[PATCH 2/3] KVM: x86: Reject writes to PERF_CAPABILITIES feature MSR after KVM_RUN

[Like Xu]

From: Like Xu <likexu@xxxxxxxxxxx>

KVM may do the "wrong" thing if userspace changes PERF_CAPABILITIES
after running the vCPU, i.e. after KVM_RUN. Similar to disallowing CPUID
changes after KVM_RUN, KVM should also disallow changing the feature MSRs
(conservatively starting from PERF_CAPABILITIES) after KVM_RUN to prevent
unexpected behavior.

Applying the same logic to most feature msrs in do_set_msr() may
reduce the flexibility (one odd but reasonable user space may want
per-vcpu control, feature by feature) and also increases the overhead.

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 33560bfa0cac..3fb933bfb3bd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3540,6 +3540,9 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 
 		if (!msr_info->host_initiated)
 			return 1;
+		if (vcpu->arch.last_vmentry_cpu != -1 &&
+		    vcpu->arch.perf_capabilities != data)
+			return 1;
 		if (kvm_get_msr_feature(&msr_ent))
 			return 1;
 		if (data & ~msr_ent.data)
-- 
2.37.1