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commit1c728719a4upstream. 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:a24fa22ce2("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> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>