Date: Fri, 12 Jan 2018 10:58:11 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Ashok Raj <ashok.raj@intel.com>, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>
Subject: Re: [PATCH 4/5] x86/svm: Direct access to MSR_IA32_SPEC_CTRL
Message-ID: <20180112095811.GQ3040@hirez.programming.kicks-ass.net>
References: <1515720739-43819-1-git-send-email-ashok.raj@intel.com>
 <1515720739-43819-5-git-send-email-ashok.raj@intel.com>
 <1515741833.22302.408.camel@infradead.org>
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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.