Introduction to Android Verified Boot 2.0

Android Verified Boot (AVB) 2.0, often referred to as dm-verity, is a critical security feature implemented across modern Android devices. Its primary purpose is to ensure the integrity of the boot process, verifying that all executed code from the bootloader to the system image has not been tampered with. This is achieved through cryptographic verification, using a chain of trust that starts from a hardware-rooted public key. While essential for user security, AVB 2.0 presents significant challenges and considerations for custom ROM developers who need to modify or replace system partitions.

For developers creating custom firmware, kernels, or entirely new Android distributions like LineageOS, the default AVB 2.0 protections can prevent the device from booting modified images, resulting in error messages like “Your device is corrupt” or boot loops. This guide delves into advanced techniques for temporarily disabling and safely re-enabling AVB 2.0, providing the necessary knowledge for seamless custom ROM development and testing without compromising long-term device security.

Understanding AVB 2.0 Fundamentals

Before proceeding with modification, it’s crucial to grasp the core components of AVB 2.0:

• Root of Trust: A hardware-backed public key, usually fused into the SoC, which validates the initial bootloader.
• Verification Chain: Each stage of the boot process cryptographically verifies the next. The bootloader verifies the vbmeta partition, which in turn contains hashes and signatures for partitions like boot, system, vendor, etc.
• vbmeta Partition: This critical partition holds the metadata (hashes, signatures, and configuration flags) for all verified partitions. It’s the primary target for disabling verification.
• Rollback Protection: AVB 2.0 incorporates a rollback index to prevent an attacker from flashing an older, potentially vulnerable version of the OS. Modifying this can lead to hard-brick situations if not handled correctly.
• State (Locked/Unlocked): The bootloader’s state dictates its behavior. An UNLOCKED bootloader allows flashing custom images but usually triggers a warning. A LOCKED bootloader only boots signed, trusted images.

Prerequisites for AVB 2.0 Modification

To follow this guide, you will need:

• A Linux-based development environment (Ubuntu, Debian, etc.).
• ADB and Fastboot tools installed and configured.
• The Android source code (AOSP) or your custom ROM source code cloned and ready for building.
• Device-specific factory images or a full payload dump.
• The avbtool utility, typically found in the AOSP build environment (out/host/linux-x86/bin/avbtool).
• Basic understanding of shell commands and Android partitions.

Method 1: Disabling AVB via fastboot flashing unlock_critical (Limited Scope)

Many modern devices separate the bootloader unlock from critical partition unlock. While fastboot flashing unlock allows flashing most partitions, fastboot flashing unlock_critical is often required to truly disable AVB warnings and allow modification of partitions like vbmeta, boot, and system_a/b. This command typically wipes your device.

adb reboot bootloaderfastboot flashing unlock_critical

Confirm the unlock on your device screen. While this step is essential for flashing, it often doesn’t fully disable AVB warnings for modified images without further steps. It primarily enables flashing of critical partitions, which is a prerequisite for advanced AVB modifications.

Method 2: Patching the vbmeta Image

This is the most common and effective method for temporarily disabling AVB for custom ROM development. It involves modifying the vbmeta partition to include specific flags.

Step 2.1: Obtain the vbmeta Image

You can extract the vbmeta.img from your device’s factory image or a built AOSP image. For many devices, it’s a standalone partition.

# If you have factory images (example for Pixel devices)unzip <device_factory_image>.zipcd <image_folder> # e.g., crosshatch-tq1a.230205.002tar -xf image-<device_codename>-<build_id>.zip vbmeta.img# Alternatively, pull from a rooted device (if vbmeta is mounted)adb pull /dev/block/by-name/vbmeta vbmeta.img

Step 2.2: Patching vbmeta.img with avbtool

The avbtool utility is specifically designed for manipulating AVB images. We’ll use it to add the --disable-verification and --enable-loading-unverified-boot flags.

# Ensure avbtool is in your PATH or specify its full path# Example: source build/envsetup.sh && lunch <target># Navigate to where your vbmeta.img is locatedcp vbmeta.img vbmeta_original.img # Always keep a backup!avbtool disable_verification --partition_name vbmeta --image vbmeta.img --output_image vbmeta_patched.img

Alternatively, for more granular control or if the above doesn’t fully suppress warnings, you might need to recreate the vbmeta image with specific flags:

avbtool make_vbmeta_image --output vbmeta_patched.img 	--flag 2 	--padding_size 4096 	--set_hashtree_disabled_flag 	--set_verity_disabled_flag 	--set_boot_ap_verity_disabled 	--include_descriptors_from_image vbmeta_original.img

• --flag 2 (AVB_VBMETA_IMAGE_FLAGS_ROLLBACK_INDEX_DISABLED) disables rollback index for this image. Use with caution.
• --set_hashtree_disabled_flag disables dm-verity for data partitions.
• --set_verity_disabled_flag disables verity for boot/system partitions.
• --set_boot_ap_verity_disabled explicitly disables verity for the boot image.

The exact flags depend on the device and what warning you are trying to suppress. Experimentation might be required.

Step 2.3: Flashing the Patched vbmeta

Once you have your vbmeta_patched.img, flash it using fastboot:

adb reboot bootloaderfastboot flash vbmeta vbmeta_patched.imgfastboot reboot

After rebooting, your device should now boot into custom ROMs or modified images without AVB warnings. Note that on some devices, flashing a patched vbmeta might reset the device or trigger a factory reset on the next boot due to changes in verified state.

Method 3: Source Code Modification (Custom ROM/Kernel Builds)

For custom ROM developers who compile their entire system, modifying the build system directly is a cleaner approach to manage AVB.

Step 3.1: Modifying BoardConfig.mk

In your device’s device/<vendor>/<device> folder, locate the BoardConfig.mk file. You will typically find AVB-related flags there.

# Example of AVB configuration in BoardConfig.mk# To disable:BOARD_AVB_ENABLE := false# If you see specific partition flags, you might need to disable them too# BOARD_AVB_BOOT_ADD_HASH_FOOTER := false# BOARD_AVB_SYSTEM_ADD_HASH_FOOTER := false# BOARD_AVB_VENDOR_ADD_HASH_FOOTER := false# Add these lines if not present, or set existing ones to falseBOARD_AVB_DISABLE_VERITY := trueBOARD_AVB_MAKE_VBMETA_IMAGE_ARGS += --flags 2 # Disables rollback index for built vbmeta

Setting BOARD_AVB_ENABLE := false will instruct the build system not to generate AVB metadata for the built images. If you still encounter issues, ensure that BOARD_AVB_VBMETA_ADD_HASHTREE_FOOTER and similar flags are not set or explicitly set to false for testing.

Step 3.2: Rebuilding and Flashing

After modifying BoardConfig.mk, rebuild your custom ROM or the specific images (like boot.img, system.img, vbmeta.img) that you modified.

# Example commands for AOSP/LineageOS buildsource build/envsetup.shlunch <your_device_target>make -j$(nproc) # Or make bootimage, make vbmetaimage# Then flash the newly built imagesfastboot flash boot out/target/product/<device>/boot.imgfastboot flash vbmeta out/target/product/<device>/vbmeta.imgfastboot reboot

Re-enabling AVB 2.0

Re-enabling AVB 2.0 is crucial for restoring device security, enabling future OTA updates, and sometimes for relocking the bootloader. This process essentially reverses the steps of disabling it.

Step 4.1: Flash Stock Partitions

The most straightforward way to re-enable AVB is to flash the stock, untouched vbmeta.img and other critical partitions (boot.img, system.img, vendor.img, etc.) from your device’s official factory image. This ensures all components are cryptographically signed and verified correctly.

adb reboot bootloaderfastboot flash vbmeta <path_to_stock_vbmeta.img>fastboot flash boot <path_to_stock_boot.img># Repeat for system, vendor, etc., if they were modifiedfastboot reboot

Step 4.2: Relocking the Bootloader (Optional, with caution)

If you intend to return to a fully stock and secure state, you might want to relock your bootloader. This is only possible if all partitions are in a verifiable state (i.e., you’ve flashed all stock images). If any partition remains modified, relocking will result in a hard brick or a continuous