Summary of the blog post "Zenbleed" by Tavis Ormandy: The blog post discusses the discovery of a vulnerability called "Zenbleed" in certain AMD Zen 2 processor

Summary of the blog post "Zenbleed" by Tavis Ormandy: The blog post discusses the discovery of a vulnerability called "Zenbleed" in certain AMD Zen 2 processors. It revolves around the use of AVX2 and the vzeroupper instruction, which zeroes upper bits in vector registers (YMM) to avoid dependencies and stalls during superscalar execution. The vulnerability arises from a misprediction involving vzeroupper, which can be exploited with precise scheduling and triggering the XMM Register Merge Optimization, leading to a use-after-free-like situation. This allows attackers to monitor operations using vector registers, potentially leaking sensitive information like encryption keys and passwords. The author found the bug through fuzzing and developed a new approach called Oracle Serialization to detect CPU execution errors during testing. The vulnerability (CVE-2023-20593) affects various AMD Zen 2 processors, but AMD released a microcode update to address the issue. For systems unable to apply the update, a software workaround exists by setting the DE_CFG[9] "chicken bit." The post concludes with acknowledgments to individuals who contributed to the discovery and analysis of the Zenbleed vulnerability.