KVM: nVMX: Suppress external interrupt VM-Exit injection if there's no IRQ

In the should-be-impossible scenario that kvm_cpu_get_interrupt() doesn't
return a valid vector after checking kvm_cpu_has_interrupt(), skip VM-Exit
injection to reduce the probability of crashing/confusing L1.  Now that
KVM gets the IRQ _before_ calling nested_vmx_vmexit(), squashing the
VM-Exit injection is trivial since there are no actions that need to be
undone.

Reviewed-by: Chao Gao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: sean-jc

arch/x86/kvm/vmx/nested.c

@@ -4305,7 +4305,8 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
 		}
 
 		irq = kvm_apic_has_interrupt(vcpu);
-		WARN_ON_ONCE(irq < 0);
+		if (WARN_ON_ONCE(irq < 0))
+			goto no_vmexit;
 
 		nested_vmx_vmexit(vcpu, EXIT_REASON_EXTERNAL_INTERRUPT,
 				  INTR_INFO_VALID_MASK | INTR_TYPE_EXT_INTR | irq, 0);