This repository contains tools and libraries for working with Android Verified Boot 2.0. Usually AVB is used to refer to this codebase.
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
).
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]
andavb_sha.[ch]
files. System dependencies expected to be provided by the platform is defined inavb_sysdeps.h
. If the platform provides the standard C runtimeavb_sysdeps_posix.c
can be used.
- 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
libavb_ab/
- An A/B implementation for use in boot loaders.
libavb_atx/
- An Android Things Extension for validating public key metadata.
libavb_user/
- Contains an AvbOps implementation suitable for use in userspace on the device (used in boot_control.avb and avbctl).
boot_control/
- An implemementation of the Android boot_control HAL for use with
boot loaders using
libavb_ab
.
- An implemementation of the Android boot_control HAL for use with
boot loaders using
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
, andlibavb_atx
.
- Unit tests for
tools/avbctl/
- Contains the source-code for a tool that can be used to control AVB at runtime.
examples/uefi/
- Contains the source-code for a UEFI-based boot-loader utilizing
libavb/
andlibavb_ab/
.
- Contains the source-code for a UEFI-based boot-loader utilizing
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.
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.
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.
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
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