This article will discuss securing your phone after you’ve rooted it and installed your preferred os (it will not discuss how to root your phone or change the OS). Re-securing your phone requires the installation of a custom AVB key, which not all phones support, so I’ll only be discussing Google Pixel phones (which support this) and the LineageOS distribution (which is the one I use). The reason for wanting to do this is that by default LineageOS runs with the debug keys (i.e. known to everyone) with an unlocked bootloader, meaning OS updates and even binary changes to the system partition are easy to do. Since most android phones are fully locked, this isn’t a standard attack vector for malicious apps, but if someone is targetting you directly it may become one.

This article will cover how android verified boot (AVB) works, how to install your own custom AVB key and get a stock LineageOS distribution to use it and how to turn DM verity back on to make /system immutable.

A Brief Tour of Android Verified Boot (AVB)

We’ll actually be covering the 2.0 version of AVB, but that’s what most phones use today. The proprietary bootloader of a Pixel (the fastboot capable one you get to with adb reboot bootloader) uses a vbmeta partition to find the boot/recovery system and from there either enter recovery or boot the standard OS. vbmeta contains hashes for both this boot partition and the system partition. If your phone is unlocked the bootloader will simply boot the partitions vbmeta points to without any verification. If it is locked, the vbmeta partition must be signed by a key the phone knows. Pixel phones have two keyslots: a built in one which houses either the Google key or an OEM one and the custom slot, which is blank. In the unlocked mode, you can flash your own key into the custom slot using the fastboot flash avb_custom_key command.

The vbmeta partition also contains a boot flags region which tells the bootloader how to boot the OS. The two flags the OS knows about are in external/avb/libavb/avb_vbmeta_image.h:

/* Flags for the vbmeta image.
 *
 * AVB_VBMETA_IMAGE_FLAGS_HASHTREE_DISABLED: If this flag is set,
 * hashtree image verification will be disabled.
 *
 * AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED: If this flag is set,
 * verification will be disabled and descriptors will not be parsed.
 */
typedef enum {
  AVB_VBMETA_IMAGE_FLAGS_HASHTREE_DISABLED = (1 << 0),
  AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED = (1 << 1)
} AvbVBMetaImageFlags;

if the first flag is set then dm-verity is disabled and if the second one is set, the bootloader doesn’t pass the hash of the vbmeta partition on the kernel command line. In a standard LineageOS build, both these flags are set.

The reason for passing the vbmeta hash on the command line is so the android init process can load the vbmeta partition, hash it and verify against what the bootloader passed in, thus confirming there hasn’t been a time of check to time of use (TOCTOU) security breach. The init process cannot verify the signature for itself because the vbmeta signing public key isn’t built into the OS (which allows the OS to be signed after the images are build).

The description of the AVB_VBMETA_IMAGE_FLAGS_HASHTREE_DISABLED flag is slightly wrong. A standard android build always seems to calculate the dm-verity hash tree and insert it into the vbmeta partition (where it is verified by the vbmeta signature) it’s just that if this flag is set, the android init process won’t load the dm-verity hash tree and the system partition will thus be mutable.

Creating and Using your own custom Boot Key

Obviously android doesn’t use any standard tool form for its keys, so you have to create your own RSA 2048 (the literature implies it will work with 4096 as well but I haven’t tried it) AVB custom key using say openssl, then use avbtool (found in external/avb as a python script) to convert your RSA public key to a form that can be flashed in the phone:

avbtool extract_public_key --key pubkey.pem --output pkmd.bin

This can then be flashed to the unlocked phone (in the bootloader fastboot) with

fastboot flash avb_custom_key pkmd.bin

And you’re all set up to boot a custom signed OS.

Signing your LineageOS

There is a wrinkle to this: to re-sign the OS, you need the target-files.zip intermediate build, not the ROM install file. Unfortunately, this is pretty big (38GB for lineage-19.1) and doesn’t seem to be available for download any more. If you can find it, you can re-sign the stock LineageOS, but if not you have to build it yourself. Instructions for both building and re-signing can be found here. You need to follow this but in addition you must add two extra flags to the sign_target_files_apks command:

--avb_vbmeta_key=/path/to/private/avb.key
--avb_vbmeta_algorithm=SHA256_RSA2048

Which will ensure the vbmeta partition is signed with the key you created above.

Optionally Enabling dm-verity

If you want to enable dm-verity, you have to change the vbmeta flags to 0 (enable both hashtree and vbmeta verification) before you execute the signing command above. These flags are stored in the META/misc_info.txt file which you can extract from target-files.zip with

unzip target-files.zip META/misc_info.txt

And once done you can vi this file to find the line

avb_vbmeta_args=--flags 3 --padding_size 4096 --rollback_index 1804212800

If you update the 3 to 0 this will unset the two disable flags and allow you to do a dm-verity verified boot. Then use zip to replace this updated file

zip -u target-files.zip META/misc_info.txt

And then proceed with signing the updated target-files.zip

Wrinkle for Android-12 (lineage-19.1) and above

For all these versions, this patch ensures that if the vbmeta was signed then the vbmeta hash must be verified, otherwise the system will crash in early init, so you have no choice and must alter the avb_vbmeta_args above to either --flags 1 or --flags 0 so the vbmeta hash is passed in to init. Since you have to alter the flags anyway, you might as well enable dm-verity (set to 0) at the same time.

Re-Lock the Bootloader

Once you have installed both your custom keys and your custom signed boot image, you are ready to re-lock the bootloader. Beware that some phones will erase your data partition when you do this (the Google advice says they shouldn’t, but not all manufacturers bother to follow it), so make sure you have a backup (or are playing with a newly rooted phone).

fastboot flashing lock

Check with a reboot (the phone should now display a yellow warning triangle saying it is booting a custom OS rather than the orange unsigned OS one). If everything goes according to plan you can enter the developer settings and click the “OEM Unlocking” settings to disabled and your phone can no longer be unlocked without your say so.

Conclusions and Caveats

Following the above instructions, you can updated your phone so it will verify images you signed with your AVB key, turn on dm-verity if you wish and generally make your phone much more secure. However, remember that you haven’t erased the original AVB key, so the phone can still be updated to an image signed with that key and, worse, the recovery partition of LineageOS is modified to allow rollback, so it will allow the flashing of any signed image without triggering an erase of the data partition. There are also a few more problems like, thanks to a bug in AOSP, the recovery version of fastboot will actually allow commands that are usually forbidden if the phone is locked.