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?

source: L