USB: fix urb-poison imbalance

The calls to usb_poison_urb and usb_unpoison_urb are expected to be
balanced. However, if an urb that has not yet been submitted is
poisoned, its reject counter will not be increased as its ep-field is
NULL. A consecutive call to unpoison will thus in fact poison the urb
as its reject counter will be decremented to a negative value,
effectively preventing the urb from being submitted.

Note that there are currently no in-kernel drivers affected by this.

Cc: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/urb.c

@@ -683,10 +683,13 @@ EXPORT_SYMBOL_GPL(usb_kill_urb);
 void usb_poison_urb(struct urb *urb)
 {
 	might_sleep();
-	if (!(urb && urb->dev && urb->ep))
+	if (!urb)
 		return;
 	atomic_inc(&urb->reject);
 
+	if (!urb->dev || !urb->ep)
+		return;
+
 	usb_hcd_unlink_urb(urb, -ENOENT);
 	wait_event(usb_kill_urb_queue, atomic_read(&urb->use_count) == 0);
 }