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

[Andi Kleen]

On Sun, Nov 25, 2018 at 11:20:50PM +0100, Thomas Gleixner wrote:
> 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. 

Ok fair enough. I guess it's acceptable if you add a comment explaining it.

-Andi