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    Subject[PATCH 5.10 695/717] xen-blkback: set ring->xenblkd to NULL after kthread_stop()
    Date
    From: Pawel Wieczorkiewicz <wipawel@amazon.de>

    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@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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    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
    @@ -274,6 +274,7 @@ static int xen_blkif_disconnect(struct x

    if (ring->xenblkd) {
    kthread_stop(ring->xenblkd);
    + ring->xenblkd = NULL;
    wake_up(&ring->shutdown_wq);
    }


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