Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware

Ingo Molnar
2018-02-13T07:58:30Z

* David Woodhouse <dwmw2@infradead.org> wrote:

> On Mon, 2018-02-12 at 12:50 +0100, Peter Zijlstra wrote:
> > On Mon, Feb 12, 2018 at 11:22:11AM +0100, Ingo Molnar wrote:
> > > > +static inline void firmware_restrict_branch_speculation_start(void)
> > > > +{
> > > > +   alternative_msr_write(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS,
> > > > +                         X86_FEATURE_USE_IBRS_FW);
> > > > +}
> > > > +
> > > > +static inline void firmware_restrict_branch_speculation_end(void)
> > > > +{
> > > > +   alternative_msr_write(MSR_IA32_SPEC_CTRL, 0,
> > > > +                         X86_FEATURE_USE_IBRS_FW);
> > >
> > > BTW., there's a detail that only occurred to me today, this enabling/disabling
> > > sequence is not NMI safe, and it might be called from NMI context:
> >
> > Wait, we're doing firmware from NMI? That sounds like a _REALLY_ bad
> > idea.
>
> And spin_lock_irqsave() too. Which is probably why I missed the fact
> that this was being called in NMI context.
>
> Yay for HP and their persistent attempts to "value subtract" in their
> firmware offerings.
>
> I'm tempted to drop that part of the patch and declare that if you're
> using this driver, the potential for stray branch prediction when you
> call into the firmware from the NMI handler is the *least* of your
> problems.
>
> I *will* go back over the other parts of the patch and audit them for
> preempt safety though; there could potentially be a similar issue
> there. I think I put them close enough to the actual firmware calls
> that if we aren't already preempt-safe then we were screwed anyway, but
> *maybe* there's merit in making the macros explicitly bump the preempt
> count anyway.

Ok, meanwhile I'm removing this patch from the x86/pti branch, and since the
branch has to be rebased anyway, I'll merge these into a single patch:

85d8426e0720: x86/speculation: Correct Speculation Control microcode blacklist again
1751342095f0: x86/speculation: Update Speculation Control microcode blacklist

Thanks,

Ingo