Zen and the Art of Microcode Hacking (Google Bug Hunters)

The Google Bug Hunters blog has adetailed description of how a vulnerability in AMD's microcode-patchingfunctionality was discovered and exploited; the authors have also releaseda set of tools to assist with this kind of research in the future.

Secure hash functions are designed in such a way that there is nosecret key, and there is no way to use knowledge of theintermediate state in order to generate a collision. However, CMACwas not designed as a hash function, and therefore it is a weakhash function against an adversary who has the key. Remember thatevery AMD Zen CPU has to have the same AES-CMAC key in order tosuccessfully calculate the hash of the AMD public key and themicrocode patch contents. Therefore, the key only needs to berevealed from a single CPU in order to compromise all other CPUsusing the same key. This opens up the potential for hardwareattacks (e.g., reading the key from ROM with a scanning electronmicroscope), side-channel attacks (e.g., using Correlation PowerAnalysis to leak the key during validation), or other software orhardware attacks that can somehow reveal the key. In summary, it isa safe assumption that such a key will not remain secret forever.