FAILED: patch "[PATCH] xen-blkback: set ring->xenblkd to NULL after kthread_stop()" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 1c728719a4da6e654afb9cc047164755072ed7c9 Mon Sep 17 00:00:00 2001
From: Pawel Wieczorkiewicz <[email protected]>
Date: Mon, 14 Dec 2020 10:25:57 +0100
Subject: [PATCH] xen-blkback: set ring->xenblkd to NULL after kthread_stop()

When xen_blkif_disconnect() is called, the kernel thread behind the block interface is stopped by calling kthread_stop(ring->xenblkd). The ring->xenblkd thread pointer being non-NULL determines if the thread has been already stopped.
Normally, the thread's function xen_blkif_schedule() sets the ring->xenblkd to NULL, when the thread's main loop ends.

However, when the thread has not been started yet (i.e. wake_up_process() has not been called on it), the xen_blkif_schedule() function would not be called yet.

In such case the kthread_stop() call returns -EINTR and the ring->xenblkd remains dangling.
When this happens, any consecutive call to xen_blkif_disconnect (for example in frontend_changed() callback) leads to a kernel crash in kthread_stop() (e.g. NULL pointer dereference in exit_creds()).

This is XSA-350.

index 1d8b8d24496c..9860d4842f36 100644
@@ -274,6 +274,7 @@ static int xen_blkif_disconnect(struct xen_blkif *blkif)
 
 		if (ring->xenblkd) {
 			kthread_stop(ring->xenblkd);
+			ring->xenblkd = NULL;
 			wake_up(&ring->shutdown_wq);
 		}