usbnet: increase URB reference count before usb_unlink_urb

Commit 4231d47e(net/usbnet: avoid
recursive locking in usbnet_stop()) fixes the recursive locking
problem by releasing the skb queue lock, but it makes usb_unlink_urb
racing with defer_bh, and the URB to being unlinked may be freed before
or during calling usb_unlink_urb, so use-after-free problem may be
triggerd inside usb_unlink_urb.

The patch fixes the use-after-free problem by increasing URB
reference count with skb queue lock held before calling
usb_unlink_urb, so the URB won't be freed until return from
usb_unlink_urb.

Cc: stable@kernel.org
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Oliver Neukum <oliver@neukum.org>
Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/usbnet.c

@@ -589,6 +589,14 @@ static int unlink_urbs (struct usbnet *dev, struct sk_buff_head *q)
 		entry = (struct skb_data *) skb->cb;
 		urb = entry->urb;
 
+		/*
+		 * Get reference count of the URB to avoid it to be
+		 * freed during usb_unlink_urb, which may trigger
+		 * use-after-free problem inside usb_unlink_urb since
+		 * usb_unlink_urb is always racing with .complete
+		 * handler(include defer_bh).
+		 */
+		usb_get_urb(urb);
 		spin_unlock_irqrestore(&q->lock, flags);
 		// during some PM-driven resume scenarios,
 		// these (async) unlinks complete immediately
@@ -597,6 +605,7 @@ static int unlink_urbs (struct usbnet *dev, struct sk_buff_head *q)
 			netdev_dbg(dev->net, "unlink urb err, %d\n", retval);
 		else
 			count++;
+		usb_put_urb(urb);
 		spin_lock_irqsave(&q->lock, flags);
 	}
 	spin_unlock_irqrestore (&q->lock, flags);