xen-netback: take a reference to the RX task thread

Do this in order to prevent the task from being freed if the thread
returns (which can be triggered by the frontend) before the call to
kthread_stop done as part of the backend tear down. Not taking the
reference will lead to a use-after-free in that scenario. Such
reference was taken before but dropped as part of the rework done in
2ac061ce.

Reintroduce the reference taking and add a comment this time
explaining why it's needed.

This is XSA-374 / CVE-2021-28691.

Fixes: 2ac061ce ('xen/netback: cleanup init and deinit code')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: stable@vger.kernel.org
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/net/xen-netback/interface.c

@@ -684,6 +684,7 @@ static void xenvif_disconnect_queue(struct xenvif_queue *queue)
 {
 	if (queue->task) {
 		kthread_stop(queue->task);
+		put_task_struct(queue->task);
 		queue->task = NULL;
 	}
 
@@ -745,6 +746,11 @@ int xenvif_connect_data(struct xenvif_queue *queue,
 	if (IS_ERR(task))
 		goto kthread_err;
 	queue->task = task;
+	/*
+	 * Take a reference to the task in order to prevent it from being freed
+	 * if the thread function returns before kthread_stop is called.
+	 */
+	get_task_struct(task);
 
 	task = kthread_run(xenvif_dealloc_kthread, queue,
 			   "%s-dealloc", queue->name);