[PATCH] usb-gadget-ether: Prevent oops caused by error interrupt race -V2 (comments update)

[Thomas Gleixner <tglx@linutronix.de>]

From: Benedikt Spranger <bene@linutronix.de>
 
eth_start_xmit() can race against a disconnect interrupt in the gadget
device driver, which nukes all pending request. Right now we access the
pending request list unconditionally and dereference the request list
head itself in such a case, which results in an Oops.

Check whether the list is empty before actually dereferencing
dev->tx_reqs.next. Also add a comment for the second list_empty check
further down to avoid confusion.

Long standing bug. Patch should be applied to 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..f2a7bd5 100644
--- a/drivers/usb/gadget/ether.c
+++ b/drivers/usb/gadget/ether.c
@@ -1989,8 +1989,21 @@ 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. We raced against a disconnect
+	 * interrupt in the gadget device driver, which nuked all
+	 * pending 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);