Re: [PATCH 4/5] x86/svm: Direct access to MSR_IA32_SPEC_CTRL

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
Date: Fri, 12 Jan 2018 10:58:11 +0100	[thread overview]
Message-ID: <20180112095811.GQ3040@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1515741833.22302.408.camel@infradead.org>

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.