Re: [PATCH 09/14] arm64: ssbd: Introduce thread flag to control userspace mitigation

[Will Deacon]

On Thu, May 24, 2018 at 01:16:38PM +0100, Marc Zyngier wrote:
> On 24/05/18 13:01, Mark Rutland wrote:
> > On Tue, May 22, 2018 at 04:06:43PM +0100, Marc Zyngier wrote:
> >> In order to allow userspace to be mitigated on demand, let's
> >> introduce a new thread flag that prevents the mitigation from
> >> being turned off when exiting to userspace, and doesn't turn
> >> it on on entry into the kernel (with the assumtion that the
> > 
> > Nit: s/assumtion/assumption/
> > 
> >> mitigation is always enabled in the kernel itself).
> >>
> >> This will be used by a prctl interface introduced in a later
> >> patch.
> >>
> >> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> > 
> > On the assumption that this flag cannot be flipped while a task is in
> > userspace:
> 
> Well, that's the case unless you get into the seccomp thing, which does
> change TIF_SSBD on all threads of the task, without taking it to the
> kernel first. That nicely breaks the state machine, and you end-up
> running non-mitigated in the kernel. Oops.
> 
> I have a couple of patches fixing that, using a second flag
> (TIF_SSBD_PENDING) that gets turned into the real thing on exit to
> userspace. It's pretty ugly though.

... which introduces the need for atomics on the entry path too :(

I would /much/ rather kill the seccomp implicit enabling of the mitigation,
or at least have a way to opt-out per arch since it doesn't seem to be
technically justified imo.

Will