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

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Date:   Tue, 13 Feb 2018 08:58:24 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Mingarelli <thomas.mingarelli@....com>, hpa@...or.com,
        tglx@...utronix.de, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Dave Hansen <dave@...1.net>, Borislav Petkov <bp@...en8.de>,
        Arjan van de Ven <arjan@...radead.org>
Subject: Re: [tip:x86/pti] x86/speculation: Use IBRS if available before
 calling into firmware


* David Woodhouse <dwmw2@...radead.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