Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

[David Woodhouse]

On Fri, 2018-01-26 at 09:19 -0800, Linus Torvalds wrote:
> On Fri, Jan 26, 2018 at 1:11 AM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> 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.

That's the vmexit one. The one on context switch is in
commitÂc995efd5a7 and has its own X86_FEATURE_RSB_CTXSW which in
kernel/cpu/bugs.c is turned on for (!SMEP || Skylake).

The "low bits of the BTB" issue probably means that wants to be
X86_FEATURE_RETPOLINE too. Despite Intel's doc saying otherwise.

(Intel's doc also says to do it on kernel entry, but we elected to do
it on context switch instead since *that's* when the imbalances show up
in the RSB.)

Attachment: smime.p7s
Description: S/MIME cryptographic signature