linux-kernel - Re: [PATCH 4/5] x86/svm: Direct access to MSR_IA32_SPEC

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Date:   Fri, 12 Jan 2018 10:58:11 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Ashok Raj <ashok.raj@...el.com>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>
Subject: Re: [PATCH 4/5] x86/svm: Direct access to MSR_IA32_SPEC_CTRL

On Fri, Jan 12, 2018 at 07:23:53AM +0000, David Woodhouse wrote:
> On Thu, 2018-01-11 at 17:32 -0800, Ashok Raj wrote:
> > 
> > @@ -4910,6 +4935,14 @@ static void svm_vcpu_run(struct kvm_vcpu
> > *vcpu)
> >  
> >         clgi();
> >  
> > +       if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) {
> > +               /*
> > +                * FIXME: lockdep_assert_irqs_disabled();
> > +                */
> > +               WARN_ON_ONCE(!irqs_disabled());
> > +               spec_ctrl_set(svm->spec_ctrl);
> > +       }
> > +
> >         local_irq_enable();
> >  
> 
> Same comments here as we've had previously. If you do this without an
> 'else lfence' then you need a comment showing that you've proved it's
> safe.
> 
> And I don't think even using static_cpu_has() is good enough. We don't
> already "rely" on that for anything but optimisations, AFAICT. Turning
> a missed GCC optimisation into a security hole is not a good idea.

I disagree, and if you worry about that, we should write a testcase. But
we rely on GCC for correct code generation in lots of places, this isn't
different.