commit	c9b6e163084eb906cb597c7aa131d2fc3c6486d2	[log] [tgz]
author	Tom Cherry <tomcherry@google.com>	Thu May 18 20:34:51 2017 +0000
committer	android-build-merger <android-build-merger@google.com>	Thu May 18 20:34:51 2017 +0000
tree	691ce873c7e20df40de4c43084bd3197add4939e
parent	710367fc6bf2e19b75e9f0d6490cc462e392739a [diff]
parent	ac8f2206b6dd37000693ac38c16d883d9214473c [diff]

tree: 691ce873c7e20df40de4c43084bd3197add4939e

README.md

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.

What is it?
Tools and Libraries
Device Integration

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:

AVB with boot, system, and vendor

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:

AVB with a chained partition

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 either the vbmeta partition or - via a chain partition descriptor - in the partition that they are used to integrity check. This allows for a wide range of organizational and trust relationships.

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:

AVB rollback indexes

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:

AVB stored rollback indexes

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 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:

AVB with A/B partitions

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].

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.

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_ab/
- An experimental A/B implementation for use in boot loaders and AVB examples.
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.
boot_control/
- An implementation of the Android boot_control HAL for use with boot loaders using the experimental libavb_ab A/B stack.
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/.
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.

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]        \
    [--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]                                \
    [--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_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]                                \
    [--append_to_release_string STR]                                           \
    [--calc_max_image_size]

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_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]                                \
    [--append_to_release_string STR]                                           \
    [--calc_max_image_size]

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.

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

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 verify_image command verifies that the signature on the vbmeta struct is valid and that it was made with the embedded public key. This can be used to check that a custom signing helper works as intended.

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

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 vendor.img is being built, a hash-tree will also be appended to this image and its hash-tree descriptor will be included in vbmeta.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, --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.

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, and device state (whether the device is LOCKED or UNLOCKED). 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.

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:

// 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, &current_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:

if (is_slot_is_marked_as_successful(slot->ab_suffix)) {
    if (!update_stored_rollback_indexes_for_slot(ops, slot)) {
        // TODO: handle error.
    }
}

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 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:

Recommended AVB boot flow

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.).

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).
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_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.

The value passed in hashtree_error_mode is essentially just passed on through to the HLOS through the the androidboot.veritymode and androidboot.vbmeta.invalidate_on_error kernel command-line parameters. The HLOS - including the Linux kernel when using CONFIG_DM_VERITY_AVB - will then act upon hashtree verification errors as specified.

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 example, consider the EIO mode in an earlier version of Android Verified Boot (see the "Recovering from dm-verity errors" section). In a nutshell this mode uses AVB_HASHTREE_ERROR_MODE_RESTART mode until an error is encounted and then it switches to AVB_HASHTREE_ERROR_MODE_EIO mode on the reboot. Additionally when in AVB_HASHTREE_ERROR_MODE_EIO mode the user is informed that the device experienced corruption and then asked to click through a screen to continue.

To implement this mode in a boot loader, a combination of the AVB_HASHTREE_ERROR_MODE_RESTART mode and AVB_HASHTREE_ERROR_MODE_EIO mode could be used along with persistent storage recording what mode the bootloader is currently in. This would need to include transition rules e.g. if the kernel indicates that it rebooted because of a dm-verity error the bootloader would need to transition from the AVB_HASHTREE_ERROR_MODE_RESTART mode to the AVB_HASHTREE_ERROR_MODE_EIO mode. Ditto, when the slot is updated the bootloader needs to transition from the AVB_HASHTREE_ERROR_MODE_EIO mode back to the AVB_HASHTREE_ERROR_MODE_RESTART mode so the user doesn't have to click through a screen on every boot.

On the other hand, 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.