KVM: x86: Mask off reserved bit from #DB exception payload

KVM defines the #DB payload as compatible with the 'pending debug
exceptions' field under VMX, not DR6. Mask off bit 12 when applying the
payload to DR6, as it is reserved on DR6 but not the 'pending debug
exceptions' field.

Fixes: f10c729f ("kvm: vmx: Defer setting of DR6 until #DB delivery")
Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -438,6 +438,14 @@ void kvm_deliver_exception_payload(struct kvm_vcpu *vcpu)
 		 * for #DB exceptions under VMX.
 		 */
 		vcpu->arch.dr6 ^= payload & DR6_RTM;
+
+		/*
+		 * The #DB payload is defined as compatible with the 'pending
+		 * debug exceptions' field under VMX, not DR6. While bit 12 is
+		 * defined in the 'pending debug exceptions' field (enabled
+		 * breakpoint), it is reserved and must be zero in DR6.
+		 */
+		vcpu->arch.dr6 &= ~BIT(12);
 		break;
 	case PF_VECTOR:
 		vcpu->arch.cr2 = payload;