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

[Thomas Gleixner]

On Sun, 25 Nov 2018, Andi Kleen wrote:
> 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.

Will do.

Thanks,

	tglx