BaselineWatch
Desk
Your fleet and your work Fleetyour releases and the hardware they run onTriagewhat needs a decisionWatchlistthe products and platforms you runSaved searchesthe lists you come back to
This instance Sourceswhat is read, how often, under what termsDigestthe daily or weekly summary, and alerts
How it is drawn Appearancethe look and the colour schemeBrandingthe mark this instance wears
Owner sign-in

5 The RTU ensures that the BIOS flash memory is locked prior to transferring control to code

NIST platform security: roots of trust, firmware and secure enclaves SP 800-147B
All changes in this version
first version loaded 5 The RTU ensures that the BIOS flash memory is locked prior to transferring control to code

Title

The RTU ensures that the BIOS flash memory is locked prior to transferring control to code

Control

outside the RTU (e.g., Option ROMs), per the Firmware Integrity Protection requirements in Section 3.3. See Appendix B for an example of this update mechanism.

4.2 Update Mechanism 2: Secure BIOS Update at Reboot

With this BIOS update mechanism, the BIOS flash memory process is initiated at server runtime. However, the actual update of the BIOS flash memory does not occur until a server reboot. This BIOS update mechanism prevents inauthentic code from ever being written to the BIOS flash memory.

The relevant guidelines for implementing this BIOS update mechanism are listed below:  The BIOS update image must be digitally signed in accordance with the BIOS Update Authentication requirements in Section 3.1. An implementation may divide the BIOS update image into multiple, individually digitally signed parts.  A locking mechanism must exist for the BIOS flash memory such that no entity except an RTU has write access to the BIOS flash memory at runtime. Although it is not necessary for an RTU to have write access to the BIOS flash memory for this update mechanism, additional BIOS update mechanisms may be supported that do have this requirement.  An RTU, as defined in Section 2.5, must be available during system boot that can update the BIOS flash memory at runtime. The RTU must execute on reboot before the BIOS flash memory is updated. The RTU will verify the BIOS update image’s digital signature before any changes to the flash occur.  A memory location whose contents are preserved on reboot must exist in which the signed BIOS update image can be buffered at runtime and which the RTU can access during the reboot to verify the BIOS update image’s digital signature before updating the contents of the BIOS flash memory. During reboot, the memory location holding the BIOS update image during verification must not be accessible by any entities other than an RTU.

The general steps for implementing this mechanism are: 1. A digitally signed BIOS update image is buffered into a storage location whose contents are preserved when the server is rebooted. 2. When the server is rebooted, execution is transferred to the RTU. 3. The RTU verifies the BIOS update image is authentic. If found to be authentic, the RTU will unlock the BIOS flash memory if necessary and write the update into the BIOS flash memory. If the BIOS update image is determined to be inauthentic, the BIOS flash memory will not be updated. 4. The locking mechanism for the BIOS flash memory is enabled before executing untrusted code, including Option ROMs.

See Appendix C for an example of this update mechanism.

4.3 Update Mechanism 3: Secure BIOS Update Requiring Verification at Boot

With this BIOS update mechanism, a locking mechanism to protect the BIOS flash memory at runtime does not exist or has limitations such that writes to the BIOS flash memory by an entity other than an RTU are not prevented. A rogue update of the BIOS flash memory might occur as the BIOS flash memory is not write-protected or locked. However, the contents of the BIOS flash memory shall be authenticated before being executed on every boot. If the BIOS flash memory is determined to be inauthentic during boot, a recovery process is initiated and the inauthentic BIOS is not executed.

The relevant guidelines for implementing this BIOS update mechanism are listed below:  The BIOS image must be digitally signed in accordance with the BIOS Update Authentication requirements in Section 3.1.  An RTU, as defined in Section 2.5, must verify the digital signature of the BIOS update image before writing the image to the BIOS flash memory.  As the system BIOS is not protected from modification at runtime, the RTU must include a verification component capable of verifying the system BIOS prior to execution, per the Firmware Integrity Protection requirements in Section 3.3. The verification component must execute on boot and verify the system BIOS before any updatable BIOS code is executed.  If it is determined that the system BIOS is inauthentic, the RTU must initiate a recovery process, and the inauthentic system BIOS must never be executed.

The general steps for implementing this mechanism are: 1. A digitally signed BIOS image is verified and written into the BIOS flash memory. 2. On every boot, execution is transferred to the RTU. 3. If the verification component of the RTU determines the BIOS flash memory is authentic, execution is transferred to the BIOS. 4. If it is determined that the BIOS flash memory is inauthentic, the RTU initiates a recovery process, and the inauthentic BIOS is never executed.

See Appendix D for an example of this update mechanism. 5. Guidelines for Service Processors

A key distinction between servers and clients is the inclusion of a Service Processor in the system. The Service Processor plays a critical role in managing and monitoring the server, and may have a role in updating the system BIOS. This section describes the Service Processor and provides more detailed security requirements for systems that contain a Service Processor.