[PATCH] usb-gadget-ether: Prevent oops caused by error interrupt race

[Thomas Gleixner <tglx@linutronix.de>]

From: Benedikt Spranger <bene@linutronix.de>

An USB error interrupt (e.g. disconnect) nukes the pending requests for
an ethernet gadget device asynchronously. This can race against
eth_start_xmit(), where we end up dereferencing the list head itself.

The nuke code is serialized against eth_start_xmit via dev->req_lock,
but we need to check the list for empty first instead of unconditionally
accessing dev->tx_reqs.next.

This is a long standing bug, which should be fixed in stable as well.

Signed-off-by: Benedikt Spranger <bene@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/drivers/usb/gadget/ether.c b/drivers/usb/gadget/ether.c
index 593e235..f1d7c82 100644
--- a/drivers/usb/gadget/ether.c
+++ b/drivers/usb/gadget/ether.c
@@ -1989,8 +1989,20 @@ static int eth_start_xmit (struct sk_buff *skb, struct net_device *net)
 	}
 
 	spin_lock_irqsave(&dev->req_lock, flags);
+	/*
+	 * dev->tx_reqs may be empty due to an error interrupt which
+	 * nuked all requests.
+	 */
+	if (list_empty(&dev->tx_reqs)) {
+		netif_stop_queue(net);
+		spin_unlock_irqrestore(&dev->req_lock, flags);
+		return 1;
+	}
+
 	req = container_of (dev->tx_reqs.next, struct usb_request, list);
 	list_del (&req->list);
+
+	/* last request in list: stop queue */
 	if (list_empty (&dev->tx_reqs))
 		netif_stop_queue (net);
 	spin_unlock_irqrestore(&dev->req_lock, flags);