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Android Verified Boot 2.0

This repository contains tools and libraries for working with Android Verified Boot 2.0. Usually AVB is used to refer to this codebase.

Introduction

The main job of avbtool is to create vbmeta.img which is the top-level object for verified boot. This image is designed to go into the vbmeta partition (or, if using A/B, the slot in question e.g. vbmeta_a or vbmeta_b) and be of minimal size (for out-of-band updates). The vbmeta image is cryptographically signed and contains verification data (e.g. cryptographic digests) for verifying boot.img, system.img, and other partitions/images.

The vbmeta image can also contain references to other partitions where verification data is stored as well as a public key indicating who should sign the verification data. This indirection provides delegation, that is, it allows a 3rd party to control content on a given partition by including their public key in vbmeta.img. By design, this authority can be easily revoked by simply updating vbmeta.img with new descriptors for the partition in question.

Storing signed verification data on other images - for example boot.img and system.img - is also done with avbtool.

In addition to avbtool, a library - libavb - is provided. This library performs all verification on the device side e.g. it starts by loading the vbmeta partition, checks the signature, and then goes on to load the boot partition for verification. This library is intended to be used in both boot loaders and inside Android. It has a simple abstraction for system dependencies (see avb_sysdeps.h) as well as operations that the boot loader or OS is expected to implement (see avb_ops.h). The main entry point for verification is avb_slot_verify().

It is expected that most devices will use A/B (e.g. multiple copies of the OS in separate so-called ‘slots’) in addition to AVB. While managing A/B metadata and associated metadata (e.g. managing stored_rollback_index[n] locations) is outside the scope of libavb, enough interfaces are exposed so the boot loader can integrate its A/B stack with libavb. In particular avb_slot_verify() takes a slot_suffix parameter and its result struct AvbSlotVerifyData convey the rollback indexes in the image that was verified.

AVB also includes an A/B implementation that boot loaders may optionally use. This implementation is in the libavb_ab library and integrates with image verification including updating the stored_rollback_index[n] locations on the device as needed. The bootloader can use this through the avb_ab_flow() function which in turn calls avb_slot_verify() as needed.

In libavb_ab, A/B metadata is stored in the misc partition using a format private to libavb_ab in the location on misc reserved for this. For more information about the misc.img file format see the bootloader_message.h file in AOSP. A/B metadata can be written to misc.img using the set_ab_metadata sub-command of avbtool. A/B metadata is comprised of data for each slo and per-slot metadata has a priority field (0 to 15), number of tries remaining for attempting to boot the slot (0 to 7), and a flag to indicate whether the slot has successfully booted.

A/B metadata integrity is provided by a simple magic marker and a CRC-32 checksum. If invalid A/B metadata is detected, the behavior is to reset the A/B metadata to a known state where both slots are given seven boot tries.

An implementation of a boot_control HAL using AVB-specific A/B metadata is also provided.

Android Things has specific requirements and validation logic for the vbmeta public key. An extension is provided in libavb_atx which performs this validation as an implementatio of libavb's public key validation operation (see avb_validate_vbmeta_public_key() in avb_ops.h).

Files and Directories

  • libavb/
    • An implementation of image verification. This code is designed to be highly portable so it can be used in as many contexts as possible. This code requires a C99-compliant C compiler. Part of this code is considered internal to the implementation and should not be used outside it. For example, this applies to the avb_rsa.[ch] and avb_sha.[ch] files. System dependencies expected to be provided by the platform is defined in avb_sysdeps.h. If the platform provides the standard C runtime avb_sysdeps_posix.c can be used.
  • libavb_ab/
    • An A/B implementation for use in boot loaders.
  • libavb_atx/
    • An Android Things Extension for validating public key metadata.
  • boot_control/
    • An implemementation of the Android boot_control HAL for use with boot loaders using libavb_ab.
  • Android.mk
    • Build instructions for building libavb (a static library for use on the device), host-side libraries (for unit tests), and unit tests.
  • avbtool
    • A tool written in Python for working with images related to verified boot.
  • test/
    • Unit tests for abvtool, libavb, libavb_ab, and libavb_atx.
  • examples/uefi/
    • Contains the source-code for a UEFI-based boot-loader utilizing libavb/ and libavb_ab/.

Audience and portability notes

This code is intended to be used in bootloaders in devices running Android. The suggested approach is to copy the appropriate header and C files mentioned in the previous section into the boot loader and integrate as appropriate.

The libavb/ and libavb_ab/ codebase will evolve over time so integration should be as non-invasive as possible. The intention is to keep the API of the library stable however it will be broken if necessary. As for portability, the library is intended to be highly portable, work on both little- and big-endian architectures and 32- and 64-bit. It's also intended to work in non-standard environments without the standard C library and runtime.

If the AVB_ENABLE_DEBUG preprocessor symbol is set, the code will include useful debug information and run-time checks. Production builds should not use this. The preprocessor symbol AVB_COMPILATION should be set only when compiling the libraries. The code must be compiled into a separate libraries.

Applications using the compiled libavb library must only include the libavb/libavb.h file (which will include all public interfaces) and must not have the AVB_COMPILATION preprocessor symbol set. This is to ensure that internal code that may be change in the future (for example avb_sha.[ch] and avb_rsa.[ch]) will not be visible to application code.

Versioning and compatibility

AVB uses a version number with three fields - the major, minor, and sub version. Here's an example version number

                     1.4.3
                     ^ ^ ^
                     | | |
the major version ---+ | |
the minor version -----+ |
  the sub version -------+

The major version number is bumped only if compatibility is broken, e.g. a struct field has been removed or changed. The minor version number is bumped only if a new feature is introduced, for example a new algorithm or descriptor has been added. The sub version number is bumped when bugs are fixed or other changes not affecting compatibility are made.

The AvbVBMetaImageHeader struct (as defined in the avb_vbmeta_image.h) carries the major and minor version number of libavb required to verify the struct in question. This is stored in the required_libavb_version_major and required_libavb_version_minor fields. Additionally this struct contains a textual field with the version of avbtool used to create the struct, for example “avbtool 1.4.3” or “avbtool 1.4.3 some_board Git-4589fbec”.

Note that it's entirely possible to have a AvbVBMetaImageHeader struct with

required_libavb_version_major = 1
required_libavb_version_minor = 0
avbtool_release_string = "avbtool 1.4.3"

if, for example, creating an image that does not use any features added after AVB version 1.0.

Adding new features

If adding a new feature for example a new algorithm or a new descriptor then AVB_VERSION_MINOR in avb_version.h and avbtool must be bumped and AVB_VERSION_SUB should be set to zero.

Unit tests MUST be added to check that

  • The feature is used if - and only if - suitable commands/options are passed to avbtool.
  • The required_version_minor field is set to the bumped value if - and only if - the feature is used.

If AVB_VERSION_MINOR has already been bumped since the last release there is obviously no need to bump it again.

Usage

The content for the vbmeta partition can be generated as follows:

$ avbtool make_vbmeta_image                                                    \
    --output OUTPUT                                                            \
    [--algorithm ALGORITHM] [--key /path/to/key_used_for_signing_or_pub_key]   \
    [--public_key_metadata /path/to/pkmd.bin] [--rollback_index NUMBER]        \
    [--include_descriptors_from_footer /path/to/image.bin]                     \
    [--setup_rootfs_from_kernel /path/to/image.bin]                            \
    [--chain_partition part_name:rollback_index_location:/path/to/key1.bin]    \
    [--signing_helper /path/to/external/signer]                                \
    [--append_to_release_string STR]

An integrity footer containing the hash for an entire partition can be added to an existing image as follows:

$ avbtool add_hash_footer                                                      \
    --image IMAGE                                                              \
    --partition_name PARTNAME --partition_size SIZE                            \
    [--algorithm ALGORITHM] [--key /path/to/key_used_for_signing_or_pub_key]   \
    [--public_key_metadata /path/to/pkmd.bin] [--rollback_index NUMBER]        \
    [--hash_algorithm HASH_ALG] [--salt HEX]                                   \
    [--include_descriptors_from_footer /path/to/image.bin]                     \
    [--setup_rootfs_from_kernel /path/to/image.bin]                            \
    [--output_vbmeta_image OUTPUT_IMAGE] [--do_not_append_vbmeta_image]        \
    [--signing_helper /path/to/external/signer]                                \
    [--append_to_release_string STR]

An integrity footer containing the root digest and salt for a hashtree for a partition can be added to an existing image as follows. The hashtree is also appended to the image.

$ avbtool add_hashtree_footer                                                  \
    --image IMAGE                                                              \
    --partition_name PARTNAME --partition_size SIZE                            \
    [--algorithm ALGORITHM] [--key /path/to/key_used_for_signing_or_pub_key]   \
    [--public_key_metadata /path/to/pkmd.bin] [--rollback_index NUMBER]        \
    [--hash_algorithm HASH_ALG] [--salt HEX] [--block_size SIZE]               \
    [--include_descriptors_from_footer /path/to/image.bin]                     \
    [--setup_rootfs_from_kernel /path/to/image.bin]                            \
    [--output_vbmeta_image OUTPUT_IMAGE] [--do_not_append_vbmeta_image]        \
    [--generate_fec] [--fec_num_roots FEC_NUM_ROOTS]                           \
    [--signing_helper /path/to/external/signer]                                \
    [--append_to_release_string STR]

The integrity footer on an image can be removed from an image. The hashtree can optionally be kept in place.

$ avbtool erase_footer --image IMAGE [--keep_hashtree]

For hash- and hashtree-images the vbmeta struct can also be written to an external file via the --output_vbmeta_image option and one can also specify that the vbmeta struct and footer not be added to the image being operated on.

To calculate the maximum size of an image that will fit in a partition of a given size after having used the avbtool add_hashtree_footer command on it, use the --calc_max_image_size option:

$ avbtool add_hashtree_footer --partition_size $((10*1024*1024)) \
    --calc_max_image_size
10330112

The --signing_helper option can be used in make_vbmeta_image, add_hash_footer and add_hashtree_footer commands to specify any external program for signing hashes. The data to sign (including padding e.g. PKCS1-v1.5) is fed via STDIN and the signed data is returned via STDOUT. If --signing_helper is present in a command line, the --key option need only contain a public key. Arguments for a signing helper are algorithm and public key. If the signing helper exits with a non-zero exit code, it means failure.

Here's an example invocation:

/path/to/my_signing_program SHA256_RSA2048 /path/to/publickey.pem

The append_vbmeta_image command can be used to append an entire vbmeta blob to the end of another image. This is useful for cases when not using any vbmeta partitions, for example:

$ cp boot.img boot-with-vbmeta-appended.img
$ avbtool append_vbmeta_image                       \
    --image boot-with-vbmeta-appended.img           \
    --partition_size SIZE_OF_BOOT_PARTITION         \
    --vbmeta_image vbmeta.img
$ fastboot flash boot boot-with-vbmeta-appended.img

Build system integration notes

Android Verified Boot is enabled by the BOARD_AVB_ENABLE variable

BOARD_AVB_ENABLE := true

This will make the build system create vbmeta.img which will contain a hash descriptor for boot.img, a hashtree descriptor for system.img, a kernel-cmdline descriptor for setting up dm-verity for system.img and append a hash-tree to system.img.

By default, the algorithm SHA256_RSA4096 is used with a test key from the external/avb/test/data directory. This can be overriden by the BOARD_AVB_ALGORITHM and BOARD_AVB_KEY_PATH variables to use e.g. a 4096-bit RSA key and SHA-512:

BOARD_AVB_ALGORITHM := SHA512_RSA4096
BOARD_AVB_KEY_PATH := /path/to/rsa_key_4096bits.pem

Remember that the public part of this key needs to be available to the bootloader of the device expected to verify resulting images. Use avbtool extract_public_key to extract the key in the expected format (AVB_pk in the following). If the device is using a different root of trust than AVB_pk the --public_key_metadata option can be used to embed a blob (AVB_pkmd in the following) that can be used to e.g. derive AVB_pk. Both AVB_pk and AVB_pkmd are passed to the validate_vbmeta_public_key() operation when verifying a slot.

To prevent rollback attacks, the rollback index should be increased on a regular basis. The rollback index can be set with the BOARD_AVB_ROLLBACK_INDEX variable:

 BOARD_AVB_ROLLBACK_INDEX := 5

If this is not set, the rollback index defaults to 0.

The variable BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS can be used to specify additional options passed to avbtool make_vbmeta_image. Typical options to be used here include --prop, --prop_from_file, and --chain_partition.

The variable BOARD_AVBTOOL_BOOT_ADD_HASH_FOOTER_ARGS can be used to specify additional options passed to avbtool add_hash_footer for boot.img. Typical options to be used here include --hash_algorithm and --salt.

The variable BOARD_AVBTOOL_SYSTEM_ADD_HASHTREE_FOOTER_ARGS can be used to specify additional options passed to avbtool add_hashtree_footer for system.img. Typical options to be used here include --hash_algorithm, --salt, --block_size, and --generate_fec.

Build system variables (such as PRODUCT_SUPPORTS_VERITY_FEC) used for previous version of Verified Boot in Android are not used in AVB