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

[<gregkh@linuxfoundation.org>]

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 <stable@vger.kernel.org>.

thanks,

greg k-h

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

From 1c728719a4da6e654afb9cc047164755072ed7c9 Mon Sep 17 00:00:00 2001
From: Pawel Wieczorkiewicz <wipawel@amazon.de>
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.

Cc: <stable@vger.kernel.org> # 4.12
Fixes: a24fa22ce22a ("xen/blkback: don't use xen_blkif_get() in xen-blkback kthread")
Reported-by: Olivier Benjamin <oliben@amazon.com>
Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Signed-off-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 1d8b8d24496c..9860d4842f36 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -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);
 		}