[patch 21/40] usb-gadget-ether: prevent oops caused by error interrupt race

-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Benedikt Spranger <[email protected]>

patch 5395353e0c8272fe73ac914acd7e4add0da2bef0 in mainline.

Fix a longstanding race in the Ethernet gadget driver, which can cause an
oops on device disconnect.  The fix is just to make the TX path check
whether its freelist is empty.  That check is otherwise not necessary,
since the queue is always stopped when that list empties (and restarted
when request completion puts an entry back on that freelist).

The race window starts when the network code decides to transmit a packet,
and ends when hard_start_xmit() grabs the freelist lock.  When disconnect()
is called inside that window, it shuts down the TX queue and breaks the
otherwise-solid assumption that packets are never sent through a TX queue
that's stopped.

Signed-off-by: Benedikt Spranger <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Signed-off-by: David Brownell <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

---
 drivers/usb/gadget/ether.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/drivers/usb/gadget/ether.c
+++ b/drivers/usb/gadget/ether.c
@@ -1989,8 +1989,20 @@ static int eth_start_xmit (struct sk_buf
 	}
 
 	spin_lock_irqsave(&dev->req_lock, flags);
+	/*
+	 * this freelist can be empty if an interrupt triggered disconnect()
+	 * and reconfigured the gadget (shutting down this queue) after the
+	 * network stack decided to xmit but before we got the spinlock.
+	 */
+	if (list_empty(&dev->tx_reqs)) {
+		spin_unlock_irqrestore(&dev->req_lock, flags);
+		return 1;
+	}
+
 	req = container_of (dev->tx_reqs.next, struct usb_request, list);
 	list_del (&req->list);
+
+	/* temporarily stop TX queue when the freelist empties */
 	if (list_empty (&dev->tx_reqs))
 		netif_stop_queue (net);
 	spin_unlock_irqrestore(&dev->req_lock, flags);

-- 

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