hci_usb.h: fix hard-to-trigger race

If someone tries to _urb_unlink while _urb_queue_head is running, he'll see _urb->queue == NULL and fail to do any locking. Prevent that from happening by strategically placed barriers. Signed-off-by: Pavel Machek <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: David S. Miller <[email protected]>
ambv · May 2, 2008 · 026672d · 026672d
1 parent 84994e1
commit 026672d
Showing 1 changed file with 13 additions and 8 deletions.
diff --git a/drivers/bluetooth/hci_usb.h b/drivers/bluetooth/hci_usb.h
@@ -70,27 +70,32 @@ static inline void _urb_queue_head(struct _urb_queue *q, struct _urb *_urb)
 {
 	unsigned long flags;
 	spin_lock_irqsave(&q->lock, flags);
-	list_add(&_urb->list, &q->head); _urb->queue = q;
+	/* _urb_unlink needs to know which spinlock to use, thus mb(). */
+	_urb->queue = q; mb(); list_add(&_urb->list, &q->head);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 
 static inline void _urb_queue_tail(struct _urb_queue *q, struct _urb *_urb)
 {
 	unsigned long flags;
 	spin_lock_irqsave(&q->lock, flags);
-	list_add_tail(&_urb->list, &q->head); _urb->queue = q;
+	/* _urb_unlink needs to know which spinlock to use, thus mb(). */
+	_urb->queue = q; mb(); list_add_tail(&_urb->list, &q->head);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 
 static inline void _urb_unlink(struct _urb *_urb)
 {
-	struct _urb_queue *q = _urb->queue;
+	struct _urb_queue *q;
 	unsigned long flags;
-	if (q) {
-		spin_lock_irqsave(&q->lock, flags);
-		list_del(&_urb->list); _urb->queue = NULL;
-		spin_unlock_irqrestore(&q->lock, flags);
-	}
+
+	mb();
+	q = _urb->queue;
+	/* If q is NULL, it will die at easy-to-debug NULL pointer dereference.
+	   No need to BUG(). */
+	spin_lock_irqsave(&q->lock, flags);
+	list_del(&_urb->list); _urb->queue = NULL;
+	spin_unlock_irqrestore(&q->lock, flags);
 }
 
 struct hci_usb {