linux-kernel - hci_usb.h: fix hard-to-trigger race

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Message-ID: <20080401133212.GA24598@elf.ucw.cz>
Date:	Tue, 1 Apr 2008 15:32:12 +0200
From:	Pavel Machek <pavel@....cz>
To:	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, marcel@...tmann.org,
	maxk@...lcomm.com
Subject: 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@...e.cz>

diff --git a/drivers/bluetooth/hci_usb.h b/drivers/bluetooth/hci_usb.h
index 414080a..1790cc8 100644
--- a/drivers/bluetooth/hci_usb.h
+++ b/drivers/bluetooth/hci_usb.h
@@ -70,7 +70,8 @@ static inline void _urb_queue_head(struc
 {
 	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(struc
 {
 	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 {


-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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