Verified boot is an important security feature, primarily aimed at making it substantially harder for an attacker to persistently compromise the OS. It also provides basic resistance against tampering with a device after gaining physical access. For more user-oriented information such as details on the user experience and instructions on using the Auditor app for attestation, see the section in the usage guide.
In the primary use case, it serves as an extra line of defence after an attacker has gained remote code execution (either with an exploit chain or by getting the user to install an app) and then escalated to kernel level access. Verified boot aims to prevent the attacker from making modifications to any firmware or the operating system, along with preventing downgrades to past vulnerable versions. It aims to force the attacker into persisting via state outside the operating system (i.e. the userdata partition) and exploiting the OS again from there each time it boots. An important security property it aims to provide is that a factory reset can fully wipe away malware even if it has used exploits to gain root access. A much harder path to privileged persistence would be to exploit the verification process as it happens.
CopperheadOS implements full verified boot like stock Android operating systems, and also improves it by reducing the trust placed in persistent state (userdata partition).
The main weakness of verified boot is not the verification implementation itself but the fact that there’s a lot of persistent state in userdata. As a thought experiment, consider what would end up happening if the OS supported app-accessible root access (which isn’t even provided by either AOSP or CopperheadOS in a debug build). An attacker could simply persist as root by granting root access to their malicious app, which would wipe out many of the benefits of verified boot. The situation is much better than that, but still far from ideal:
Early firmware (i.e. loaded before the OS) is verified from a hardware root of trust. Standard mobile SoC platforms allow the device vendor to irreversibly flash their keys into fuses so that early firmware is signed by the device vendor rather than the SoC vendor. Early firmware includes the boot chain leading up to the OS kernel, TrustZone, the cellular baseband, fastboot mode, etc.
Each component of the boot chains verifies the next component before it’s loaded, leading up to the late stage bootloader (such as Qualcomm’s lk-based bootloader or TianoCore-based bootloader) which is responsible for verifying and loading the OS. Sample verification implementations are available in Qualcomm’s open source bootloader projects on the CodeAurora site (see the previous links). Device vendors usually make tweaks to the bootloaders, but Qualcomm’s reference implementations aren’t very far from what most devices using their SoC are shipping so it’s a good place to look.
Later firmware components and TrustZone apps may have their rollback protection enforced via the Replay Protected Memory Block instead of fuses if the device vendor wants more flexibility, such as being able to unlock the bootloader and downgrade components since increasing the version in the fuses cannot be undone. Ideally, fuses would be used for everything, but security needs to coexist with development needs, especially on devices supporting unlocking.
The main limitation of the early verified boot process is reluctance from device vendors to make use of the available rollback protection by actually increasing the rollback version. They may want to support using the devices for testing past releases of the OS paired with older firmware and this form of security is in conflict with that.
Verified boot becomes more interesting with the OS, where it shifts into Android Verified Boot 2.0 from the SoC platform implementation of verified boot.
The late stage bootloader starts by verifying vbmeta, which is a tiny signed operating system partition containing metadata needed to verify the rest of the OS. RSA2048 / SHA256 is the baseline standard signature algorithm and is used by the Pixel 2. The vbmeta data structure contains a rollback index, which is used to provide rollback protection for the OS. On the Pixel 2, the rollback index gets stored in the Replay Protected Memory Block.
If the device isn’t unlockable, verifying the OS uses a hard-wired key. If the device is unlockable like the Pixel 2, it also supports a non-verified mode when the bootloader is unlocked which shows a warning for at least 5s (10s on the Pixel 2). Devices supporting locking the bootloader with another OS (like the Pixel 2) have support for flashing a public key when the bootloader is unlocked and using that as an alternative to the hard-wired key, with a notice about an alternate OS being installed showing the fingerprint of the key on boot (for at least 5s, and the Pixel 2 also uses 10s for this). The Pixel 2 stores the public key in the Replay Protected Memory Block with the rollback indexes.
The vbmeta data structure hash hashes needed to verify the other partitions and to bootstrap verification of large partitions via hash trees. On the Pixel 2, it has hashes for the boot and dtbo partitions and hash tree metadata for system and vendor, which together with vbmeta are the full set of OS partitions.
Once the bootloader is finished with verification, it passes the verified boot state and verified boot public key to the Trusted Execution Environment TEE and loads / runs the kernel from the boot image. The kernel is responsible for using dm-verity to verify the rest of the OS via hash trees.
For more details see the official AVB documentation.
The TEE uses the verified boot key as input for disk encryption keys. It also makes use of the verified boot state, OS version and OS patch level to provide protection for the hardware-backed keystore as an indirect form of verified boot enforcement and downgrade protection. Even if the rollback index isn’t incremented for a new version, there’s still downgrade protection.
The integration with the keystore means that hardware-backed keys can be used for remote attestation of the verified boot state. In fact, there’s a fancy key attestation feature for having the TEE sign the public key certificate of a newly generated hardware-backed key with an attestation key chaining to a known attestation root. An attacker that has gained root / kernel access can’t fake this, but they could exploit the TEE to bypass this. Android 9.0 brings support for a StrongBox keymaster implemented via a dedicated HSM, which will probably be present on the Pixel 3. Ideally, various other improvements will be made to these attestation capabilities too. The current implementation is far from ideal, but it’s already a very useful feature and a huge improvement over not being able to meaningfully inspect devices like this.
Copperhead uses the hardware-backed keystore with key attestation to implement our Auditor app which provides both local verification from another Android device (via QR codes). The app also has support for regularly scheduled remote verification using our attestation server hosted at https://attestation.copperhead.co/.
Our Auditor app builds on the baseline verified boot for firmware and the entire operating system by gathering information from the OS. For example, it can detect that an attacker persisted via an accessibility service or device administrator. The TEE attestation provides the app id and key fingerprint, which is what bootstraps the OS enforced checks. An attacker with root / kernel access could fake the OS enforced checks, but the OS version / OS patch level are provided by the key attestation feature and mitigate simply holding back the OS version to continue reliably exploiting it. The Auditor app capabilities will be substantially expanded in the future. It’s not magic, but it does offer real security properties and it will be very useful for monitoring the health of a fleet of Android devices, or just one device.
For more details on the Auditor app, see the documentation on the attestation protocol in the sources.