Secure Boot is a crucial security feature designed to ensure that only trusted software is executed during the boot process. However, as vulnerabilities emerge, more advanced mechanisms like Secure Boot Advanced Targeting (SBAT) have been introduced to enhance its security. This article explains the key concepts behind SBAT and how it works to maintain the integrity of the boot process.

How Traditional Secure Boot Works #

Secure Boot functions by validating each component of the boot process, starting with the firmware verifying the bootloader, which in turn validates the kernel and so on. This chain of trust ensures that only verified and signed components are allowed to run. If a vulnerability is found, the system revokes untrustworthy components by adding their hash to a blacklist. This method has been effective, but it faces challenges in handling the scale and complexity of modern boot environments.

At a high level, the Secure Boot process follows a chain-of-trust model:
1. UEFI Firmware: Verifies the bootloader signature.
2. Bootloader: Verifies the kernel signature.
3. Kernel: Validates dynamically loaded modules and any additional kernel code.
4. Runtime Environment: Trust is carried forward, securing the system’s execution environment.

This model ensures that only components signed with a trusted key are executed. However, when a vulnerability is discovered in any of these components (e.g., the bootloader), a traditional Secure Boot implementation requires adding the component’s hash to a blacklist, which prevents that binary from running in the future. The problem? Scale.

The Problem: Managing Large Numbers of Binaries #

Each Linux distribution, for example, may compile its own version of the bootloader (e.g., GRUB), creating many distinct binaries with unique hashes. When a vulnerability is identified in the underlying bootloader code, every affected binary must be revoked individually. Given that the storage available for revocation hashes (typically stored in NVRAM) is limited, this approach is not sustainable for large-scale environments.

SBAT: Enhancing Secure Boot Scalability #

Secure Boot Advanced Targeting (SBAT) is designed to solve this scalability issue by shifting away from revoking individual binary hashes and instead introducing security generations. The SBAT mechanism works as follows:
1. Security Generation Number: Each boot component (bootloader, kernel, etc.) is assigned a generation number. This number is embedded into the signed binary during the build process.
2. SBAT Variable: A new firmware variable is introduced to store the minimum allowed security generation for each component.
3. Validation Logic: During the boot process, the system checks the next component’s security generation number against the minimum generation defined in the SBAT variable. If the component’s generation is below the threshold, it is rejected and the boot process halts.

SBAT Flow #

• The firmware initiates the bootloader and checks its SBAT security generation.
• The bootloader validates the kernel’s generation number.
• The kernel may validate other modules’ security generation numbers as required.

This approach allows for revoking multiple versions of vulnerable binaries by simply raising the minimum generation number in the SBAT variable, without the need to store individual hashes.

Implementing SBAT: Key Components #

SBAT Section in Signed Binaries #

Every component in the boot chain must include an SBAT metadata section that contains:

• The component’s name.
• The component’s current security generation number.

Firmware Integration #

The system firmware needs to be updated to recognize and handle the SBAT variable. This includes:

• Storing the SBAT variable in NVRAM.
• Checking SBAT metadata in boot components.
• Rejecting any components with outdated generation numbers.

Security Policy Management #

Updating the SBAT variable is part of the overall security policy management of the system. Any updates must increment the generation number across all relevant components in the boot chain to ensure continuous protection.

SBAT and Secure Boot Hardening #

By adopting SBAT, organizations can:
1. Reduce Revocation Overhead: Instead of storing many hashes of vulnerable binaries, only the generation number needs to be updated.
2. Increase Security: Vulnerable components can be blocked more efficiently, mitigating risks without exhausting firmware storage.
3. Easier Rollouts: System administrators can manage firmware updates and Secure Boot configurations more easily, ensuring quicker response times to vulnerabilities.

Conclusion #

Secure Boot Advanced Targeting (SBAT) is an effective method for handling security updates in the Secure Boot chain without overwhelming the system’s storage or security infrastructure. By embedding generation numbers into each boot component and using a single update mechanism to enforce those generations, SBAT provides scalability and flexibility for managing the ever-evolving landscape of boot security.

For modern Linux and Windows systems, SBAT ensures a stronger, more resilient Secure Boot environment capable of responding quickly to threats with minimal operational overhead.