KVM: x86: Protect MSR-based index computations in pmu.h from Spectre-v1/L1TF attacks

This fixes a Spectre-v1/L1TF vulnerability in the get_gp_pmc() and
get_fixed_pmc() functions.
They both contain index computations based on the (attacker-controlled)
MSR number.

Fixes: 25462f7f ("KVM: x86/vPMU: Define kvm_pmu_ops to support vPMU function dispatch")
Signed-off-by: Nick Finco <nifi@google.com>
Signed-off-by: Marios Pomonis <pomonis@google.com>
Reviewed-by: Andrew Honig <ahonig@google.com>
Cc: stable@vger.kernel.org
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/pmu.h

@@ -2,6 +2,8 @@
 #ifndef __KVM_X86_PMU_H
 #define __KVM_X86_PMU_H
 
+#include <linux/nospec.h>
+
 #define vcpu_to_pmu(vcpu) (&(vcpu)->arch.pmu)
 #define pmu_to_vcpu(pmu)  (container_of((pmu), struct kvm_vcpu, arch.pmu))
 #define pmc_to_pmu(pmc)   (&(pmc)->vcpu->arch.pmu)
@@ -102,8 +104,12 @@ static inline bool kvm_valid_perf_global_ctrl(struct kvm_pmu *pmu,
 static inline struct kvm_pmc *get_gp_pmc(struct kvm_pmu *pmu, u32 msr,
 					 u32 base)
 {
-	if (msr >= base && msr < base + pmu->nr_arch_gp_counters)
-		return &pmu->gp_counters[msr - base];
+	if (msr >= base && msr < base + pmu->nr_arch_gp_counters) {
+		u32 index = array_index_nospec(msr - base,
+					       pmu->nr_arch_gp_counters);
+
+		return &pmu->gp_counters[index];
+	}
 
 	return NULL;
 }
@@ -113,8 +119,12 @@ static inline struct kvm_pmc *get_fixed_pmc(struct kvm_pmu *pmu, u32 msr)
 {
 	int base = MSR_CORE_PERF_FIXED_CTR0;
 
-	if (msr >= base && msr < base + pmu->nr_arch_fixed_counters)
-		return &pmu->fixed_counters[msr - base];
+	if (msr >= base && msr < base + pmu->nr_arch_fixed_counters) {
+		u32 index = array_index_nospec(msr - base,
+					       pmu->nr_arch_fixed_counters);
+
+		return &pmu->fixed_counters[index];
+	}
 
 	return NULL;
 }