linux-kernel - Re: [PATCH 03/15] KVM: SVM: Inject #UD on RDTSCP when it should be disabled in the guest

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Date:   Tue, 4 May 2021 14:56:40 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Reiji Watanabe <reijiw@...gle.com>
Subject: Re: [PATCH 03/15] KVM: SVM: Inject #UD on RDTSCP when it should be
 disabled in the guest

On Tue, May 4, 2021 at 2:53 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, May 04, 2021, Jim Mattson wrote:
> > On Tue, May 4, 2021 at 10:17 AM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > Intercept RDTSCP to inject #UD if RDTSC is disabled in the guest.
> > >
> > > Note, SVM does not support intercepting RDPID.  Unlike VMX's
> > > ENABLE_RDTSCP control, RDTSCP interception does not apply to RDPID.  This
> > > is a benign virtualization hole as the host kernel (incorrectly) sets
> > > MSR_TSC_AUX if RDTSCP is supported, and KVM loads the guest's MSR_TSC_AUX
> > > into hardware if RDTSCP is supported in the host, i.e. KVM will not leak
> > > the host's MSR_TSC_AUX to the guest.
> > >
> > > But, when the kernel bug is fixed, KVM will start leaking the host's
> > > MSR_TSC_AUX if RDPID is supported in hardware, but RDTSCP isn't available
> > > for whatever reason.  This leak will be remedied in a future commit.
> > >
> > > Fixes: 46896c73c1a4 ("KVM: svm: add support for RDTSCP")
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > ---
> > ...
> > > @@ -4007,8 +4017,7 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > >         svm->nrips_enabled = kvm_cpu_cap_has(X86_FEATURE_NRIPS) &&
> > >                              guest_cpuid_has(vcpu, X86_FEATURE_NRIPS);
> > >
> > > -       /* Check again if INVPCID interception if required */
> > > -       svm_check_invpcid(svm);
> > > +       svm_recalc_instruction_intercepts(vcpu, svm);
> >
> > Does the right thing happen here if the vCPU is in guest mode when
> > userspace decides to toggle the CPUID.80000001H:EDX.RDTSCP bit on or
> > off?
>
> I hate our terminology.  By "guest mode", do you mean running the vCPU, or do
> you specifically mean running in L2?

I mean is_guest_mode(vcpu) is true (i.e. running L2).