linux-kernel - [PATCH] KVM: x86/vPMU: Forbid writing to MSR_F15H_PERF MSRs when guest doesn't have X86_FEATURE_PERFCTR

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Message-Id: <20210323084515.1346540-1-vkuznets@redhat.com>
Date:   Tue, 23 Mar 2021 09:45:15 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wei Huang <wei.huang2@....com>, Joerg Roedel <joro@...tes.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: x86/vPMU: Forbid writing to MSR_F15H_PERF MSRs when guest doesn't have X86_FEATURE_PERFCTR_CORE

MSR_F15H_PERF_CTL0-5, MSR_F15H_PERF_CTR0-5 MSRs are only available when
X86_FEATURE_PERFCTR_CORE CPUID bit was exposed to the guest. KVM, however,
allows these MSRs unconditionally because kvm_pmu_is_valid_msr() ->
amd_msr_idx_to_pmc() check always passes and because kvm_pmu_set_msr() ->
amd_pmu_set_msr() doesn't fail.

In case of a counter (CTRn), no big harm is done as we only increase
internal PMC's value but in case of an eventsel (CTLn), we go deep into
perf internals with a non-existing counter.

Note, kvm_get_msr_common() just returns '0' when these MSRs don't exist
and this also seems to contradict architectural behavior which is #GP
(I did check one old Opteron host) but changing this status quo is a bit
scarier.

Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
---
 arch/x86/kvm/svm/pmu.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
index 035da07500e8..fdf587f19c5f 100644
--- a/arch/x86/kvm/svm/pmu.c
+++ b/arch/x86/kvm/svm/pmu.c
@@ -98,6 +98,8 @@ static enum index msr_to_index(u32 msr)
 static inline struct kvm_pmc *get_gp_pmc_amd(struct kvm_pmu *pmu, u32 msr,
 					     enum pmu_type type)
 {
+	struct kvm_vcpu *vcpu = pmu_to_vcpu(pmu);
+
 	switch (msr) {
 	case MSR_F15H_PERF_CTL0:
 	case MSR_F15H_PERF_CTL1:
@@ -105,6 +107,9 @@ static inline struct kvm_pmc *get_gp_pmc_amd(struct kvm_pmu *pmu, u32 msr,
 	case MSR_F15H_PERF_CTL3:
 	case MSR_F15H_PERF_CTL4:
 	case MSR_F15H_PERF_CTL5:
+		if (!guest_cpuid_has(vcpu, X86_FEATURE_PERFCTR_CORE))
+			return NULL;
+		fallthrough;
 	case MSR_K7_EVNTSEL0 ... MSR_K7_EVNTSEL3:
 		if (type != PMU_TYPE_EVNTSEL)
 			return NULL;
@@ -115,6 +120,9 @@ static inline struct kvm_pmc *get_gp_pmc_amd(struct kvm_pmu *pmu, u32 msr,
 	case MSR_F15H_PERF_CTR3:
 	case MSR_F15H_PERF_CTR4:
 	case MSR_F15H_PERF_CTR5:
+		if (!guest_cpuid_has(vcpu, X86_FEATURE_PERFCTR_CORE))
+			return NULL;
+		fallthrough;
 	case MSR_K7_PERFCTR0 ... MSR_K7_PERFCTR3:
 		if (type != PMU_TYPE_COUNTER)
 			return NULL;
-- 
2.30.2