Tech

BIOS Chronomancy


Expo Theatre (Hall G) October 22, 2013 - Feedback   

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John Butterworth

In 2011 the National Institute of Standard and Technology (NIST) released a draft of special publication 800-155. This document provides a more detailed description than the Trusted Platform Module (TPM) PC client specification for content that should be measured in the BIOS to provide an adequate Static Root of Trust for Measurement (SRTM). To justify the importance of 800-155, in this talk we look at the implementation of the SRTM from a vendor’s pre-800-155 laptop. We discuss how the BIOS and thus SRTM can be manipulated either due to a configuration that does not enable signed BIOS updates, or via an exploit we discovered that allows for BIOS reflash even in the presence of a signed update requirement.

 

We also show how a 51 byte patch to the SRTM can cause it to provide a forged measurement to the TPM indicating that the BIOS is pristine. If a TPM Quote is used to query the boot state of the system, this TPM-signed falsification will then serve as the root of misplaced trust. We also show how reflashing the BIOS may not necessarily remove this trust-subverting malware. To fix the un-trustworthy SRTM we apply an academic technique whereby the BIOS software indicates its integrity through a timing side-channel.