this post was submitted on 10 Jul 2025
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I remember the days when bugs in x86 CPUs were almost unheard of. The Pentium FDIV bug and the F00F bug were considered these unicorn things.
This isn't really the same kind of bug. Those bugs made instructions emit the wrong answer, which is obviously really bad, and they're really rare. The bugs in the article make instructions take different amounts of time depending on what else the CPU has done recently, which isn't something anyone would notice except that by asking the kernel to do something and measuring the time to execute affected instructions, an attacker that only had usermode access could learn secrets that should only be available to the kernel.