USB: fix use-after-free bug in usb_hcd_unlink_urb()

commit c9919790 upstream.

The usb_hcd_unlink_urb() routine in hcd.c contains two possible
use-after-free errors.  The dev_dbg() statement at the end of the
routine dereferences urb and urb->dev even though both structures may
have been deallocated.

This patch fixes the problem by storing urb->dev in a local variable
(avoiding the dereference of urb) and moving the dev_dbg() up before
the usb_put_dev() call.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Joe Lawrence <joe.lawrence@stratus.com>
Tested-by: Joe Lawrence <joe.lawrence@stratus.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/usb/core/hcd.c

@@ -1621,6 +1621,7 @@ static int unlink1(struct usb_hcd *hcd, struct urb *urb, int status)
 int usb_hcd_unlink_urb (struct urb *urb, int status)
 {
 	struct usb_hcd		*hcd;
+	struct usb_device	*udev = urb->dev;
 	int			retval = -EIDRM;
 	unsigned long		flags;
 
@@ -1632,20 +1633,19 @@ int usb_hcd_unlink_urb (struct urb *urb, int status)
 	spin_lock_irqsave(&hcd_urb_unlink_lock, flags);
 	if (atomic_read(&urb->use_count) > 0) {
 		retval = 0;
-		usb_get_dev(urb->dev);
+		usb_get_dev(udev);
 	}
 	spin_unlock_irqrestore(&hcd_urb_unlink_lock, flags);
 	if (retval == 0) {
 		hcd = bus_to_hcd(urb->dev->bus);
 		retval = unlink1(hcd, urb, status);
-		usb_put_dev(urb->dev);
-	}
-
 		if (retval == 0)
 			retval = -EINPROGRESS;
 		else if (retval != -EIDRM && retval != -EBUSY)
-		dev_dbg(&urb->dev->dev, "hcd_unlink_urb %p fail %d\n",
+			dev_dbg(&udev->dev, "hcd_unlink_urb %p fail %d\n",
 					urb, retval);
+		usb_put_dev(udev);
+	}
 	return retval;
 }