xhci: fix null pointer dereference on ring_doorbell_for_active_rings

commit d66eaf9f89502971fddcb0de550b01fa6f409d83 upstream.

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

1 files changed, 1 insertions, 1 deletions

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 1969c001b3f9..cc3bfc5d590d 100644
--- 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;
         }