KVM: ARM: vgic: plug irq injection race

As it stands, nothing prevents userspace from injecting an interrupt before the guest's GIC is actually initialized.

This goes unnoticed so far (as everything is pretty much statically allocated), but ends up exploding in a spectacular way once we switch to a more dynamic allocation (the GIC data structure isn't there yet).

The fix is to test for the "ready" flag in the VGIC distributor before trying to inject the interrupt. Note that in order to avoid breaking userspace, we have to ignore what is essentially an error.

Also, move the setting of the flag out of the critical section, which will ensure the visibility of the initialized data-structure before the flag is actually set.

---
 virt/kvm/arm/vgic.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

index f29761b..4850e87 100644
@@ -1431,7 +1431,8 @@ out:
 int kvm_vgic_inject_irq(struct kvm *kvm, int cpuid, unsigned int irq_num,
 			bool level)
 {
-	if (vgic_update_irq_state(kvm, cpuid, irq_num, level))
+	if (likely(vgic_initialized(kvm)) &&
+	    vgic_update_irq_state(kvm, cpuid, irq_num, level))
 		vgic_kick_vcpus(kvm);
 
 	return 0;

@@ -1581,9 +1582,10 @@ int kvm_vgic_init(struct kvm *kvm)
 	for (i = VGIC_NR_PRIVATE_IRQS; i < VGIC_NR_IRQS; i += 4)
 		vgic_set_target_reg(kvm, 0, i);
 
-	kvm->arch.vgic.ready = true;
 out:
 	mutex_unlock(&kvm->lock);
+	if (!ret)
+		kvm->arch.vgic.ready = true;
 	return ret;
 }
 

1.8.3.4