[PATCH 4.9 174/175] xen-blkback: set ring->xenblkd to NULL after kthread_stop()

[Greg Kroah-Hartman]

From: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>

commit 1c728719a4da6e654afb9cc047164755072ed7c9 upstream.

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@xxxxxxxxxxxxxxx> # 4.12
Fixes: a24fa22ce22a ("xen/blkback: don't use xen_blkif_get() in xen-blkback kthread")
Reported-by: Olivier Benjamin <oliben@xxxxxxxxxx>
Reported-by: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>
Signed-off-by: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>
Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/block/xen-blkback/xenbus.c |    1 +
 1 file changed, 1 insertion(+)

--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -262,6 +262,7 @@ static int xen_blkif_disconnect(struct x
 
 		if (ring->xenblkd) {
 			kthread_stop(ring->xenblkd);
+			ring->xenblkd = NULL;
 			wake_up(&ring->shutdown_wq);
 		}