On Fri, 2018-01-26 at 09:59 -0800, Andi Kleen wrote:
> On Fri, Jan 26, 2018 at 09:19:09AM -0800, Linus Torvalds wrote:
> > 
> > On Fri, Jan 26, 2018 at 1:11 AM, David Woodhouse  wrote:
> > > 
> > > 
> > > Do we need to look again at the fact that we've disabled the RSB-
> > > stuffing for SMEP?
> > Absolutely. SMEP helps make people a lot less worried about things,
> > but it doesn't fix the "BTB only contains partial addresses" case.
> > 
> > But did we do that "disable stuffing with SMEP"? I'm not seeing it. In
> > my tree, it's only conditional on X86_FEATURE_RETPOLINE.
>
> For Skylake we need RSB stuffing even with SMEP to avoid falling back to the
> BTB on underflow.

I am *actively* ignoring Skylake right now. This is about per-SKL
userspace even with SMEP, because we think Intel's document lies to us.

If the RSB only holds the low bits of the target, then a userspace
attacker can populate an RSB entry which points to a kernel gadget of
her choice, even with SMEP or KPTI enabled.