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

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Message-ID: <YJHCadSIQ/cK/RAw@google.com>
Date:   Tue, 4 May 2021 21:53:45 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...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 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?