Re: [RFC PATCH 24/41] KVM: x86/pmu: Zero out unexposed Counters/Selectors to avoid information leakage

[Jim Mattson]

On Thu, Apr 11, 2024 at 2:36 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > From: Mingwei Zhang <mizhang@xxxxxxxxxx>
> >
> > Zero out unexposed counters/selectors because even though KVM intercepts
> > all accesses to unexposed PMU MSRs, it does pass through RDPMC instruction
> > which allows guest to read all GP counters and fixed counters. So, zero out
> > unexposed counter values which might contain critical information for the
> > host.
>
> This belongs in the previous patch, it's effectively a bug fix.  I appreciate
> the push for finer granularity, but introducing a blatant bug and then immediately
> fixing it goes too far.
>
> > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/vmx/pmu_intel.c | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > index f79bebe7093d..4b4da7f17895 100644
> > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > @@ -895,11 +895,27 @@ static void intel_restore_pmu_context(struct kvm_vcpu *vcpu)
> >               wrmsrl(MSR_ARCH_PERFMON_EVENTSEL0 + i, pmc->eventsel);
> >       }
> >
> > +     /*
> > +      * Zero out unexposed GP counters/selectors to avoid information leakage
> > +      * since passthrough PMU does not intercept RDPMC.
>
> Zeroing the selectors is unnecessary.  KVM still intercepts MSR_CORE_PERF_GLOBAL_CTRL,
> so just ensure the PMCs that aren't exposed the guest are globally enabled.
>
> > +      */
> > +     for (i = pmu->nr_arch_gp_counters; i < kvm_pmu_cap.num_counters_gp; i++) {
> > +             wrmsrl(MSR_IA32_PMC0 + i, 0);
> > +             wrmsrl(MSR_ARCH_PERFMON_EVENTSEL0 + i, 0);
> > +     }
> > +
> >       wrmsrl(MSR_CORE_PERF_FIXED_CTR_CTRL, pmu->fixed_ctr_ctrl);
> >       for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
> >               pmc = &pmu->fixed_counters[i];
> >               wrmsrl(MSR_CORE_PERF_FIXED_CTR0 + i, pmc->counter);
> >       }
> > +
> > +     /*
> > +      * Zero out unexposed fixed counters to avoid information leakage
> > +      * since passthrough PMU does not intercept RDPMC.
>
> I would call out that RDPMC interception is all or nothing, i.e. KVM can't
> selectively intercept _some_ PMCs, and the MSR bitmaps don't apply to RDPMC.

Yes. RDPMC must be intercepted, unless KVM knows that all possible
PMCs are passthrough. It must also be intercepted if
enable_vmware_backdoor is true.

> > +      */
> > +     for (i = pmu->nr_arch_fixed_counters; i < kvm_pmu_cap.num_counters_fixed; i++)
> > +             wrmsrl(MSR_CORE_PERF_FIXED_CTR0 + i, 0);
> >  }
> >
> >  struct kvm_pmu_ops intel_pmu_ops __initdata = {
> > --
> > 2.34.1
> >