Re: [PATCH] KVM: x86: Fix array_index_nospec() protection in kvm_vcpu_ioctl_x86_set_mce()

["Carlos López" <clopez@suse.de>]

On 6/2/26 9:56 PM, Carlos López wrote:
> ...
> * Non-UCNA error logging is broken, at least on the count that logging
>   is gated based on MCi_CTL, when it should not. The spec is ambiguous
>   on whether MCG_CTL controls logging for these. Current behavior in KVM
>   is to *do* the gating based on MCG_CTL; the spec suggests that this
>   may be correct at least on AMD (but it is not symmetric with MCi_CTL
>   behavior).

So something like this?

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c9e40f2a5996..259dba2a3299 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5505,24 +5505,31 @@ static int kvm_vcpu_ioctl_x86_set_mce(struct kvm_vcpu *vcpu,
 	if ((mce->status & MCI_STATUS_UC) && (mcg_cap & MCG_CTL_P) &&
 	    vcpu->arch.mcg_ctl != ~(u64)0)
 		return 0;
-	/*
-	 * if IA32_MCi_CTL is not all 1s, the uncorrected error
-	 * reporting is disabled for the bank
-	 */
-	if ((mce->status & MCI_STATUS_UC) && banks[0] != ~(u64)0)
-		return 0;
+
 	if (mce->status & MCI_STATUS_UC) {
+		/*
+		 * Per the SDM, error logging always happens independently of
+		 * IA32_MCi_CTL
+		 */
+		if (banks[1] & MCI_STATUS_VAL)
+			mce->status |= MCI_STATUS_OVER;
+		banks[2] = mce->addr;
+		banks[3] = mce->misc;
+		banks[1] = mce->status;
+
+		/*
+		 * if IA32_MCi_CTL is not all 1s, the uncorrected error
+		 * reporting is disabled for the bank
+		 */
+		if (banks[0] != ~(u64)0)
+			return 0;
+
 		if ((vcpu->arch.mcg_status & MCG_STATUS_MCIP) ||
 		    !kvm_is_cr4_bit_set(vcpu, X86_CR4_MCE)) {
 			kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
 			return 0;
 		}
-		if (banks[1] & MCI_STATUS_VAL)
-			mce->status |= MCI_STATUS_OVER;
-		banks[2] = mce->addr;
-		banks[3] = mce->misc;
 		vcpu->arch.mcg_status = mce->mcg_status;
-		banks[1] = mce->status;
 		kvm_queue_exception(vcpu, MC_VECTOR);
 	} else if (!(banks[1] & MCI_STATUS_VAL)
 		   || !(banks[1] & MCI_STATUS_UC)) {