On Sun, 25 Nov 2018, Andi Kleen wrote:
> > The current check whether two tasks belong to the same context is using the
> > tasks context id. While correct, it's simpler to use the mm pointer because
> > it allows to mangle the TIF_SPEC_IB bit into it. The context id based
> > mechanism requires extra storage, which creates worse code.
>
> [We tried similar in some really early versions, but it was replaced
> with the context id later.]
>
> One issue with using the pointer is that the pointer can be reused
> when the original mm_struct is freed, and then gets reallocated
> immediately to an attacker. Then the attacker may avoid the IBPB.
>
> Given it's probably hard to generate any reasonable leak bandwidth with
> such a complex scenario, but it still seemed better to close the hole.
Sorry, but that's really a purely academic exercise.
Thanks,
tglx
