[PATCH net 3/5] rose: fix race between loopback timer and module removal

[Bernard Pidoux <bernard.f6bvp@gmail.com>]

rose_loopback_clear() called timer_delete() which returns immediately
without waiting for any running callback to complete.  If the timer
fired concurrently with module removal, rose_loopback_timer() could
re-arm the timer after timer_delete() returned and then access
rose_loopback_neigh after it was freed.

Two complementary changes close the race:

1. Add a loopback_stopping atomic flag.  rose_loopback_timer() checks
   it at entry (before acquiring a reference) and again inside the
   loop; when set it drains the queue and exits without re-arming the
   timer.

2. Switch rose_loopback_clear() to timer_delete_sync() so it blocks
   until any in-flight callback has returned before freeing resources.

The smp_mb() between setting the flag and calling timer_delete_sync()
ensures the flag is visible to any callback that is about to run.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Tested-by: Bernard Pidoux <bernard.f6bvp@gmail.com>
Signed-off-by: Bernard Pidoux <bernard.f6bvp@gmail.com>
---
 net/rose/rose_loopback.c | 31 ++++++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/net/rose/rose_loopback.c b/net/rose/rose_loopback.c
index d66913df360d..80d7879ef36a 100644
--- a/net/rose/rose_loopback.c
+++ b/net/rose/rose_loopback.c
@@ -12,13 +12,15 @@
 #include <net/rose.h>
 #include <linux/init.h>
 
-static struct sk_buff_head loopback_queue;
 #define ROSE_LOOPBACK_LIMIT 1000
-static struct timer_list loopback_timer;
 
+static struct timer_list loopback_timer;
+static struct sk_buff_head loopback_queue;
 static void rose_set_loopback_timer(void);
 static void rose_loopback_timer(struct timer_list *unused);
 
+static atomic_t loopback_stopping = ATOMIC_INIT(0);
+
 void rose_loopback_init(void)
 {
 	skb_queue_head_init(&loopback_queue);
@@ -66,6 +68,9 @@ static void rose_loopback_timer(struct timer_list *unused)
 	unsigned int lci_i, lci_o;
 	int count;
 
+	if (atomic_read(&loopback_stopping))
+		return;
+
 	if (rose_loopback_neigh)
 		rose_neigh_hold(rose_loopback_neigh);
 	else
@@ -75,6 +80,13 @@ static void rose_loopback_timer(struct timer_list *unused)
 		skb = skb_dequeue(&loopback_queue);
 		if (!skb)
 			goto out;
+
+		if (atomic_read(&loopback_stopping)) {
+			kfree_skb(skb);
+			skb_queue_purge(&loopback_queue);
+			goto out;
+		}
+
 		if (skb->len < ROSE_MIN_LEN) {
 			kfree_skb(skb);
 			continue;
@@ -118,7 +130,7 @@ static void rose_loopback_timer(struct timer_list *unused)
 out:
 	rose_neigh_put(rose_loopback_neigh);
 
-	if (!skb_queue_empty(&loopback_queue))
+	if (!atomic_read(&loopback_stopping) && !skb_queue_empty(&loopback_queue))
 		mod_timer(&loopback_timer, jiffies + 1);
 }
 
@@ -126,10 +138,15 @@ void __exit rose_loopback_clear(void)
 {
 	struct sk_buff *skb;
 
-	timer_delete(&loopback_timer);
+	atomic_set(&loopback_stopping, 1);
+	/* Pairs with atomic_read() in rose_loopback_timer(): ensure the
+	 * stopping flag is visible before we cancel, so a concurrent
+	 * callback aborts its loop early rather than re-arming the timer.
+	 */
+	smp_mb();
+
+	timer_delete_sync(&loopback_timer);
 
-	while ((skb = skb_dequeue(&loopback_queue)) != NULL) {
-		skb->sk = NULL;
+	while ((skb = skb_dequeue(&loopback_queue)) != NULL)
 		kfree_skb(skb);
-	}
 }
-- 
2.51.0