Android Verified Boot 2.0

This file contains information about the current support of Android Verified Boot 2.0 in U-Boot.

Overview

Verified Boot establishes a chain of trust from the bootloader to system images:

Provides integrity checking for:
- Android Boot image: Linux kernel + ramdisk. RAW hashing of the whole partition is done and the hash is compared with the one stored in the VBMeta image
- system/vendor partitions: verifying root hash of dm-verity hashtrees
Provides capabilities for rollback protection

Integrity of the bootloader (U-Boot BLOB and environment) is out of scope.

For additional details check [1].

AVB using OP-TEE (optional)

If AVB is configured to use OP-TEE (see Enable on your board) rollback indexes and device lock state are stored in RPMB. The RPMB partition is managed by OP-TEE (see [2] for details) which is a secure OS leveraging ARM TrustZone.

AVB 2.0 U-Boot shell commands

Provides CLI interface to invoke AVB 2.0 verification + misc. commands for different testing purposes:

avb init <dev> - initialize avb 2 for <dev>
avb read_rb <num> - read rollback index at location <num>
avb write_rb <num> <rb> - write rollback index <rb> to <num>
avb is_unlocked - returns unlock status of the device
avb get_uuid <partname> - read and print uuid of partition <part>
avb read_part <partname> <offset> <num> <addr> - read <num> bytes from
    partition <partname> to buffer <addr>
avb read_part_hex <partname> <offset> <num> - read <num> bytes from
    partition <partname> and print to stdout
avb write_part <partname> <offset> <num> <addr> - write <num> bytes to
    <partname> by <offset> using data from <addr>
avb read_pvalue <name> <bytes> - read a persistent value <name>
avb write_pvalue <name> <value> - write a persistent value <name>
avb verify [slot_suffix] - run verification process using hash data
    from vbmeta structure
    [slot_suffix] - _a, _b, etc (if vbmeta partition is slotted)

Partitions tampering (example)

Boot or system/vendor (dm-verity metadata section) is tampered:

=> avb init 1
=> avb verify
avb_slot_verify.c:175: ERROR: boot: Hash of data does not match digest in
descriptor.
Slot verification result: ERROR_IO

Vbmeta partition is tampered:

=> avb init 1
=> avb verify
avb_vbmeta_image.c:206: ERROR: Hash does not match!
avb_slot_verify.c:388: ERROR: vbmeta: Error verifying vbmeta image:
HASH_MISMATCH
Slot verification result: ERROR_IO

Enable on your board

The following options must be enabled:

CONFIG_LIBAVB=y
CONFIG_AVB_VERIFY=y
CONFIG_CMD_AVB=y

In addtion optionally if storing rollback indexes in RPMB with help of OP-TEE:

CONFIG_TEE=y
CONFIG_OPTEE=y
CONFIG_OPTEE_TA_AVB=y
CONFIG_SUPPORT_EMMC_RPMB=y

Then add avb verify invocation to your android boot sequence of commands, e.g.:

=> avb_verify=avb init $mmcdev; avb verify;
=> if run avb_verify; then                       \
        echo AVB verification OK. Continue boot; \
        set bootargs $bootargs $avb_bootargs;    \
   else                                          \
        echo AVB verification failed;            \
        exit;                                    \
   fi;                                           \

=> emmc_android_boot=                                   \
       echo Trying to boot Android from eMMC ...;       \
       ...                                              \
       run avb_verify;                                  \
       mmc read ${fdtaddr} ${fdt_start} ${fdt_size};    \
       mmc read ${loadaddr} ${boot_start} ${boot_size}; \
            bootm $loadaddr $loadaddr $fdtaddr;         \

If partitions you want to verify are slotted (have A/B suffixes), then current slot suffix should be passed to avb verify sub-command, e.g.:

=> avb verify _a

To switch on automatic generation of vbmeta partition in AOSP build, add these lines to device configuration mk file:

BOARD_AVB_ENABLE := true
BOARD_AVB_ALGORITHM := SHA512_RSA4096
BOARD_BOOTIMAGE_PARTITION_SIZE := <boot partition size>

After flashing U-Boot don’t forget to update environment and write new partition table:

=> env default -f -a
=> setenv partitions $partitions_android
=> env save
=> gpt write mmc 1 $partitions_android