KVM: VMX: Fix interrupt loss during race with NMI

If an interrupt cannot be injected for some reason (say, page fault
when fetching the IDT descriptor), the interrupt is marked for
reinjection.  However, if an NMI is queued at this time, the NMI
will be injected instead and the NMI will be lost.

Fix by deferring the NMI injection until the interrupt has been
injected successfully.

Analyzed by Jan Kiszka.
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/vmx.c

@@ -3149,7 +3149,9 @@ static void vmx_intr_assist(struct kvm_vcpu *vcpu)
 
 	if (cpu_has_virtual_nmis()) {
 		if (vcpu->arch.nmi_pending && !vcpu->arch.nmi_injected) {
-			if (vmx_nmi_enabled(vcpu)) {
+			if (vcpu->arch.interrupt.pending) {
+				enable_nmi_window(vcpu);
+			} else if (vmx_nmi_enabled(vcpu)) {
 				vcpu->arch.nmi_pending = false;
 				vcpu->arch.nmi_injected = true;
 			} else {