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).

Link: https://lore.kernel.org/r/20240802185511.305849-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -3795,8 +3795,9 @@ 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:
@@ -4256,9 +4257,8 @@ 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: