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); }