KVM: nVMX: Get to-be-acknowledge IRQ for nested VM-Exit at injection site

Move the logic to get the to-be-acknowledge IRQ for a nested VM-Exit from
nested_vmx_vmexit() to vmx_check_nested_events(), which is subtly the one
and only path where KVM invokes nested_vmx_vmexit() with
EXIT_REASON_EXTERNAL_INTERRUPT.  A future fix will perform a last-minute
check on L2's nested posted interrupt notification vector, just before
injecting a nested VM-Exit.  To handle that scenario correctly, KVM needs
to get the interrupt _before_ injecting VM-Exit, as simply querying the
highest priority interrupt, via kvm_cpu_has_interrupt(), would result in
TOCTOU bug, as a new, higher priority interrupt could arrive between
kvm_cpu_has_interrupt() and kvm_cpu_get_interrupt().

Unfortunately, simply moving the call to kvm_cpu_get_interrupt() doesn't
suffice, as a VMWRITE to GUEST_INTERRUPT_STATUS.SVI is hiding in
kvm_get_apic_interrupt(), and acknowledging the interrupt before nested
VM-Exit would cause the VMWRITE to hit vmcs02 instead of vmcs01.

Open code a rough equivalent to kvm_cpu_get_interrupt() so that the IRQ
is acknowledged after emulating VM-Exit, taking care to avoid the TOCTOU
issue described above.

Opportunistically convert the WARN_ON() to a WARN_ON_ONCE().  If KVM has
a bug that results in a false positive from kvm_cpu_has_interrupt(),
spamming dmesg won't help the situation.

Note, nested_vmx_reflect_vmexit() can never reflect external interrupts as
they are always "wanted" by L0.

Link: https://lore.kernel.org/r/20240906043413.1049633-3-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/include/asm/kvm_host.h

@@ -2256,6 +2256,7 @@ int kvm_cpu_has_injectable_intr(struct kvm_vcpu *v);
 int kvm_cpu_has_interrupt(struct kvm_vcpu *vcpu);
 int kvm_cpu_has_extint(struct kvm_vcpu *v);
 int kvm_arch_interrupt_allowed(struct kvm_vcpu *vcpu);
+int kvm_cpu_get_extint(struct kvm_vcpu *v);
 int kvm_cpu_get_interrupt(struct kvm_vcpu *v);
 void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event);
 

arch/x86/kvm/irq.c

@@ -108,7 +108,7 @@ EXPORT_SYMBOL_GPL(kvm_cpu_has_interrupt);
  * Read pending interrupt(from non-APIC source)
  * vector and intack.
  */
-static int kvm_cpu_get_extint(struct kvm_vcpu *v)
+int kvm_cpu_get_extint(struct kvm_vcpu *v)
 {
 	if (!kvm_cpu_has_extint(v)) {
 		WARN_ON(!lapic_in_kernel(v));
@@ -131,6 +131,7 @@ static int kvm_cpu_get_extint(struct kvm_vcpu *v)
 	} else
 		return kvm_pic_read_irq(v->kvm); /* PIC */
 }
+EXPORT_SYMBOL_GPL(kvm_cpu_get_extint);
 
 /*
  * Read pending interrupt vector and intack.

arch/x86/kvm/vmx/nested.c

@@ -4285,14 +4285,40 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
 	}
 
 	if (kvm_cpu_has_interrupt(vcpu) && !vmx_interrupt_blocked(vcpu)) {
+		int irq;
+
 		if (block_nested_events)
 			return -EBUSY;
 		if (!nested_exit_on_intr(vcpu))
 			goto no_vmexit;
+
+		if (!nested_exit_intr_ack_set(vcpu)) {
 			nested_vmx_vmexit(vcpu, EXIT_REASON_EXTERNAL_INTERRUPT, 0, 0);
 			return 0;
 		}
 
+		irq = kvm_cpu_get_extint(vcpu);
+		if (irq != -1) {
+			nested_vmx_vmexit(vcpu, EXIT_REASON_EXTERNAL_INTERRUPT,
+					  INTR_INFO_VALID_MASK | INTR_TYPE_EXT_INTR | irq, 0);
+			return 0;
+		}
+
+		irq = kvm_apic_has_interrupt(vcpu);
+		WARN_ON_ONCE(irq < 0);
+
+		nested_vmx_vmexit(vcpu, EXIT_REASON_EXTERNAL_INTERRUPT,
+				  INTR_INFO_VALID_MASK | INTR_TYPE_EXT_INTR | irq, 0);
+
+		/*
+		 * ACK the interrupt _after_ emulating VM-Exit, as the IRQ must
+		 * be marked as in-service in vmcs01.GUEST_INTERRUPT_STATUS.SVI
+		 * if APICv is active.
+		 */
+		kvm_apic_ack_interrupt(vcpu, irq);
+		return 0;
+	}
+
 no_vmexit:
 	return vmx_complete_nested_posted_interrupt(vcpu);
 }
@@ -4970,14 +4996,6 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 	vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
 
 	if (likely(!vmx->fail)) {
-		if ((u16)vm_exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT &&
-		    nested_exit_intr_ack_set(vcpu)) {
-			int irq = kvm_cpu_get_interrupt(vcpu);
-			WARN_ON(irq < 0);
-			vmcs12->vm_exit_intr_info = irq |
-				INTR_INFO_VALID_MASK | INTR_TYPE_EXT_INTR;
-		}
-
 		if (vm_exit_reason != -1)
 			trace_kvm_nested_vmexit_inject(vmcs12->vm_exit_reason,
 						       vmcs12->exit_qualification,