[79/84] xhci: fix null pointer dereference on ring_doorbell_for_active_rings

[Ben Hutchings]

3.2.50-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Oleksij Rempel <linux@xxxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxxx>
Signed-off-by: Sarah Sharp <sarah.a.sharp@xxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/usb/host/xhci-ring.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -463,7 +463,7 @@ static void ring_doorbell_for_active_rin
 
 	/* 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;
 	}

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