[PATCH 6/7] KVM: x86: Ignore benign host accesses to "unsupported" PEBS and BTS MSRs

[Sean Christopherson]

Ignore host userspace reads and writes of '0' to PEBS and BTS MSRs that
KVM reports in the MSR-to-save list, but the MSRs are ultimately
unsupported.  All MSRs in said list must be writable by userspace, e.g.
if userspace sends the list back at KVM without filtering out the MSRs it
doesn't need.

8183a538cd95 ("KVM: x86/pmu: Add IA32_DS_AREA MSR emulation to support guest DS")
902caeb6841a ("KVM: x86/pmu: Add PEBS_DATA_CFG MSR emulation to support adaptive PEBS")
c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS")
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 25f471adb8b8..655fb0b3bba4 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3786,6 +3786,16 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		vcpu->arch.guest_fpu.xfd_err = data;
 		break;
 #endif
+	case MSR_IA32_PEBS_ENABLE:
+	case MSR_IA32_DS_AREA:
+	case MSR_PEBS_DATA_CFG:
+		if (kvm_pmu_is_valid_msr(vcpu, msr))
+			return kvm_pmu_set_msr(vcpu, msr_info);
+		/*
+		 * Userspace is allowed to write '0' to MSRs that KVM reports
+		 * as to-be-saved, even if an MSRs isn't fully supported.
+		 */
+		return !msr_info->host_initiated || data;
 	default:
 		if (kvm_pmu_is_valid_msr(vcpu, msr))
 			return kvm_pmu_set_msr(vcpu, msr_info);
@@ -4122,6 +4132,16 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		msr_info->data = vcpu->arch.guest_fpu.xfd_err;
 		break;
 #endif
+	case MSR_IA32_PEBS_ENABLE:
+	case MSR_IA32_DS_AREA:
+	case MSR_PEBS_DATA_CFG:
+		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
+			return kvm_pmu_get_msr(vcpu, msr_info);
+		/*
+		 * Userspace is allowed to read MSRs that KVM reports as
+		 * to-be-saved, even if an MSR isn't fully supported.
+		 */
+		return !msr_info->host_initiated;
 	default:
 		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
 			return kvm_pmu_get_msr(vcpu, msr_info);
-- 
2.36.1.476.g0c4daa206d-goog