[PATCH v3 08/21] KVM: x86/pmu: Zero out LBR capabilities during PMU refresh

[Sean Christopherson]

Zero out the LBR capabilities during PMU refresh to avoid exposing LBRs
to the guest against userspace's wishes. If userspace modifies the
guest's CPUID model or invokes KVM_CAP_PMU_CAPABILITY to disable vPMU
after an initial KVM_SET_CPUID2, but before the first KVM_RUN, KVM will
retain the previous LBR info due to bailing before refreshing the LBR
descriptor.

Note, this is a very theoretical bug, there is no known use case where a
VMM would deliberately enable the vPMU via KVM_SET_CPUID2, and then later
disable the vPMU.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/vmx/pmu_intel.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
index e8a3be0b9df9..d889bb2a1de5 100644
--- a/arch/x86/kvm/vmx/pmu_intel.c
+++ b/arch/x86/kvm/vmx/pmu_intel.c
@@ -531,6 +531,16 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
 	pmu->pebs_enable_mask = ~0ull;
 	pmu->pebs_data_cfg_mask = ~0ull;
 
+	memset(&lbr_desc->records, 0, sizeof(lbr_desc->records));
+
+	/*
+	 * Setting passthrough of LBR MSRs is done only in the VM-Entry loop,
+	 * and PMU refresh is disallowed after the vCPU has run, i.e. this code
+	 * should never be reached while KVM is passing through MSRs.
+	 */
+	if (KVM_BUG_ON(lbr_desc->msr_passthrough, vcpu->kvm))
+		return;
+
 	entry = kvm_find_cpuid_entry(vcpu, 0xa);
 	if (!entry || !vcpu->kvm->arch.enable_pmu)
 		return;
-- 
2.40.0.rc1.284.g88254d51c5-goog