Introduction to Custom Kernels and Android Verified Boot 2.0

For Android enthusiasts and power users, customizing their device often involves flashing a custom kernel. A custom kernel can unlock new features, improve performance, extend battery life, or enable root access. However, modern Android devices, especially those running Android 9.0 (Pie) and newer, incorporate robust security features like Android Verified Boot (AVB) 2.0, which can make flashing custom components a more complex endeavor. This guide will walk you through the process of flashing custom kernels on devices utilizing AVB 2.0, specifically for users with an already unlocked bootloader.

Android Verified Boot (AVB) 2.0 is a critical security mechanism designed to prevent malicious tampering with the device’s software. It ensures that all executed code, from the bootloader to the system partitions, comes from a trusted source (typically the device manufacturer). While essential for security, AVB 2.0 introduces challenges for custom development because flashing a modified kernel or other system images will trigger verification failures, often resulting in boot loops, a warning screen, or even rendering the device unbootable. For users with unlocked bootloaders, however, there are specific methods to bypass or disable AVB 2.0’s verification for custom kernel flashing.

Understanding Android Verified Boot (AVB) 2.0

AVB 2.0, also known as “dm-verity,” uses cryptographic integrity checks across various partitions to ensure the authenticity and integrity of the OS. Key components of AVB 2.0 include:

• `vbmeta` Partition: This dedicated partition (or a specific area within another partition) stores metadata about the verification process, including hashes of other partitions (`boot`, `system`, `vendor`, `dtbo`, etc.) and the root public key used for verification.
• Hash Tree Verification: Instead of verifying a single hash for an entire partition, AVB 2.0 employs a hash tree (Merkle tree) approach. This allows for block-level verification, meaning even a single altered block can be detected.
• Rollback Protection: AVB 2.0 prevents downgrading to an older, potentially vulnerable version of the OS. This is achieved by storing the OS version in anti-rollback counters (often in the `vbmeta` or SoC-specific fuses) that can only increase.

When you flash a custom kernel (typically as `boot.img` and sometimes `dtbo.img`), its cryptographic signature and/or hashes will not match those expected by the `vbmeta` partition, leading to a verification failure. To successfully flash a custom kernel, we must instruct AVB 2.0 to either ignore these integrity checks or verify against a modified `vbmeta` that acknowledges the custom components.

Prerequisites for Flashing Custom Kernels

Before proceeding, ensure you have the following:

1. Unlocked Bootloader:

   This is paramount. Without an unlocked bootloader, you cannot flash custom images or modify critical partitions. The process of unlocking varies by manufacturer and often voids warranties and wipes all user data. Ensure this step is completed before attempting any custom kernel flashing.

2. ADB and Fastboot Tools:

   Ensure you have the latest Android SDK Platform Tools installed on your computer and that ADB and Fastboot commands are accessible from your terminal/command prompt. You can download them from the official Android developer website.

3. Device-Specific Custom Kernel Files:

   You will need the custom `boot.img` file for your specific device model and Android version. Sometimes, a separate `dtbo.img` (Device Tree Blob Overlay) may also be required. Obtain these from trusted sources like XDA Developers forums or the kernel developer’s official repository.

4. Stock or Patched `vbmeta.img`:

   Depending on your device and the custom kernel, you might need the stock `vbmeta.img` from your current ROM’s factory image, or a specially prepared `vbmeta.img` that has already been patched to disable AVB verification. Always try with the stock `vbmeta.img` first, using the disable flags.

5. Backup Your Device:

   Always perform a full backup of your device’s data and, if possible, a Nandroid backup via a custom recovery (like TWRP). Flashing custom kernels carries inherent risks, and a backup can save you from data loss or a bricked device.

Method: Flashing with `fastboot –disable-verity –disable-verification`

This is the most common and recommended method for devices with an unlocked bootloader, as it temporarily disables AVB’s integrity checks on the `vbmeta` partition during the flashing process.

Step 1: Download and Prepare Files

Download your custom `boot.img` and any accompanying `dtbo.img` to the directory where your ADB and Fastboot tools are located on your computer for easy access. If you need a stock `vbmeta.img`, extract it from your device’s full factory image.

Step 2: Boot Your Device into Fastboot Mode

Power off your Android device. Then, boot it into Fastboot mode. This usually involves holding down a combination of buttons (e.g., Power + Volume Down) while powering on. Connect your device to your computer via USB.

Verify your device is recognized by Fastboot:

fastboot devices

You should see your device’s serial number listed.

Step 3: Disable AVB and Flash `vbmeta.img`

This is the critical step for bypassing AVB 2.0. We use specific Fastboot flags to instruct the bootloader to disable AVB verification for the `vbmeta` partition. This essentially tells the bootloader to ignore any integrity mismatches that might occur on other partitions, allowing your custom kernel to boot.

fastboot --disable-verity --disable-verification flash vbmeta vbmeta.img

• --disable-verity: This flag disables `dm-verity`, which is responsible for enforcing read-only integrity on partitions like `system` and `vendor`.
• --disable-verification: This flag disables the signature verification of the boot image and potentially other verified partitions.
• `flash vbmeta vbmeta.img`: This command flashes the (stock or patched) `vbmeta.img` to the `vbmeta` partition. It’s crucial that these flags precede the `flash vbmeta` command.

Note: If your device does not have a dedicated `vbmeta` partition, the `vbmeta` data might be embedded within the `boot` partition or another location. In such rare cases, the flashing process might be slightly different or involve custom tools to patch the `boot.img` directly. However, for most modern devices with AVB 2.0, a separate `vbmeta` partition exists.

Step 4: Flash the Custom Kernel Components

Now, flash your custom kernel’s `boot.img` and `dtbo.img` (if applicable) to their respective partitions:

fastboot flash boot boot.img fastboot flash dtbo dtbo.img

Replace `boot.img` and `dtbo.img` with the actual filenames of your custom kernel files.

Step 5: Reboot Your Device

Once flashing is complete, reboot your device:

fastboot reboot

Your device should now boot into the system using your newly flashed custom kernel. The first boot might take a bit longer than usual.

Step 6: Verify Installation (Optional but Recommended)

After your device boots, you can verify that the custom kernel is active. Navigate to “Settings” > “About phone” > “Android version” (or similar path depending on your ROM). Look for “Kernel version” or “Build number.” It should display information pertaining to your custom kernel.

Troubleshooting Common Issues

• Boot Loops or “Your Device Is Corrupt” Warning:

  This often indicates that AVB verification somehow still failed, or the custom kernel is incompatible with your current ROM/device. Double-check that you used the `–disable-verity –disable-verification` flags correctly. If issues persist, try re-flashing your stock `boot.img` and `vbmeta.img` from the factory image to restore functionality.

• Fastboot Not Recognizing Device:

  Ensure your USB drivers are correctly installed on your computer. Try a different USB port or cable. On some Windows systems, you might need to manually install specific Fastboot drivers for your device.

• `dtbo.img` Not Found or Required:

  Not all devices use a separate `dtbo` partition. If your custom kernel package doesn’t include a `dtbo.img` or your device’s architecture embeds DTBs within the `boot.img`, you can omit the `fastboot flash dtbo dtbo.img` step.

Conclusion

Flashing custom kernels on modern Android devices with AVB 2.0 requires an understanding of the verification process and the specific commands needed to bypass it. By utilizing the `fastboot –disable-verity –disable-verification` flags on an unlocked bootloader, you can successfully install custom kernels and enjoy the benefits they offer. Always remember the importance of backing up your data and sourcing kernel files from reputable developers. While this process empowers you to customize your device, proceed with caution and at your own risk.