linux-kernel - [PATCH 4/9] KVM: x86: Reject userspace attempts to access PERF

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Message-ID: <20240802185511.305849-5-seanjc@google.com>
Date: Fri,  2 Aug 2024 11:55:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 4/9] KVM: x86: Reject userspace attempts to access
 PERF_CAPABILITIES w/o PDCM

Reject userspace accesses to PERF_CAPABILITIES if PDCM isn't set in guest
CPUID, i.e. if the vCPU doesn't actually have PERF_CAPABILITIES.  But!  Do
so via KVM_MSR_RET_UNSUPPORTED, so that reads get '0' and writes of '0'
are ignored if KVM advertised support PERF_CAPABILITIES.

KVM's ABI is that userspace must set guest CPUID prior to setting MSRs,
and that setting MSRs that aren't supposed exist is disallowed (modulo the
'0' exemption).

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9b52d8f3304f..dbb5e06ef264 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3803,8 +3803,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		vcpu->arch.arch_capabilities = data;
 		break;
 	case MSR_IA32_PERF_CAPABILITIES:
-		if (!msr_info->host_initiated)
-			return 1;
+		if (!msr_info->host_initiated ||
+		    !guest_cpuid_has(vcpu, X86_FEATURE_PDCM))
+			return KVM_MSR_RET_UNSUPPORTED;
+
 		if (data & ~kvm_caps.supported_perf_cap)
 			return 1;
 
@@ -4263,9 +4265,8 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		msr_info->data = vcpu->arch.arch_capabilities;
 		break;
 	case MSR_IA32_PERF_CAPABILITIES:
-		if (!msr_info->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_PDCM))
-			return 1;
+		if (!guest_cpuid_has(vcpu, X86_FEATURE_PDCM))
+			return KVM_MSR_RET_UNSUPPORTED;
 		msr_info->data = vcpu->arch.perf_capabilities;
 		break;
 	case MSR_IA32_POWER_CTL:
-- 
2.46.0.rc2.264.g509ed76dc8-goog