Re: [Patch v5 11/16] x86/speculation: Add Spectre v2 app to app protection modes

[Jiri Kosina]

On Mon, 19 Nov 2018, Thomas Gleixner wrote:

> > On Sat, 17 Nov 2018, Jiri Kosina wrote:
> 
> > Subject: [PATCH] x86/speculation: enforce STIBP for SECCOMP tasks in lite mode
> > 
> > If 'lite' mode of app2app protection from spectre_v2 is selected on
> > kernel command-line, we are currently applying STIBP protection to
> > non-dumpable tasks, and tasks that have explicitly requested such
> > protection via
> > 
> > 	prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIR_BRANCH, PR_SPEC_ENABLE, 0, 0);
> > 
> > Let's extend this to cover also SECCOMP tasks (analogically to how we
> > apply SSBD protection).
> 
> Right. And SSBD does not fiddle with dumpable.
> 
> Willy had concerns about the (ab)use of dumpable so I'm holding off on that
> bit for now.

Yeah. IBPB implementation used to check the dumpability of tasks during 
rescheduling, but that went away later.

I still think that ideally that 'app2app' setting would toggle how IBPB is 
being used as well, something along the lines:

lite:
	- STIBP for the ones marked via prctl() and SECCOMP with the TIF_ 
	  flag
	- ibpb_needed() returning true for the same

strict:
	- STIBP: as currently implemented
	- ibpb_needed() returning always true

off:
	- neither STIBP nor IBPB applied ever

That's give us also some % of performance lost via IBPB back.

Makes sense?

Thanks,

-- 
Jiri Kosina
SUSE Labs