On 24/05/18 13:01, Mark Rutland wrote:

On Tue, May 22, 2018 at 04:06:43PM +0100, Marc Zyngier wrote:

quoted

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/

quoted

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 <redacted>

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.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...