# 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. # Table of Contents * [What is it?](#What-is-it) + [The VBMeta struct](#The-VBMeta-struct) + [Rollback Protection](#Rollback-Protection) + [A/B Support](#A_B-Support) + [The VBMeta Digest](#The-VBMeta-Digest) * [Tools and Libraries](#Tools-and-Libraries) + [avbtool and libavb](#avbtool-and-libavb) + [Files and Directories](#Files-and-Directories) + [Portability](#Portability) + [Versioning and Compatibility](#Versioning-and-Compatibility) + [Adding New Features](#Adding-New-Features) + [Using avbtool](#Using-avbtool) + [Build System Integration](#Build-System-Integration) * [Device Integration](#Device-Integration) + [System Dependencies](#System-Dependencies) + [Locked and Unlocked mode](#Locked-and-Unlocked-mode) + [Tamper-evident Storage](#Tamper_evident-Storage) + [Named Persistent Values](#Named-Persistent-Values) + [Persistent Digests](#Persistent-Digests) + [Updating Stored Rollback Indexes](#Updating-Stored-Rollback-Indexes) + [Recommended Bootflow](#Recommended-Bootflow) + [Booting Into Recovery](#Booting-Into-Recovery) + [Handling dm-verity Errors](#Handling-dm_verity-Errors) + [Android Specific Integration](#Android-Specific-Integration) + [Device Specific Notes](#Device-Specific-Notes) * [Version History](#Version-History) # What is it? Verified boot is the process of assuring the end user of the integrity of the software running on a device. It typically starts with a read-only portion of the device firmware which loads code and executes it only after cryptographically verifying that the code is authentic and doesn't have any known security flaws. AVB is one implementation of verified boot. ## The VBMeta struct The central data structure used in AVB is the VBMeta struct. This data structure contains a number of descriptors (and other metadata) and all of this data is cryptographically signed. Descriptors are used for image hashes, image hashtree metadata, and so-called *chained partitions*. A simple example is the following:  where the `vbmeta` partition holds the hash for the `boot` partition in a hash descriptor. For the `system` and `vendor` partitions a hashtree follows the filesystem data and the `vbmeta` partition holds the root hash, salt, and offset of the hashtree in hashtree descriptors. Because the VBMeta struct in the `vbmeta` partition is cryptographically signed, the boot loader can check the signature and verify it was made by the owner of `key0` (by e.g. embedding the public part of `key0`) and thereby trust the hashes used for `boot`, `system`, and `vendor`. A chained partition descriptor is used to delegate authority - it contains the name of the partition where authority is delegated as well as the public key that is trusted for signatures on this particular partition. As an example, consider the following setup:  In this setup the `xyz` partition has a hashtree for integrity-checking. Following the hashtree is a VBMeta struct which contains the hashtree descriptor with hashtree metadata (root hash, salt, offset, etc.) and this struct is signed with `key1`. Finally, at the end of the partition is a footer which has the offset of the VBMeta struct. This setup allows the bootloader to use the chain partition descriptor to find the footer at the end of the partition (using the name in the chain partition descriptor) which in turns helps locate the VBMeta struct and verify that it was signed by `key1` (using `key1_pub` stored in the chain partition descriptor). Crucially, because there's a footer with the offset, the `xyz` partition can be updated without the `vbmeta` partition needing any changes. The VBMeta struct is flexible enough to allow hash descriptors and hashtree descriptors for any partition to live in the `vbmeta` partition, the partition that they are used to integrity check (via a chain partition descriptor), or any other partition (via a chain partition descriptor). This allows for a wide range of organizational and trust relationships. Chained partitions need not use a footer - it is permissible to have a chained partition point to a partition where the VBMeta struct is at the beginning (e.g. just like the `vbmeta` partition). This is useful for use-cases where all hash- and hashtree-descriptors for the partitions owned by an entire organization are stored in a dedicated partition, for example `vbmeta_google`. In this example the hashtree descriptor for `system` is in the `vbmeta_google` partition meaning that the bootloader doesn't need to access the `system` partition at all which is helpful if the `system` partition is managed as a logical partition (via e.g. [LVM techniques](https://en.wikipedia.org/wiki/Logical_volume_management) or similar). ## Rollback Protection AVB includes Rollback Protection which is used to protect against known security flaws. Each VBMeta struct has a *rollback index* baked into it like the following:  These numbers are referred to as `rollback_index[n]` and are increased for each image as security flaws are discovered and fixed. Additionally the device stores the last seen rollback index in tamper-evident storage:  and these are referred to as `stored_rollback_index[n]`. Rollback protection is having the device reject an image unless `rollback_index[n]` >= `stored_rollback_index[n]` for all `n`, and having the device increase `stored_rollback_index[n]` over time. Exactly how this is done is discussed in the [Updating Stored Rollback Indexes](#Updating-Stored-Rollback-Indexes) section. ## A/B Support AVB has been designed to work with A/B by requiring that the A/B suffix is never used in any partition names stored in descriptors. Here's an example with two slots:  Note how the rollback indexes differ between slots - for slot A the rollback indexes are `[42, 101]` and for slot B they are `[43, 103]`. In version 1.1 or later, avbtool supports `--do_not_use_ab` for `add_hash_footer` and `add_hashtree_footer` operations. This makes it possible to work with a partition that does not use A/B and should never have the prefix. This corresponds to the `AVB_HASH[TREE]_DESCRIPTOR_FLAGS_DO_NOT_USE_AB` flags. ## The VBMeta Digest The VBMeta digest is a digest over all VBMeta structs including the root struct (e.g. in the `vbmeta` partition) and all VBMeta structs in chained partitions. This digest can be calculated at build time using `avbtool calculate_vbmeta_digest` and also at runtime using the `avb_slot_verify_data_calculate_vbmeta_digest()` function. It is also set on the kernel command-line as `androidboot.vbmeta.digest`, see the `avb_slot_verify()` documentation for exact details. This digest can be used together with `libavb` in userspace inside the loaded operating system to verify authenticity of the loaded vbmeta structs. This is useful if the root-of-trust and/or stored rollback indexes are only available while running in the boot loader. Additionally, if the VBMeta digest is included in [hardware-backed attestation data](https://developer.android.com/training/articles/security-key-attestation) a relying party can extract the digest and compare it with list of digests for known good operating systems which, if found, provides additional assurance about the device the application is running on. For [factory images of Pixel 3 and later devices](https://developers.google.com/android/images), the `pixel_factory_image_verify.py` located in `tools/transparency` is a convenience tool for downloading, verifying and calcuating VBMeta Digests. $ pixel_factory_image_verify.py https://dl.google.com/dl/android/aosp/image.zip Fetching file from: https://dl.google.com/dl/android/aosp/image.zip Successfully downloaded file. Successfully unpacked factory image. Successfully unpacked factory image partitions. Successfully verified VBmeta. Successfully calculated VBMeta Digest. The VBMeta Digest for factory image is: 1f329b20a2dd69425e7a29566ca870dad51d2c579311992d41c9ba9ba05e170e If the given argument is not an URL it considered to be a local file: $ pixel_factory_image_verify.py image.zip # Tools and Libraries This section contains information about the tools and libraries included in AVB. ## avbtool and libavb 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`. The minimum requirement for running `avbtool` is to either have Python 3.5 installed or build the avbtool with the embedded launcher using `m avbtool` and then run it out of the build artifact directory: `out/soong/host/linux-x86/bin/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()`. 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 implementation 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_atx/` + An Android Things Extension for validating public key metadata. * `libavb_user/` + Contains an `AvbOps` implementation suitable for use in Android userspace. This is used in `boot_control.avb` and `avbctl`. * `libavb_ab/` + An experimental A/B implementation for use in boot loaders and AVB examples. **NOTE**: This code is *DEPRECATED* and you must define `AVB_AB_I_UNDERSTAND_LIBAVB_AB_IS_DEPRECATED` to use it. The code will be removed Jun 1 2018. * `boot_control/` + An implementation of the Android `boot_control` HAL for use with boot loaders using the experimental `libavb_ab` A/B stack. **NOTE**: This code is *DEPRECATED* and will be removed Jun 1 2018. * `Android.bp` + 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`. * `tools/avbctl/` + Contains the source-code for a tool that can be used to control AVB at runtime in Android. * `examples/uefi/` + Contains the source-code for a UEFI-based boot-loader utilizing `libavb/` and `libavb_ab/`. * `examples/things/` + Contains the source-code for a slot verification suitable for Android Things. * `README.md` + This file. * `docs/` + Contains documentation files. ## Portability The `libavb` code is intended to be used in bootloaders in devices that will load Android or other operating systems. 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. As the `libavb/` codebase will evolve over time 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 library. 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. Also add tests to check that the correct value is output when `--print_required_libavb_version` is used. If `AVB_VERSION_MINOR` has already been bumped since the last release there is obviously no need to bump it again. ## Using avbtool 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] [--rollback_index_location NUMBER] \ [--include_descriptors_from_image /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] \ [--signing_helper_with_files /path/to/external/signer_with_files] \ [--print_required_libavb_version] \ [--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 \ --partition_name PARTNAME --partition_size SIZE \ [--image IMAGE] \ [--algorithm ALGORITHM] [--key /path/to/key_used_for_signing_or_pub_key] \ [--public_key_metadata /path/to/pkmd.bin] \ [--rollback_index NUMBER] [--rollback_index_location NUMBER] \ [--hash_algorithm HASH_ALG] [--salt HEX] \ [--include_descriptors_from_image /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] \ [--signing_helper_with_files /path/to/external/signer_with_files] \ [--print_required_libavb_version] \ [--append_to_release_string STR] \ [--calc_max_image_size] \ [--do_not_use_ab] \ [--use_persistent_digest] Valid values for `HASH_ALG` above include `sha1` and `sha256`. 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 \ --partition_name PARTNAME --partition_size SIZE \ [--image IMAGE] \ [--algorithm ALGORITHM] [--key /path/to/key_used_for_signing_or_pub_key] \ [--public_key_metadata /path/to/pkmd.bin] \ [--rollback_index NUMBER] [--rollback_index_location NUMBER] \ [--hash_algorithm HASH_ALG] [--salt HEX] [--block_size SIZE] \ [--include_descriptors_from_image /path/to/image.bin] \ [--setup_rootfs_from_kernel /path/to/image.bin] \ [--setup_as_rootfs_from_kernel] \ [--output_vbmeta_image OUTPUT_IMAGE] [--do_not_append_vbmeta_image] \ [--do_not_generate_fec] [--fec_num_roots FEC_NUM_ROOTS] \ [--signing_helper /path/to/external/signer] \ [--signing_helper_with_files /path/to/external/signer_with_files] \ [--print_required_libavb_version] \ [--append_to_release_string STR] \ [--calc_max_image_size] \ [--do_not_use_ab] \ [--no_hashtree] \ [--use_persistent_digest] \ [--check_at_most_once] Valid values for `HASH_ALG` above include `sha1`, `sha256`, and `blake2b-256`. The size of an image with integrity footers can be changed using the `resize_image` command: $ avbtool resize_image \ --image IMAGE \ --partition_size SIZE 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. The hashtree and FEC data in an image can be zeroed out with the following command: $ avbtool zero_hashtree --image IMAGE This is useful for trading compressed image size for having to reculculate the hashtree and FEC at runtime. If this is done the hashtree and FEC data is set to zero except for the first eight bytes which are set to the magic `ZeRoHaSH`. Either the hashtree or FEC data or both may be zeroed this way so applications should check for the magic both places. Applications can use the magic to detect if recalculation is needed. To calculate the maximum size of an image that will fit in a partition of a given size after having used the `avbtool add_hash_footer` or `avbtool add_hashtree_footer` commands on it, use the `--calc_max_image_size` option: $ avbtool add_hash_footer --partition_size $((10*1024*1024)) \ --calc_max_image_size 10416128 $ avbtool add_hashtree_footer --partition_size $((10*1024*1024)) \ --calc_max_image_size 10330112 To calculate the required libavb version that would be put in the vbmeta struct when using `make_vbmeta_image`, `add_hash_footer`, and `add_hashtree_footer` commands use the `--print_required_libavb_version` option: $ avbtool make_vbmeta_image \ --algorithm SHA256_RSA2048 --key /path/to/key.pem \ --include_descriptors_from_image /path/to/boot.img \ --include_descriptors_from_image /path/to/system.img \ --print_required_libavb_version 1.0 Alternatively, `--no_hashtree` can be used with `avbtool add_hashtree_footer` command. If `--no_hashtree` is given, the hashtree blob is omitted and only its descriptor is added to the vbmeta struct. The descriptor says the size of hashtree is 0, which tells an application the need to recalculate hashtree. 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 `--signing_helper_with_files` is similar to `--signing_helper` except that a temporary file is used to communicate with the helper instead of `STDIN` and `STDOUT`. This is useful in situations where the signing helper is using code which is outputting diagnostics on `STDOUT` instead of `STDERR`. Here's an example invocation /path/to/my_signing_program_with_files SHA256_RSA2048 \ /path/to/publickey.pem /tmp/path/to/communication_file where the last positional argument is a file that contains the data to sign. The helper should write the signature in this file. 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 Information about an image can be obtained using the `info_image` command. The output of this command should not be relied on and the way information is structured may change. The `verify_image` command can be used to verify the contents of several image files at the same time. When invoked on an image the following checks are performed: * If the image has a VBMeta struct the signature is checked against the embedded public key. If the image doesn't look like `vbmeta.img` then a footer is looked for and used if present. * If the option `--key` is passed then a `.pem` file is expected and it's checked that the embedded public key in said VBMeta struct matches the given key. * All descriptors in the VBMeta struct are checked in the following way: + For a hash descriptor the image file corresponding to the partition name is loaded and its digest is checked against that in the descriptor. + For a hashtree descriptor the image file corresponding to the partition name is loaded and the hashtree is calculated and its root digest compared to that in the descriptor. + For a chained partition descriptor its contents is compared against content that needs to be passed in via the `--expected_chain_partition` options. The format for this option is similar to that of the `--chain_partition` option. If there is no `--expected_chain_partition` descriptor for the chain partition descriptor the check fails. Here's an example for a setup where the digests for `boot.img` and `system.img` are stored in `vbmeta.img` which is signed with `my_key.pem`. It also checks that the chain partition for partition `foobar` uses rollback index 8 and that the public key in AVB format matches that of the file `foobar_vendor_key.avbpubkey`: $ avbtool verify_image \ --image /path/to/vbmeta.img \ --key my_key.pem \ --expect_chained_partition foobar:8:foobar_vendor_key.avbpubkey Verifying image /path/to/vbmeta.img using key at my_key.pem vbmeta: Successfully verified SHA256_RSA4096 vbmeta struct in /path_to/vbmeta.img boot: Successfully verified sha256 hash of /path/to/boot.img for image of 10543104 bytes system: Successfully verified sha1 hashtree of /path/to/system.img for image of 1065213952 bytes foobar: Successfully verified chain partition descriptor matches expected data In this example the `verify_image` command verifies the files `vbmeta.img`, `boot.img`, and `system.img` in the directory `/path/to`. The directory and file extension of the given image (e.g. `/path/to/vbmeta.img`) is used together with the partition name in the descriptor to calculate the filenames of the images holding hash and hashtree images. The `verify_image` command can also be used to check that a custom signing helper works as intended. The `calculate_vbmeta_digest` command can be used to calculate the vbmeta digest of several image files at the same time. The result is printed as a hexadecimal string either on `STDOUT` or a supplied path (using the `--output` option). $ avbtool calculate_vbmeta_digest \ --hash_algorithm sha256 \ --image /path/to/vbmeta.img a20fdd01a6638c55065fe08497186acde350d6797d59a55d70ffbcf41e95c2f5 In this example the `calculate_vbmeta_digest` command loads the `vbmeta.img` file. If this image has one or more chain partition descriptors, the same logic as the `verify_image` command is used to load files for these (e.g. it assumes the same directory and file extension as the given image). Once all vbmeta structs have been loaded, the digest is calculated (using the hash algorithm given by the `--hash_algorithm` option) and printed out. To print hash and hashtree digests embedded in the verified metadata, use the `print_partition_digests` command like this: $ avbtool print_partition_digests --image /path/to/vbmeta.img system: ddaa513715fd2e22f3c1cea3c1a1f98ccb515fc6 boot: 5cba9a418e04b5f9e29ee6a250f6cdbe30c6cec867c59d388f141c3fedcb28c1 vendor: 06993a9e85e46e53d3892881bb75eff48ecadaa8 For partitions with hash descriptors, this prints out the digest and for partitions with hashtree descriptors the root digest is printed out. Like the `calculate_vbmeta_digest` and `verify_image` commands, chain partitions are followed. To use JSON for the output, use the `--json` option. In case you would like to log all command lines for all avbtool invocations for debugging integrations with other tooling, you can configure the envirionment variable AVB_INVOCATION_LOGFILE with the name of the log file: $ export AVB_INVOCATION_LOGFILE='/tmp/avb_invocation.log' $ ./avbtool version $ ./avbtool version $ cat /tmp/avb_invocation.log ./avbtool version ./avbtool version ## Build System Integration In Android, AVB 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`. If the build system is set up such that one or many of `vendor.img` / `product.img` / `system_ext.img` / `odm.img` are being built, the hash-tree for each of them will also be appended to the image respectively, and their hash-tree descriptors will be included into `vbmeta.img` accordingly. 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. Some devices may support the end-user configuring the root of trust to use, see the [Device Specific Notes](#Device-Specific-Notes) section for details. Devices can be configured to create additional `vbmeta` partitions as [chained partitions](#The-VBMeta-struct) in order to update a subset of partitions without changing the top-level `vbmeta` partition. For example, the following variables create `vbmeta_system.img` as a chained `vbmeta` image that contains the hash-tree descriptors for `system.img`, `system_ext.img` and `product.img`. `vbmeta_system.img` itself will be signed by the specified key and algorithm. BOARD_AVB_VBMETA_SYSTEM := system system_ext product BOARD_AVB_VBMETA_SYSTEM_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem BOARD_AVB_VBMETA_SYSTEM_ALGORITHM := SHA256_RSA2048 BOARD_AVB_VBMETA_SYSTEM_ROLLBACK_INDEX_LOCATION := 1 Note that the hash-tree descriptors for `system.img`, `system_ext.img` and `product.img` will be included only in `vbmeta_system.img`, but not `vbmeta.img`. With the above setup, partitions `system.img`, `system_ext.img`, `product.img` and `vbmeta_system.img` can be updated independently - but as a group - of the rest of the partitions, *or* as part of the traditional updates that update all the partitions. Currently build system supports building chained `vbmeta` images of `vbmeta_system.img` (`BOARD_AVB_VBMETA_SYSTEM`) and `vbmeta_vendor.img` (`BOARD_AVB_VBMETA_VENDOR`). 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`, `--chain_partition`, `--public_key_metadata`, and `--signing_helper`. The variable `BOARD_AVB_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_AVB_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 `--do_not_generate_fec`. The variable `BOARD_AVB_VENDOR_ADD_HASHTREE_FOOTER_ARGS` can be used to specify additional options passed to `avbtool add_hashtree_footer` for `vendor.img`. Typical options to be used here include `--hash_algorithm`, `--salt`, `--block_size`, and `--do_not_generate_fec`. The variable `BOARD_AVB_DTBO_ADD_HASH_FOOTER_ARGS` can be used to specify additional options passed to `avbtool add_hash_footer` for `dtbo.img`. Typical options to be used here include `--hash_algorithm` and `--salt`. Build system variables (such as `PRODUCT_SUPPORTS_VERITY_FEC`) used for previous version of Verified Boot in Android are not used in AVB. A/B related build system variables can be found [here](https://source.android.com/devices/tech/ota/ab_updates#build-variables). # Device Integration This section discusses recommendations and best practices for integrating `libavb` with a device boot loader. It's important to emphasize that these are just recommendations so the use of the word `must` should be taken lightly. Additionally term *HLOS* is used in this chapter to refer to the *High Level Operating System*. This obviously includes Android (including other form-factors than phones) but could also be other operating systems. ## System Dependencies The `libavb` library is written in a way so it's portable to any system with a C99 compiler. It does not require the standard C library however the boot loader must implement a simple set of system primitives required by `libavb` such as `avb_malloc()`, `avb_free()`, and `avb_print()`. In addition to the system primitives, `libavb` interfaces with the boot loader through the supplied `AvbOps` struct. This includes operations to read and write data from partitions, read and write rollback indexes, check if the public key used to make a signature should be accepted, and so on. ## Locked and Unlocked mode AVB has been designed to support the notion of the device being either LOCKED state or UNLOCKED state as used in Android. In the context of AVB, the LOCKED state means that verification errors are fatal whereas in UNLOCKED state they are not. If the device is UNLOCKED pass `AVB_SLOT_VERIFY_FLAGS_ALLOW_VERIFICATION_ERROR` flag in the `flags` parameter of `avb_slot_verify()` and treat verification errors including * `AVB_SLOT_VERIFY_RESULT_ERROR_PUBLIC_KEY_REJECTED` * `AVB_SLOT_VERIFY_RESULT_ERROR_VERIFICATION` * `AVB_SLOT_VERIFY_RESULT_ERROR_ROLLBACK_INDEX` as non-fatal. If the device is in the LOCKED state, don't pass the `AVB_SLOT_VERIFY_FLAGS_ALLOW_VERIFICATION_ERROR` flag in the `flags` parameter of `avb_slot_verify()` and only treat `AVB_SLOT_VERIFY_RESULT_OK` as non-fatal. On Android, device state may be altered through the fastboot interface using, e.g. `fastboot flashing lock` (to transition to the LOCKED state) and `fastboot flashing unlock` (to transition to the UNLOCKED state). The device must only allow state transitions (e.g. from LOCKED to UNLOCKED or UNLOCKED to LOCKED) after asserting physical presence of the user. If the device has a display and buttons this is typically done by showing a dialog and requiring the user to confirm or cancel using physical buttons. All user data must be cleared when transitioning from the LOCKED to the UNLOCKED state (including the `userdata` partition and any NVRAM spaces). Additionally all `stored_rollback_index[n]` locations must be cleared (all elements must be set to zero). Similar action (erasing `userdata`, NVRAM spaces, and `stored_rollback_index[n]` locations) shall also happening when transitioning from UNLOCKED to LOCKED. If the device is required to use full disk encryption, then a less intensive wipe is required for UNLOCKED to LOCKED. Depending on the device form-factor and intended use, the user should be prompted to confirm before any data is erased. ## Tamper-evident Storage In this document, *tamper-evident* means that it's possible to detect if the HLOS has tampered with the data, e.g. if it has been overwritten. Tamper-evident storage must be used for stored rollback indexes, keys used for verification, device state (whether the device is LOCKED or UNLOCKED), and named persistent values. If tampering has been detected the corresponding `AvbOps` operation should fail by e.g. returning `AVB_IO_RESULT_ERROR_IO`. It is especially important that verification keys cannot be tampered with since they represent the root-of-trust. If verification keys are mutable they must only be set by the end user, e.g. it must never be set at the factory or store or any intermediate point before the end user. Additionally, it must only be possible to set or clear a key while the device is in the UNLOCKED state. ## Named Persistent Values AVB 1.1 introduces support for named persistent values which must be tamper evident and allows AVB to store arbitrary key-value pairs. Integrators may limit support for these values to a set of fixed well-known names, a maximum value size, and / or a maximum number of values. ## Persistent Digests Using a persistent digest for a partition means the digest (or root digest in the case of a hashtree) is not stored in the descriptor but is stored in a named persistent value. This allows configuration data which may differ from device to device to be verified by AVB. It must not be possible to modify the persistent digest when the device is in the LOCKED state, except if a digest does not exist it may be initialized. To specify that a descriptor should use a persistent digest, use the `--use_persistent_digest` option for the `add_hash_footer` or `add_hashtree_footer` avbtool operations. Then, during verification of the descriptor, AVB will look for the digest in the named persistent value `avb.persistent_digest.$(partition_name)` instead of in the descriptor itself. For hashtree descriptors using a persistent digest, the digest value will be available for substitution into kernel command line descriptors using a token of the form `$(AVB_FOO_ROOT_DIGEST)` where 'FOO' is the uppercase partition name, in this case for the partition named 'foo'. The token will be replaced by the digest in hexadecimal form. By default, when the `--use_persistent_digest` option is used with `add_hash_footer` or `add_hashtree_footer`, avbtool will generate a descriptor with no salt rather than the typical default of generating a random salt equal to the digest length. This is because the digest value is stored in persistent storage and thus cannot change over time. An alternative option would be to manually provide a random salt using `--salt`, but this salt would need to remain unchanged for the life of the device once the persistent digest value was written. ## Updating Stored Rollback Indexes In order for Rollback Protection to work the bootloader will need to update the `stored_rollback_indexes[n]` array on the device prior to transferring control to the HLOS. If not using A/B this is straightforward - just update it to what's in the AVB metadata for the slot before booting. In pseudo-code it would look like this: ```c++ // The |slot_data| parameter should be the AvbSlotVerifyData returned // by avb_slot_verify() for the slot we're about to boot. // bool update_stored_rollback_indexes_for_slot(AvbOps* ops, AvbSlotVerifyData* slot_data) { for (int n = 0; n < AVB_MAX_NUMBER_OF_ROLLBACK_INDEX_LOCATIONS; n++) { uint64_t rollback_index = slot_data->rollback_indexes[n]; if (rollback_index > 0) { AvbIOResult io_ret; uint64_t current_stored_rollback_index; io_ret = ops->read_rollback_index(ops, n, ¤t_stored_rollback_index); if (io_ret != AVB_IO_RESULT_OK) { return false; } if (rollback_index > current_stored_rollback_index) { io_ret = ops->write_rollback_index(ops, n, rollback_index); if (io_ret != AVB_IO_RESULT_OK) { return false; } } } } return true; } ``` However if using A/B more care must be taken to still allow the device to fall back to the old slot if the update didn't work. For an HLOS like Android where rollback is only supported if the updated OS version is found to not work, `stored_rollback_index[n]` should only be updated from slots that are marked as SUCCESSFUL in the A/B metadata. The pseudo-code for that is as follows where `is_slot_is_marked_as_successful()` comes from the A/B stack in use: ```c++ if (is_slot_is_marked_as_successful(slot->ab_suffix)) { if (!update_stored_rollback_indexes_for_slot(ops, slot)) { // TODO: handle error. } } ``` This logic should ideally be implemented outside of the HLOS. One possible implementation is to update rollback indices in the bootloader when booting into a successful slot. This means that when booting into a new OS not yet marked as successful, the rollback indices would not be updated. The first reboot after the slot succeeded would trigger an update of the rollback indices. For an HLOS where it's possible to roll back to a previous version, `stored_rollback_index[n]` should be set to the largest possible value allowing all bootable slots to boot. This approach is implemented in AVB's experimental (and now deprecated) A/B stack `libavb_ab`, see the `avb_ab_flow()` implementation. Note that this requires verifying *all* bootable slots at every boot and this may impact boot time. ## Recommended Bootflow The recommended boot flow for a device using AVB is as follows:  Notes: * The device is expected to search through all A/B slots until it finds a valid OS to boot. Slots that are rejected in the LOCKED state might not be rejected in the UNLOCKED state, (e.g. when UNLOCKED any key can be used and rollback index failures are allowed), so the algorithm used for selecting a slot varies depending on what state the device is in. * If no valid OS (that is, no bootable A/B slot) can be found, the device cannot boot and has to enter repair mode. It is device-dependent what this looks like. If the device has a screen it must convey this state to the user. * If the device is LOCKED, only an OS signed by an embedded verification key (see the previous section) shall be accepted. Additionally, `rollback_index[n]` as stored in the verified image must be greater or equal than what's in `stored_rollback_index[n]` on the device (for all `n`) and the `stored_rollback_index[n]` array is expected to be updated as specified in the previous section. + If the key used for verification was set by the end user, and the device has a screen, it must show a warning with the key fingerprint to convey that the device is booting a custom OS. The warning must be shown for at least 10 seconds before the boot process continues. If the device does not have a screen, other ways must be used to convey that the device is booting a custom OS (lightbars, LEDs, etc.). * If the device is UNLOCKED, there is no requirement to check the key used to sign the OS nor is there any requirement to check or update rollback `stored_rollback_index[n]` on the device. Because of this the user must always be shown a warning about verification not occurring. + It is device-dependent how this is implemented since it depends on the device form-factor and intended usage. If the device has a screen and buttons (for example if it's a phone) the warning is to be shown for at least 10 seconds before the boot process continues. If the device does not have a screen, other ways must be used to convey that the device is UNLOCKED (lightbars, LEDs, etc.). ### Booting Into Recovery On Android devices not using A/B, the `recovery` partition usually isn't updated along with other partitions and therefore can't be referenced from the main `vbmeta` partition. It's still possible to use AVB to protect this partition (and others) by signing these partitions and passing the `AVB_SLOT_VERIFY_FLAGS_NO_VBMETA_PARTITION` flag to `avb_slot_verify()`. In this mode, the key used to sign each requested partition is verified by the `validate_public_key_for_partition()` operation which is also used to return the rollback index location to be used. ## Handling dm-verity Errors By design, hashtree verification errors are detected by the HLOS and not the bootloader. AVB provides a way to specify how the error should be handled through the `hashtree_error_mode` parameter in the `avb_slot_verify()` function. Possible values include * `AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE` means that the HLOS will invalidate the current slot and restart. On devices with A/B this would lead to attempting to boot the other slot (if it's marked as bootable) or it could lead to a mode where no OS can be booted (e.g. some form of repair mode). In Linux this requires a kernel built with `CONFIG_DM_VERITY_AVB`. * `AVB_HASHTREE_ERROR_MODE_RESTART` means that the OS will restart without the current slot being invalidated. Be careful using this mode unconditionally as it may introduce boot loops if the same hashtree verification error is hit on every boot. * `AVB_HASHTREE_ERROR_MODE_EIO` means that an `EIO` error will be returned to the application. * `AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO` means that either the **RESTART** or **EIO** mode is used, depending on state. This mode implements a state machine whereby **RESTART** is used by default and when the `AVB_SLOT_VERIFY_FLAGS_RESTART_CAUSED_BY_HASHTREE_CORRUPTION` is passed to `avb_slot_verify()` the mode transitions to **EIO**. When a new OS has been detected the device transitions back to the **RESTART** mode. + To do this persistent storage is needed - specifically this means that the passed in `AvbOps` will need to have the `read_persistent_value()` and `write_persistent_value()` operations implemented. The name of the persistent value used is **avb.managed_verity_mode** and 32 bytes of storage is needed. * `AVB_HASHTREE_ERROR_MODE_LOGGING` means that errors will be logged and corrupt data may be returned to applications. This mode should be used for **ONLY** diagnostics and debugging. It cannot be used unless verification errors are allowed. * `AVB_HASHTREE_ERROR_MODE_PANIC` means that the OS will **panic** without the current slot being invalidated. Be careful using this mode as it may introduce boot panic if the same hashtree verification error is hit on every boot. This mode is available since: 1.7.0 (kernel 5.9) The value passed in `hashtree_error_mode` is essentially just passed on through to the HLOS through the the `androidboot.veritymode`, `androidboot.veritymode.managed`, and `androidboot.vbmeta.invalidate_on_error` kernel command-line parameters in the following way: | | `androidboot.veritymode` | `androidboot.veritymode.managed` | `androidboot.vbmeta.invalidate_on_error` | |------|:------------------------:|:--------------------------------:|:----------------------------------------:| | `AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE` | **enforcing** | (unset) | **yes** | | `AVB_HASHTREE_ERROR_MODE_RESTART` | **enforcing** | (unset) | (unset) | | `AVB_HASHTREE_ERROR_MODE_EIO` | **eio** | (unset) | (unset) | | `AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO` | **eio** or **enforcing** | **yes** | (unset) | | `AVB_HASHTREE_ERROR_MODE_LOGGING` | **ignore_corruption** | (unset) | (unset) | | `AVB_HASHTREE_ERROR_MODE_PANIC` | **panicking** | (unset) | (unset) | The only exception to this table is that if the `AVB_VBMETA_IMAGE_FLAGS_HASHTREE_DISABLED` flag is set in the top-level vbmeta, then `androidboot.veritymode` is set to **disabled** and `androidboot.veritymode.managed` and `androidboot.vbmeta.invalidate_on_error` are unset. The different values of `hashtree_error_mode` parameter in the `avb_slot_verify()` function can be categorized into three groups: * `AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE`, which needs `CONFIG_DM_VERITY_AVB` in the kernel config for the kernel to invalidate the current slot and restart. This is kept here for legacy Android Things devices and is not recommended for other device form factors. * The bootloader handles the switch between `AVB_HASHTREE_ERROR_MODE_RESTART` and `AVB_HASHTREE_ERROR_MODE_EIO`. This would need a persistent storage on the device to store the vbmeta digest, so the bootloader can detect if a device ever gets an update or not. Once the new OS is installed and if the device is in **EIO** mode, the bootloader should switch back to **RESTART** mode. * `AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO`: `libavb` helps the bootloader manage **EIO**/**RESTART** state transition. The bootloader needs to implement the callbacks of `AvbOps->read_persistent_value()` and `AvbOps->write_persistent_value()` for `libavb` to store the vbmeta digest to detect whether a new OS is installed. ### Which mode should I use for my device? This depends entirely on the device, how the device is intended to be used, and the desired user experience. For Android devices the `AVB_HASHTREE_ERROR_MODE_MANAGED_RESTART_AND_EIO` mode should be used. Also see the [Boot Flow section on source.android.com](https://source.android.com/security/verifiedboot/boot-flow) for the kind of UX and UI the boot loader should implement. If the device doesn't have a screen or if the HLOS supports multiple bootable slots simultaneously it may make more sense to just use `AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE`. ## Android Specific Integration On Android, the boot loader must set the `androidboot.verifiedbootstate` parameter on the kernel command-line to indicate the boot state. It shall use the following values: * **green**: If in LOCKED state and the key used for verification was not set by the end user. * **yellow**: If in LOCKED state and the key used for verification was set by the end user. * **orange**: If in the UNLOCKED state. ## Device Specific Notes This section contains information about how AVB is integrated into specific devices. This is not an exhaustive list. ### Pixel 2 and later On the Pixel 2, Pixel 2 XL and later Pixel models, the boot loader supports a virtual partition with the name `avb_custom_key`. Flashing and erasing this partition only works in the UNLOCKED state. Setting the custom key is done like this: avbtool extract_public_key --key key.pem --output pkmd.bin fastboot flash avb_custom_key pkmd.bin Erasing the key is done by erasing the virtual partition: fastboot erase avb_custom_key When the custom key is set and the device is in the LOCKED state it will boot images signed with both the built-in key as well as the custom key. All other security features (including rollback-protection) are in effect, e.g. the **only** difference is the root of trust to use. When booting an image signed with a custom key, a yellow screen will be shown as part of the boot process to remind the user that the custom key is in use. # Version History ### Version 1.2 Version 1.2 adds support for the following: * `rollback_index_location` field of the main vbmeta header. * `check_at_most_once` parameter of dm-verity in a hashtree descriptor. ### Version 1.1 Version 1.1 adds support for the following: * A 32-bit `flags` element is added to hash and hashtree descriptors. * Support for partitions which don't use [A/B](#A_B-Support). * Tamper-evident [named persistent values](#Named-Persistent-Values). * [Persistent digests](#Persistent-Digests) for hash or hashtree descriptors. ### Version 1.0 All features not explicitly listed under a later version are supported by 1.0.