login password as the key.  The ssh-agent frontend speaks the ssh agent protocol and converts key and signing requests to pkcs11 which is mostly served by the ssh-store.  The gpg-agent store speaks a very cut down version of the gpg agent protocol: basically all it does is allow you to store gpg key passwords in the secret-store; it doesn’t do any cryptographic operations.

Pkcs11 Essentials

Pkcs11 is a highly complex protocol designed for opening sessions which query or operate on tokens, which roughly speaking represent a bundle of objects (If you have a USB crypto key, that’s usually represented as a token and your stored keys as objects).  There is some intermediate stuff about slots, but that mostly applies to tokens which may be offline and need insertion, which isn’t relevant to the gnome keyring.  The objects may have several levels of visibility, but the most common is public (always visible) and private (must be logged in to the token to see them).  Objects themselves have a variety of attributes, some of which depend on what type of object they are and some of which are universal (like id and label).  For instance, an object representing an RSA public key would have the public exponent and the public modulus as queryable attributes.

The pkcs11 protocol also has a reasonably comprehensive object finding protocol.  An arbitrary list of attributes and values can be passed to the query and it will return all objects that fully match.  The token identifier is a query attribute which may be present but doesn’t have to be, so if you omit it, you end up searching over every token the pkcs11 subsystem knows about.

The other operation that pkcs11 understands is logging into a token with a pin (which is pkcs11 speak for a passphrase).  The pin doesn’t have to be supplied by the entity performing the login, it may be supplied to the token via an out of band mechanism (for instance the little button on the yukikey, or even a real keypad).  The important thing for gnome keyring here is that logging into a token may be as simple as sending the login command and letting the token sort out the authorization, which is the way gnome keyring operates.

You also need to understand is conventions about searching for keys.  The pkcs11 standard recommends (but does not require) that public and private key pairs, which exist as two separate objects, should have the same id attribute.  This means that if you want to find an rsa private key, the way you do this is by searching the public objects for the exponent and modulus.  Once this is returned, you log into the token and retrieve the private key object by the public key id.

The final thing to understand is that once you have the private key object, it merely acts as an entitlement to have the token perform certain private key operations for you (like encryption, decryption or signing); it doesn’t mean you have access to the private key itself.

Gnome Keyring handling of ssh keys

Once the ssh-agent side of gnome keyring receives a challenge, it must respond by returning the private key signature of the challenge.  To do this, it searches the pkcs11 for the key used for the challenge (for RSA keys, it searches by modulus and exponent, for DSA keys it searches by signature primes, etc).  One interesting point to note is the search isn’t limited to the gnome keyring ssh token store, so if the key is found anywhere, in any pkcs11 token, it will be used.  The expectation is that they key id attribute will be the ssh fingerprint and the key label attribute will be the ssh comment, but these aren’t used in the actual search.  Once the public key is found, the agent logs into the token, retrieves the private key by label and id and proceeds to get the private key to sign the challenge.

Adding TPM key handling to the ssh store

Gnome keyring is based on GNU libgcrypt which handles all cryptographic objects via s-expressions (which are basically lisp like strings).  Gcrypt itself doesn’t seem to have an s-expression for a token, so the actual signing will be done inside the keyring code.  The s-expression I chose to represent a TPM based private key is

(private-key
  (rsa
    (tpm
      (blob <binary key blob>)
      (auth <authoriztion>))))

The rsa is necessary for it to be recognised for RSA signatures.  Now the ssh-store is modified to recognise the PEM guards for TPM keys, load the key blob up into the s-expression and ask the secret-store for the authorization passphrase which is also loaded.  In theory, it might be safer to ask the secret store for the authorization at key use time, but this method mirrors what happens to private keys, which are decrypted at load time and stored as s-expressions containing the component primes.

Now the RSA signing code is hooked to check for TPM s-expressions and divert the signature to the TPM if they’re found.  Once this is done, gnome keyring is fully enabled for TPM based ssh keys.  The initial email thread about this is here, and an openSUSE build repository of the modified code is here.

One important design constraint for this is that people (well, OK me) have a lot of ssh keys, sometimes more than could be effectively stored by the TPM in its internal shielded memory (plus if you’re like me, you’re using the TPM for other things like VPN), so the design of the keyring TPM additions is not to burden that memory further, thus TPM keys are only loaded into the TPM whenever they’re needed for an operation and are unloaded otherwise.  This means the TPM can scale to hundreds of keys, but at the expense of taking longer: Instead of simply asking the TPM to sign something, you have to first ask the TPM to load and unwrap (which is an RSA operation) the key, then use it to sign.  Effectively it’s two expensive TPM operations per real cryptographic function.

Using TPM based ssh keys

Once you’ve installed the modified gnome-keyring package, you’re ready actually to make use of it.  Since we’re going to replace all your ssh private keys with TPM equivalents, which are keyed to a given storage root key (SRK)1 which changes every time the TPM is cleared, it is prudent to take a backup of all your ssh keys on some offline encrypted USB stick, just in case you ever need to restore them (or transfer them to a new laptop2).

cd ~/.ssh/
for pub in *rsa*.pub; do
    priv=$(basename $pub .pub)
    echo $priv
    create_tpm_key -m -a -w $priv ${priv}.tpm
    mv ${priv}.tpm $priv
done

You’ll be prompted first to create a TPM authorization password for the key, then to verify it, then to give the PEM password for the ssh key (for each key).  Note, this only transfers RSA keys (the only algorithm the TPM can handle) and also note the script above is overwriting the PEM private key, so make sure you have a backup.  The create_tpm_key command comes from the openssl_tpm_engine package, which I’ve patched here to support random migration authority and well known SRK authority.

All you have to do now is log out and back in (to restart the gnome-keyring daemon with the new ssh keystore) and you’re using TPM based keys for all your ssh operations.  I’ve noticed that this adds a couple of hundred milliseconds per login, so if you batch stuff over ssh, this is why your scripts are slower.

Unprivileged Build Containers

A while ago, a goal I set myself was to be able to maintain my build and test environments for architecture emulation containers without having to do any of the tasks as root and without creating any suid binaries to do this.  One of the big problems here is that distributions get annoyed (and don’t run correctly) if root doesn’t own most of the files … for instance the installers all check to see that the file got installed with the correct ownership and permissions and fail if they don’t.  Debian has an interesting mechanism, called fakeroot, to get around this using a preload library intercepting the chmod and chown system calls, but it’s getting a bit hackish to try to extend this to work permanently for an emulation container.

The correct way to do this is with user namespaces so the rest of this post will show you how.  Before we get into how to use them, lets begin with the theory of how user namespaces actually work.

Theory of User Namespaces

A user namespace is the single namespace that can be created by an unprivileged user.  Their job is to map a set of interior (inside the user namespace) uids, gids and projids1 to a set of exterior (outside the user namespace).

The way this works is that the root user namespace simply has a 1:1 identity mapping of all 2^32 identifiers, meaning it fully covers the space.  However, any new user namespace only need remap a subset of these.  Any id that is not mapped into the user namespace becomes inaccessible to that namespace.  This doesn’t mean completely inaccessible, it just means any resource owned or accessed by an unmapped id treats an attempted access (even from root in the namespace) as though it were completely unprivileged, so if the resource is readable by any id, it can still be read even in a user namespace where its owning id is unmapped.

User namespaces can also be nested but the nested namespace can only map ids that exist in its parent, so you can only reduce but not expand the id space by nesting.  The way the nested mapping works is that it remaps through all the parent namespaces, so what appears on the resource is still the original exterior ids.

User Namespaces also come with an owner (the uid/gid of the process that created the container).  The reason for this is that this owner is allowed to execute setuid/setgid to any id mapped into the namespace, so the owning uid/gid pair is the effective “root” of the container.  Note that this setuid/setgid property works on entry to the namespace even if uid 0 is not mapped inside the namespace, but won’t survive once the first setuid/setgid is issued.

The final piece of the puzzle is that every other namespace also has an owning user namespace, so while I cannot create a mount namespace as unprivileged user jejb, I can as remapped root inside my user namespace

jejb@jarvis:~> unshare --mount
unshare: unshare failed: Operation not permitted
jejb@jarvis:~> nsenter --user=/tmp/userns
root@jarvis:~# unshare --mount
root@jarvis:~#

And once created, I can always enter this mount namespace provided I’m also in my user namespace.

Setting up Unprivileged Namespaces

Any system user can actually create a user namespace.  However a non-root (meaning not uid zero in the parent namespace) user cannot remap any exterior id except their own.  This means that, because a build container needs a range of ids, it’s not possible to set up the intial remapped namespace without the help of root.  However, once that is done, the user can pretty much do every other operation2

The way remap ranges are set up is via the uid_map, gid_map and projid_map files sitting inside the /proc/<pid> directory.  These files may only be written to once and never updated3

As an example, to set up a build container, I need a remapping for every id that would be created during installation.  Traditionally for Linux, these are ids 0-999.  I want to remap them down to something unprivileged, say 100,000 so my line entry for this is

0 100000 1000

However, I also want an identity mapping for my own id (currently I’m at uid 1000), so I can still use my home directory from within the build container.  This also means I can create the roots for the containers within my home directory.  Finally, the nobody user and nobody,nogroup groups also need to be mapped, so the final uid map entries look like

0 100000 1000
1000 1000 1
65534 101001 1

For the groups, it’s even more complex because on openSUSE, I’m a member of the users group (gid 100) which sits in the middle of the privileged 0-999 group range, so the gid_map entry I use is

0 100000 100
100 100 1
101 100100 899
65533 101000 2

Which is almost up to the kernel imposed limit of five separate lines.

Finally, here’s how to set this up and create a binding for the user namespace.  As myself (I’m uid 1000 user name jejb) I do

jejb@jarvis:~> unshare --user
nobody@jarvis:~> echo $$
20211
nobody@jarvis:~>

Note that I become nobody inside the container because currently the map files are unwritten so there are no mapped ids at all.  Now as root, I have to write the mapping files and bind the entry file to the namespace somewhere

jarvis:/home/jejb # echo 1|awk '{print "0 100000 1000\n1000 1000 1\n65534 101001 1"}' > /proc/20211/uid_map
jarvis:/home/jejb # echo 1|awk '{print "0 100000 100\n100 100 1\n101 100100 899\n65533 101000 2"}' > /proc/20211/gid_map
jarvis:/home/jejb # touch /tmp/userns
jarvis:/home/jejb # mount --bind /proc/20211/ns/user /tmp/userns

Now I can exit my user namespace because it’s permanently bound and the next time I enter it I become root inside the container (although with uid 100000 outside)

jejb@jarvis:~> nsenter --user=/tmp/userns
root@jarvis:~# id
uid=0(root) gid=0(root) groups=0(root)
root@jarvis:~# su - jejb
jejb@jarvis:~> id
uid=1000(jejb) gid=100(users) groups=100(users)

Giving me a user namespace with sufficient mapped ids to create a build container.

Unprivileged Architecture Emulation Containers

Essentially, I can use the user namespace constructed above to bootstrap and enter the entire build container and its mount namespace with one proviso that I have to have a pre-created devices directory because I don’t possess the mknod capability as myself, so my container root also doesn’t possess it.  The way I get around this is to create the initial dev directory as root and then change the ownership to 100000.100000 (my unprivileged ids)

jejb@jarvis:~/containers/debian-amd64/dev> ls -l
total 0
lrwxrwxrwx 1 100000 100000 13 Feb 20 09:45 fd -> /proc/self/fd/
crw-rw-rw- 1 100000 100000 1, 7 Feb 20 09:45 full
crw-rw-rw- 1 100000 100000 1, 3 Feb 20 09:45 null
lrwxrwxrwx 1 100000 100000 8 Feb 20 09:45 ptmx -> pts/ptmx
drwxr-xr-x 2 100000 100000 6 Feb 20 09:45 pts/
crw-rw-rw- 1 100000 100000 1, 8 Feb 20 09:45 random
drwxr-xr-x 2 100000 100000 6 Feb 20 09:45 shm/
lrwxrwxrwx 1 100000 100000 15 Feb 20 09:45 stderr -> /proc/self/fd/2
lrwxrwxrwx 1 100000 100000 15 Feb 20 09:45 stdin -> /proc/self/fd/0
lrwxrwxrwx 1 100000 100000 15 Feb 20 09:45 stdout -> /proc/self/fd/1
crw-rw-rw- 1 100000 100000 5, 0 Feb 20 09:45 tty
crw-rw-rw- 1 100000 100000 1, 9 Feb 20 09:45 urandom
crw-rw-rw- 1 100000 100000 1, 5 Feb 20 09:45 zero

This seems to be sufficient of a dev skeleton to function with.  For completeness sake, I placed my bound user namespace into /run/build-container/userns and following the original architecture container emulation post, with modified susebootstrap and build-container scripts.  The net result is that as myself I can now enter and administer and update the build and test architecture emulation container with no suid required

jejb@jarvis:~> nsenter --user=/run/build-container/userns --mount=/run/build-container/ppc64
root@jarvis:/# id
uid=0(root) gid=0(root) groups=0(root)
root@jarvis:/# uname -m
ppc64
root@jarvis:/# su - jejb
jejb@jarvis:~> id
uid=1000(jejb) gid=100(users) groups=100(users)

The only final wrinkle is that root has to set up the user namespace on every boot, but that’s only because there’s no currently defined operating system way of doing this.

Are GPLv2 and CDDL incompatible?

Canonical recently threw this issue into sharp relief by their decision to ship CDDL licensed ZFS as a module of the GPLv2 licensed Linux kernel.  Reading their legal justification for this leaves me somewhat unconvinced because it’s essentially the same “not a derivative work” argument that a number of dubious actors have used to justify binary modules.  So what I’d like to do is look at this issue from a completely different viewpoint.  First by accepting the premise that CDDL and GPLv2 are incompatible (since there’s some debate on this) and secondly by accepting the even more controversial proposition that creating a kernel module is a derivative work.  I don’t want to debate these premises because it’s a worst case assumption I’m using as inputs to make the following analysis possible.

What is compliance?

One of the curious thing about CDDL and GPLv2 is that they’re both copyleft (albeit in differing forms) and the compliance requirements: the release of complete corresponding source code for your binary containing the licensed work.  In fact, the only significant difference is the requirement for build scripts, which is in GPLv2 but not in CDDL.  Therefore you can say that if you follow the compliance regime for GPLv2 on CDDL code, you’ll be in full compliance with the CDDL.  The licences do, in fact, have compatible compliance requirements.  This fact becomes very relevant when you consider the requirements for bringing a copyright lawsuit in the first place.

Where’s the Harm?

Copyright law is something called a tort in law.  That essentially means a branch of law for resolving disputes between individual parties.  However, in order to stem what could be seen as frivolous lawsuits, bringing a claim under tort law requires not just a showing that someone broke the rules of whatever agreement they were under but also that quantifiable harm resulted1.  The essential elements of a tort claim are a showing of a violation, a theory of the harm produced and a request for damages based on the harm2.

All of the bodies which do GPL enforcement recently published a charter in which they make clear that the sole requirement from an enforcement action should be compliance with the terms of the licence.  However, as I showed above, it is perfectly possible to be compatibly in compliance with both the CDDL and the GPLv2.  So the question becomes if the party is already in compliance, even though there’s a technical violation of the terms of the licence produced by the combination, what would our theory of harm be given that we don’t really seem to have anything extra we’d ask of the violating party?

Of course, one can wax lyrical about the “harm to the licence” of allowing incompatible combinations.  However, here we’re on a very sticky wicket because there have been a lot of works published (including by the FSF itself) bemoaning the problems of licence incompatibility.  Indeed, part of the design of the GPLv3 process was to make the licence more amenable to combination with other open source licences.  So suddenly becoming ardent advocates for the benefits of licence incompatibility is probably to be unlikely to fly before a judge as a theory of harm.

Conclusion

What the above analysis shows is that even though we presumed combination of GPLv2 and CDDL works to be a technical violation, there’s no way actually to prosecute such a violation because we can’t develop a convincing theory of harm resulting.  Because this makes it impossible to take the case to court, effectively it must be concluded that the combination of GPLv2 and CDDL, provided you’re following a GPLv2 compliance regime for all the code, is allowable.  This conclusion is the same as the one Canonical reached, but the means by which I got there are very different.

Note that this conclusion would apply to mixing any open source licence with GPLv2: provided the compliance regimes are compatible and the stricter one is followed, it’s difficult to develop a theory of harm and thus the combination is allowable.

Final Thought

The above analysis is all from the point of view of the Linux kernel compliance activities.  However, with ZFS, there is another copyright holder: Oracle.  Nothing prevents Oracle suing for copyright violation with a theory of harm that says something like the CDDL was deliberately designed to be incompatible with GPLv2 to prevent ZFS being shipped in Linux and as the shipper of products base on ZFS, they’ve suffered commercial harm (which would be quantifiable) by this action.

Constructing Architecture Emulation Containers

Usually container related stuff goes on $EMPLOYER blog, but this time, I had a container need for my hobbies. the problem: how to build and test efitools for arm and aarch64 while not possessing any physical hardware.  The solution is to build an architecture emulation container using qemu and mount namespaces such that when its entered you find yourself in your home directory but with the rest of Linux running natively (well emulated natively via qemu) as a new architecture.  Binary emulation in Linux is nothing new: the binfmt_misc kernel module does it, and can execute anything provided you’ve told it what header to expect and how to do the execution.  Most distributions come with a qemu-linux-user package which will usually install the necessary binary emulators via qemu to run non-native binaries.  However, there’s a problem here: the installed binary emulator usually runs as /usr/bin/qemu-${arch}, so if you’re running a full operating system container, you can’t install any package that would overwrite that.  Unfortunately for me, the openSUSE Build Service package osc requires qemu-linux-user and would cause the overwrite of the emulator and the failure of the container.  The solution to this was to bind mount the required emulator into the / directory, where it wouldn’t be overwritten and to adjust the binfmt_misc paths accordingly.

Aside about binfmt_misc

The documentation for this only properly seems to exist in the kernel Documentation directory as binfmt_misc.txt.  However, very roughly, the format is

:name:type:offset:magic:mask:interpreter:flags

name is just a handle which will appear in /proc/sys/fs/binfmt_misc, type is M for magic or E for extension (Magic means recognise the type by the binary header, the usual UNIX way and E means recognise the type by the file extension, the Windows way). offset is where in the file to find the magic header to recognise;  magic and mask are the mask to and the binary string with and the magic to find once the masking is done.  Interpreter is the name of the interpreter to execute and flags tells binfmt_misc how to execute the interpreter.  For qemu, the flags always need to be OC meaning open the binary and generate credentials based on it (this can be seen as a security problem because the interpreter will execute with the same user and permissions as the binary, so you have to trust it).

If you’re on a systemd system, you can put all the above into /etc/binfmt.d/file.conf and systemd will feed it to binfmt_misc on boot.  Here’s an example of the aarch64 emulation file I use.

Bootstrapping

To bring up a minimal environment that’s fully native, you need to bootstrap it by installing just enough binaries using your native system before you can enter the container.  At a minimum, this is enough shared libraries and binaries to run the shell.  If you’re on a debian system, you probably already know how to use debootstrap to do this, but if you’re on openSUSE, like me, this is a much harder proposition because persuading zypper to install non native binaries isn’t easy.  The first thing you need to know is that you need to install an architectural override for libzypp in the file pointed to by the ZYPP_CONF environment variable. Here’s an example of a susebootstrap shell script that will install enough of the architecture to run zypper (so you can install all the packages you actually need).  Just run it as (note, you must have the qemu-<arch> binary installed because the installer will try to run pre and post scripts which may fail if they’re binary unless the emulation is working):

susebootstrap --arch <arch> <location>

And the bootstrap image will be build at <location> (I usually choose somewhere in my home directory, but you can use /var/tmp or anywhere else in your filesystem tree).  Note this script must be run as root because zypper can’t change ownership of files otherwise.  Now you are ready to start the architecture emulation container with <location> as the root.

Building an Architecture Emulation Container

All you really need now is a mount namespace with <location> as the real root and all the necessary Linux filesystems like /sys and /proc mounted.  Additionally, you usually want /home and I also mount /var/tmp so there’s a standard location for all my obs build directories.  Building a mount namespace is easy: simply unshare –mount and then bind mount everything you need.  Finally you use pivot_root to swap the new and old roots and unmount -l the old root (-l is necessary because the mount point is in-use outside the mount namespace as your real root, so you just need it unbinding, you don’t need to wait until no-one is using it).

All of this is easily scripted and I created this script to perform these actions.  As a final act, the script binds the process and creates an entry link in /run/build-container/<arch>.  This is the command line I used for the example below:

build-container --arch arm --location /home/jejb/tmp/arm

Now entering the build container is easy (you still have to enter the namespace as root, but you can exec su – <user> to become whatever your non-root user is).

jejb@jarvis:~> sudo -s
jarvis:/home/jejb # uname -m
x86_64
jarvis:/home/jejb # nsenter --mount=/run/build-container/arm
jarvis:/ # uname -m
armv7l
jarvis:/ # exec su - jejb
jejb@jarvis:~> uname -m
armv7l
jejb@jarvis:~> pwd
/home/jejb

And there you are, all ready to build binaries and run them on an armv7 system.

Aside about systemd and Shared Subtrees

On a normal linux system, you wouldn’t need to worry about any of this, but if you’re running systemd, you do, because systemd has some very inimical properties (to mount namespaces) you need to be aware of.

In Linux, a bind mount creates a subtree.  Because you can bind mount from practically anywhere to anywhere, you can have many such subtrees that are substantially related.  The default way to create subtrees is “private” this means that even if the subtrees are effectively the same set of files, a mount operation on one isn’t seen by any of the others.  This is great, because it’s precisely what you want for containers.  However, if a subtree is set to shared (with the mount –make-shared command) then all mount and unmount operations a propagated to every shared copy.  The reason this matters for systemd is because systemd at start of day sets every mount point in the system to shared.  Unless you re-privatise the bind mounts as you create the architecture emulation container, you’ll notice some very weird effects.  Firstly, because pivot_root won’t pivot to a shared subtree, that call will fail but secondly, you’ll notice that when you umount -l /old-root it will propagate to the real root and unmount everything (like your root /proc /dev and /sys) effectively rendering your system unusable.  the mount –make-rprivate /old-root recursively descends the /old-root and sets all the mounts to private so the umount -l simply detached the /old-root instead of propagating all the umount events.

A Modest Proposal on the DCO

In this post, I discussed why corporations are having trouble regarding the DCO as sufficient for contributions to projects using licences which require patent grants.  The fear being that rogue corporations could legitimately claim that under the DCO they were authorizing their developers as agents for copyrights but not for patents.  Rather than argue about the legality of this trick, I think it will be much more productive to move the environment forwards to a place where it simply won’t work.  The key to doing this is to change the expectations of the corporate players which moves them to the point where they expect that a corporate signoff under the DCO gives agency for both patents and copyrights because once this happens for most of them (the good actors), the usual estoppal rules would make it apply to all.

The fact is that even though corporate lawyers fear that agency might not exist for patent grants via DCO signoffs in contributions, all legitimate corporate entities who make bona fide code contributions wish to effect this anyway; that’s why they go to the additional lengths of setting up Contributor Licence Agreements and signing them.  The corollary here is that really only a bad actor in the ecosystem wishes to perpetuate the myth that patents aren’t handled by the DCO.  So if all good actors want the system to work correctly anyway, how do we make it so?

The lever that will help to make this move is a simple pledge, which can be published on a corporate website,  that allows corporations expecting to make legitimate contributions to patent binding licences under the DCO to do so properly without needing any additional Contributor Licence Agreements.  Essentially it would be an explicit statement that when their developers submit code to a project under the DCO using a corporate signoff, they’re acting as agents for the necessary patent and copyright grants, meaning you can always trust a DCO signoff from that corporation.  When enough corporations do this, it becomes standard practice and thus expectations on the DCO have moved to the point we originally assumed they were at, so here’s the proposal for what such a statement would look like.


 

Corporate Contribution Pledge

Preamble

It is our expectation that any DCO signoff from a corporate email address binds that corporation to grant all necessary copyright and, where required, patent rights to satisfy the terms of the licence.  Accordingly, we are publishing this pledge to illustrate how, as a matter of best practice, we implement this expectation.

For the purposes of this pledge, our corporate email domain is @bigcorp.com and its subdomains.

Limitations

  1. This pledge only applies to projects which use an OSI accepted Open Source  licence and which also use a developer certificate of origin (DCO).
  2. No authority is given under this pledge to sign contribution agreements on behalf of the company or otherwise bind it except by contributing code under an OSI approved licence and DCO process.
  3. No authority is given under this pledge if a developer, who may be our employee, posts patches under an email address which is not our corporate email domain above.
  4. No trademarks of this corporation may ever be bound under this pledge.
  5. Except as stated below, no other warranty, express or implied, is made on behalf of the contribution, including, but not limited to, fitness of the code for a specific purpose or merchantability.  The entire risk of the quality and performance of this contribution rests with the recipient.

Warranties

  1. Our corporation trains its Open Source contributors carefully to understand when they may and may not post patches from our corporate email domain and to obtain all necessary internal clearances according to our processes before making such a posting.
  2. When one of our developers posts a patch to a project under an OSI approved licence with a DCO Signed-off-by: from our corporate email domain, we authorise that developer to be our agent in the minimum set of patent and copyright grants that are required to satisfy the terms of the OSI approved licence for the contribution.