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

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Date:   Fri, 12 Jan 2018 07:23:53 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     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>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        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 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.
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