Re: [patch V2 27/28] x86/speculation: Add seccomp Spectre v2 user space protection mode

[Thomas Gleixner]

On Mon, 26 Nov 2018, Andrea Arcangeli wrote:

> Hello,
> 
> On Sun, Nov 25, 2018 at 11:28:59PM +0100, Thomas Gleixner wrote:
> > Indeed. Just checked the documentation again, it's also not clear whether
> > IBPB is required if STIPB is in use.
> 
> I tried to ask this question too earlier:
> 
> https://lkml.kernel.org/r/20181119234528.GJ29258@xxxxxxxxxx
> 
> If the BTB mistraining in SECCOMP context with STIBP set in SPEC_CTRL,
> can still influence the hyperthreading sibling after STIBP is cleared,
> IBPB is needed before clearing STIBP. Otherwise it's not. Unless told
> otherwise, it'd be safe to assume IBPB is needed in such case.

IBPB is still issued. I won't change that before we have clarification.

But I doubt it's necessary. STIBP seems to be a rather big hammer.

Thanks,

	tglx