From: Pavel Machek <pavel@suse.cz>
Date: Fri, 2 May 2008 23:45:10 +0000 (-0700)
Subject: hci_usb.h: fix hard-to-trigger race
X-Git-Tag: v2.6.26-rc1~5^2~15
X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=026672d0997c911c9bef9aabe862884fc0add106;p=linux-2.6

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 <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
---

diff --git a/drivers/bluetooth/hci_usb.h b/drivers/bluetooth/hci_usb.h
index 414080a4e8..1790cc8e43 100644
--- a/drivers/bluetooth/hci_usb.h
+++ b/drivers/bluetooth/hci_usb.h
@@ -70,7 +70,8 @@ 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);
 }
 
@@ -78,19 +79,23 @@ 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 {