On Fri, Jan 26, 2018 at 09:59:01AM -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 <[email protected]>
> > 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.
>
> It's also always needed with virtualization.
-ECONFUSED, see ==>
Is this incorrect then?
I see:
241 * Skylake era CPUs have a separate issue with *underflow* of the
242 * RSB, when they will predict 'ret' targets from the generic BTB.
243 * The proper mitigation for this is IBRS. If IBRS is not supported
244 * or deactivated in favour of retpolines the RSB fill on context
245 * switch is required.
246 */
which came from this:
commit c995efd5a740d9cbafbf58bde4973e8b50b4d761
Author: David Woodhouse <[email protected]>
Date: Fri Jan 12 17:49:25 2018 +0000
x86/retpoline: Fill RSB on context switch for affected CPUs
On context switch from a shallow call stack to a deeper one, as the CPU
does 'ret' up the deeper side it may encounter RSB entries (predictions for
where the 'ret' goes to) which were populated in userspace.
This is problematic if neither SMEP nor KPTI (the latter of which marks
userspace pages as NX for the kernel) are active, as malicious code in
userspace may then be executed speculatively.
Overwrite the CPU's return prediction stack with calls which are predicted
to return to an infinite loop, to "capture" speculation if this
happens. This is required both for retpoline, and also in conjunction with
IBRS for !SMEP && !KPTI.
On Skylake+ the problem is slightly different, and an *underflow* of the
RSB may cause errant branch predictions to occur. So there it's not so much
overwrite, as *filling* the RSB to attempt to prevent it getting
empty. This is only a partial solution for Skylake+ since there are many
==>other conditions which may result in the RSB becoming empty. The full
<==
==>solution on Skylake+ is to use IBRS, which will prevent the problem even
<==
when the RSB becomes empty. With IBRS, the RSB-stuffing will not be
required on context switch.
Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Arjan van de Ven <[email protected]>
The "full solution" is what is making me confused.
>
> -Andi
