Tianocore Bug 1614 – BootGuard TOCTOU vulnerability

Re: https://firmwaresecurity.com/2019/05/06/now-you-see-it-toctou-attacks-against-secure-boot-and-bootguard/

We have found a TOCTOU vulnerability which allows an attacker with physical access to achieve code execution after the Boot Guard ACM computes and validates the hash of the IBB and measured the firmware into the TPM PCR0. This means the firmware will be marked as valid and have normal PCR0 values even though unsigned code has run. The root cause is when the FSP has finished memory init and disables MTRRs (and thereby the cache) in order to switch off No Evict Mode. The code doing this (the SecCore PeiTemporaryRamDonePpi callback), is executed directly from SPI flash, allowing an attacker to intercept execution flow. As a proof of concept we demonstrated that using an FPGA to substitute a single, targeted SPI transaction we can gain code execution.

https://bugzilla.tianocore.org/show_bug.cgi?id=1614

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