Re: [patch V2 21/28] x86/speculation: Prepare for conditional IBPB in switch_mm()

[Thomas Gleixner]

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