134 lines
4.3 KiB
ReStructuredText
134 lines
4.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
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Android Verified Boot 2.0
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=========================
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This file contains information about the current support of Android Verified
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Boot 2.0 in U-Boot.
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Overview
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--------
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Verified Boot establishes a chain of trust from the bootloader to system images:
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* Provides integrity checking for:
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* Android Boot image: Linux kernel + ramdisk. RAW hashing of the whole
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partition is done and the hash is compared with the one stored in
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the VBMeta image
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* ``system``/``vendor`` partitions: verifying root hash of dm-verity hashtrees
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* Provides capabilities for rollback protection
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Integrity of the bootloader (U-Boot BLOB and environment) is out of scope.
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For additional details check [1]_.
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AVB using OP-TEE (optional)
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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If AVB is configured to use OP-TEE (see `Enable on your board`_) rollback
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indexes and device lock state are stored in RPMB. The RPMB partition is managed
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by OP-TEE (see [2]_ for details) which is a secure OS leveraging ARM
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TrustZone.
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AVB 2.0 U-Boot shell commands
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-----------------------------
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Provides CLI interface to invoke AVB 2.0 verification + misc. commands for
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different testing purposes::
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avb init <dev> - initialize avb 2.0 for <dev>
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avb verify - run verification process using hash data from vbmeta structure
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avb read_rb <num> - read rollback index at location <num>
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avb write_rb <num> <rb> - write rollback index <rb> to <num>
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avb is_unlocked - returns unlock status of the device
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avb get_uuid <partname> - read and print uuid of partition <partname>
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avb read_part <partname> <offset> <num> <addr> - read <num> bytes from
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partition <partname> to buffer <addr>
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avb write_part <partname> <offset> <num> <addr> - write <num> bytes to
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<partname> by <offset> using data from <addr>
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Partitions tampering (example)
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------------------------------
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Boot or system/vendor (dm-verity metadata section) is tampered::
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=> avb init 1
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=> avb verify
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avb_slot_verify.c:175: ERROR: boot: Hash of data does not match digest in
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descriptor.
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Slot verification result: ERROR_IO
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Vbmeta partition is tampered::
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=> avb init 1
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=> avb verify
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avb_vbmeta_image.c:206: ERROR: Hash does not match!
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avb_slot_verify.c:388: ERROR: vbmeta: Error verifying vbmeta image:
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HASH_MISMATCH
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Slot verification result: ERROR_IO
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Enable on your board
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--------------------
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The following options must be enabled::
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CONFIG_LIBAVB=y
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CONFIG_AVB_VERIFY=y
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CONFIG_CMD_AVB=y
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In addtion optionally if storing rollback indexes in RPMB with help of
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OP-TEE::
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CONFIG_TEE=y
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CONFIG_OPTEE=y
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CONFIG_OPTEE_TA_AVB=y
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CONFIG_SUPPORT_EMMC_RPMB=y
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Then add ``avb verify`` invocation to your android boot sequence of commands,
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e.g.::
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=> avb_verify=avb init $mmcdev; avb verify;
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=> if run avb_verify; then \
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echo AVB verification OK. Continue boot; \
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set bootargs $bootargs $avb_bootargs; \
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else \
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echo AVB verification failed; \
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exit; \
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fi; \
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=> emmc_android_boot= \
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echo Trying to boot Android from eMMC ...; \
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... \
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run avb_verify; \
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mmc read ${fdtaddr} ${fdt_start} ${fdt_size}; \
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mmc read ${loadaddr} ${boot_start} ${boot_size}; \
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bootm $loadaddr $loadaddr $fdtaddr; \
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If partitions you want to verify are slotted (have A/B suffixes), then current
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slot suffix should be passed to ``avb verify`` sub-command, e.g.::
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=> avb verify _a
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To switch on automatic generation of vbmeta partition in AOSP build, add these
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lines to device configuration mk file::
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BOARD_AVB_ENABLE := true
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BOARD_AVB_ALGORITHM := SHA512_RSA4096
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BOARD_BOOTIMAGE_PARTITION_SIZE := <boot partition size>
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After flashing U-Boot don't forget to update environment and write new
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partition table::
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=> env default -f -a
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=> setenv partitions $partitions_android
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=> env save
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=> gpt write mmc 1 $partitions_android
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References
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----------
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.. [1] https://android.googlesource.com/platform/external/avb/+/master/README.md
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.. [2] https://www.op-tee.org/
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