xhci: fix null pointer dereference on ring_doorbell_for_active_rings

in some cases where device is attched to xhci port and do not responding,
for example ath9k_htc with stalled firmware, kernel will
crash on ring_doorbell_for_active_rings.
This patch check if pointer exist before it is used.

This patch should be backported to kernels as old as 2.6.35, that
contain the commit e9df17eb1408cfafa3d1844bfc7f22c7237b31b8 "USB: xhci:
Correct assumptions about number of rings per endpoint"

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 1e57eafa69101aaee797b7890020c66865ef4c16..5b08cd85f8e7fe9646228da946166dfb188c10d2 100644 (file)
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -434,7 +434,7 @@ static void ring_doorbell_for_active_rings(struct xhci_hcd *xhci,
 
        /* A ring has pending URBs if its TD list is not empty */
        if (!(ep->ep_state & EP_HAS_STREAMS)) {
-               if (!(list_empty(&ep->ring->td_list)))
+               if (ep->ring && !(list_empty(&ep->ring->td_list)))
                        xhci_ring_ep_doorbell(xhci, slot_id, ep_index, 0);
                return;
        }