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.

Cc: stable@vger.kernel.org
Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://lore.kernel.org/r/20240720000138.3027780-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/nested.c

@@ -4296,7 +4296,8 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
 			int irq;
 
 			irq = kvm_cpu_get_interrupt(vcpu);
-			WARN_ON_ONCE(irq < 0);
+			if (WARN_ON_ONCE(irq < 0))
+				goto no_vmexit;
 
 			exit_intr_info = INTR_INFO_VALID_MASK | INTR_TYPE_EXT_INTR | irq;
 		} else {