linux-kernel - [PATCH 6/9] KVM: x86: Reject userspace attempts to access ARCH

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Message-ID: <20240802185511.305849-7-seanjc@google.com>
Date: Fri,  2 Aug 2024 11:55:08 -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 6/9] KVM: x86: Reject userspace attempts to access
 ARCH_CAPABILITIES w/o support

Reject userspace accesses to ARCH_CAPABILITIES if the MSR isn't supposed
to exist, according to guest CPUID.  However, "reject" accesses with
KVM_MSR_RET_UNSUPPORTED, so that reads get '0' and writes of '0' are
ignored if KVM advertised support ARCH_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 | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index dbb5e06ef264..8bce40c649b4 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3798,8 +3798,9 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			vcpu->arch.microcode_version = data;
 		break;
 	case MSR_IA32_ARCH_CAPABILITIES:
-		if (!msr_info->host_initiated)
-			return 1;
+		if (!msr_info->host_initiated ||
+		    !guest_cpuid_has(vcpu, X86_FEATURE_ARCH_CAPABILITIES))
+			return KVM_MSR_RET_UNSUPPORTED;
 		vcpu->arch.arch_capabilities = data;
 		break;
 	case MSR_IA32_PERF_CAPABILITIES:
@@ -4259,9 +4260,8 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		msr_info->data = vcpu->arch.microcode_version;
 		break;
 	case MSR_IA32_ARCH_CAPABILITIES:
-		if (!msr_info->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_ARCH_CAPABILITIES))
-			return 1;
+		if (!guest_cpuid_has(vcpu, X86_FEATURE_ARCH_CAPABILITIES))
+			return KVM_MSR_RET_UNSUPPORTED;
 		msr_info->data = vcpu->arch.arch_capabilities;
 		break;
 	case MSR_IA32_PERF_CAPABILITIES:
-- 
2.46.0.rc2.264.g509ed76dc8-goog