We have discovered a CPU vulnerability that occurs when redundant rex prefixes are added to an optimized rep movs operation. This glitch can cause unpredictable behavior such as branches to unexpected locations, ignored unconditional branches, and inaccurate recording of the instruction pointer. When multiple cores trigger this bug, the processor can report machine check exceptions and halt, posing serious security risks for cloud providers. Intel has released updated microcode to address this issue. If unable to update, disabling fast strings can serve as a workaround, although it will significantly impact performance. We welcome further study and input on this matter.
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