AMD Zenbleed chip bug leaks secrets fast and easy
, they simply mark the affected registers as zero by setting a special bit, and then allow those registers to be used for other operations. Ifis speculatively executed – the processor anticipates it will need to run that instruction – it sets this zero bit and frees the registers in the register file for reuse. This can happen if theinstruction lies right after a branch instruction; if the processor thinks the branch is unlikely to be taken, it will start thespeculatively.
instruction, it tries to rewind that decision and undo the zeroing by clearing the bit that indicates the registers are zero. Unfortunately, by that point, the registers are probably in use by some other code, and are no longer marked as zero, so their contents from the previous operation are now accessible to that other code.
As Ormandy noted,"bits and bytes are flowing into these vector registers from all over your system constantly." He continued:What should happen if the processor speculatively executed a vzeroupper, but then discovers that there was a branch misprediction? Well, we will have to revert that operation and put things back the way they were… maybe we can just unset that z-bit?
If we return to the analogy of malloc and free, you can see that it can’t be that simple - that would be like calling free on a pointer, and then changing your mind! That would be a use-after-free vulnerability, but there is no such thing as a use-after-free in a CPU… or is there?We've asked AMD for further comment. ®
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