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Date: Wed, 16 Nov 2016 15:42:15 -0500 (EST)
From: Paolo Bonzini <pbonzini@redhat.com>
To: Kyle Huey <me@kylehuey.com>
Cc: Nadav Amit <nadav.amit@gmail.com>,
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Subject: Re: [PATCH v11 7/7] KVM: x86: virtualize cpuid faulting
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> On Fri, Nov 11, 2016 at 12:54 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
> >
> >> On Nov 10, 2016, at 3:40 PM, Kyle Huey <me@kylehuey.com> wrote:
> >>
> >> Hardware support for faulting on the cpuid instruction is not required to
> >> emulate it, because cpuid triggers a VM exit anyways. KVM handles the
> >> relevant
> >> MSRs (MSR_PLATFORM_INFO and MSR_MISC_FEATURES_ENABLE) and upon a
> >> cpuid-induced VM exit checks the cpuid faulting state and the CPL.
> >> kvm_require_cpl is even kind enough to inject the GP fault for us.
> >
> > I think that for consistency you should also revise em_cpuid() of KVM
> > instruction emulator. Otherwise it may be a potential security
> > hazard (as far fetched as it currently seems).
> 
> Hmm, ok.  Do you know how I can test this code path?

Try this patch (untested!)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 5382b82462fc..06d940a7c9fa 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -5868,8 +5868,7 @@ static void vmx_set_dr7(struct kvm_vcpu *vcpu, unsigned long val)
 
 static int handle_cpuid(struct kvm_vcpu *vcpu)
 {
-	kvm_emulate_cpuid(vcpu);
-	return 1;
+	return emulate_instruction(vcpu, 0) == EMULATE_DONE;
 }
 
 static int handle_rdmsr(struct kvm_vcpu *vcpu)

Paolo