xen-netback: avoid race that can lead to NULL pointer dereference

In function xenvif_disconnect_queue(), the value of queue->rx_irq is
zeroed *before* queue->task is stopped. Unfortunately that task may call
notify_remote_via_irq(queue->rx_irq) and calling that function with a
zero value results in a NULL pointer dereference in evtchn_from_irq().

This patch simply re-orders things, stopping all tasks before zero-ing the
irq values, thereby avoiding the possibility of the race.

Fixes: 2ac061ce ("xen/netback: cleanup init and deinit code")
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

drivers/net/xen-netback/interface.c

@@ -628,18 +628,6 @@ int xenvif_connect_ctrl(struct xenvif *vif, grant_ref_t ring_ref,
 
 static void xenvif_disconnect_queue(struct xenvif_queue *queue)
 {
-	if (queue->tx_irq) {
-		unbind_from_irqhandler(queue->tx_irq, queue);
-		if (queue->tx_irq == queue->rx_irq)
-			queue->rx_irq = 0;
-		queue->tx_irq = 0;
-	}
-
-	if (queue->rx_irq) {
-		unbind_from_irqhandler(queue->rx_irq, queue);
-		queue->rx_irq = 0;
-	}
-
 	if (queue->task) {
 		kthread_stop(queue->task);
 		queue->task = NULL;
@@ -655,6 +643,18 @@ static void xenvif_disconnect_queue(struct xenvif_queue *queue)
 		queue->napi.poll = NULL;
 	}
 
+	if (queue->tx_irq) {
+		unbind_from_irqhandler(queue->tx_irq, queue);
+		if (queue->tx_irq == queue->rx_irq)
+			queue->rx_irq = 0;
+		queue->tx_irq = 0;
+	}
+
+	if (queue->rx_irq) {
+		unbind_from_irqhandler(queue->rx_irq, queue);
+		queue->rx_irq = 0;
+	}
+
 	xenvif_unmap_frontend_data_rings(queue);
 }