KVM: arm64: vgic-its: Handle errors from vgic_add_lpi

During low memory conditions, we could be dereferencing a NULL pointer
when vgic_add_lpi fails to allocate memory.

Consider for example this call sequence:

  vgic_its_cmd_handle_mapi
      itte->irq = vgic_add_lpi(kvm, lpi_nr);
          update_lpi_config(kvm, itte->irq, NULL);
              ret = kvm_read_guest(kvm, propbase + irq->intid
	                                             ^^^^
						     kaboom?

Instead, return an error pointer from vgic_add_lpi and check the return
value from its single caller.
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>

virt/kvm/arm/vgic/vgic-its.c

@@ -51,7 +51,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid)
 
 	irq = kzalloc(sizeof(struct vgic_irq), GFP_KERNEL);
 	if (!irq)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	INIT_LIST_HEAD(&irq->lpi_list);
 	INIT_LIST_HEAD(&irq->ap_list);
@@ -522,7 +522,8 @@ static void its_free_itte(struct kvm *kvm, struct its_itte *itte)
 	list_del(&itte->itte_list);
 
 	/* This put matches the get in vgic_add_lpi. */
-	vgic_put_irq(kvm, itte->irq);
+	if (itte->irq)
+		vgic_put_irq(kvm, itte->irq);
 
 	kfree(itte);
 }
@@ -713,10 +714,11 @@ static int vgic_its_cmd_handle_mapi(struct kvm *kvm, struct vgic_its *its,
 	u32 device_id = its_cmd_get_deviceid(its_cmd);
 	u32 event_id = its_cmd_get_id(its_cmd);
 	u32 coll_id = its_cmd_get_collection(its_cmd);
-	struct its_itte *itte;
+	struct its_itte *itte, *new_itte = NULL;
 	struct its_device *device;
 	struct its_collection *collection, *new_coll = NULL;
 	int lpi_nr;
+	struct vgic_irq *irq;
 
 	device = find_its_device(its, device_id);
 	if (!device)
@@ -747,13 +749,24 @@ static int vgic_its_cmd_handle_mapi(struct kvm *kvm, struct vgic_its *its,
 			return -ENOMEM;
 		}
 
+		new_itte = itte;
 		itte->event_id	= event_id;
 		list_add_tail(&itte->itte_list, &device->itt_head);
 	}
 
 	itte->collection = collection;
 	itte->lpi = lpi_nr;
-	itte->irq = vgic_add_lpi(kvm, lpi_nr);
+
+	irq = vgic_add_lpi(kvm, lpi_nr);
+	if (IS_ERR(irq)) {
+		if (new_coll)
+			vgic_its_free_collection(its, coll_id);
+		if (new_itte)
+			its_free_itte(kvm, new_itte);
+		return PTR_ERR(irq);
+	}
+	itte->irq = irq;
+
 	update_affinity_itte(kvm, itte);
 
 	/*