Re: [PATCH] sigqueue_free: fix the race with collect_signal()

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Oleg Nesterov wrote:
Spotted by taoyue <[email protected]> and Jeremy Katz <[email protected]>.

collect_signal:				sigqueue_free:

	list_del_init(&first->list);
						if (!list_empty(&q->list)) {
							// not taken
						}
						q->flags &= ~SIGQUEUE_PREALLOC;

	__sigqueue_free(first);			__sigqueue_free(q);

Now, __sigqueue_free() is called twice on the same "struct sigqueue" with the
obviously bad implications.

In particular, this double free breaks the array_cache->avail logic, so the
same sigqueue could be "allocated" twice, and the bug can manifest itself via
the "impossible" BUG_ON(!SIGQUEUE_PREALLOC) in sigqueue_free/send_sigqueue.

Hopefully this can explain these mysterious bug-reports, see

	http://marc.info/?t=118766926500003
	http://marc.info/?t=118466273000005

Alexey Dobriyan reports this patch makes the difference for the testcase, but
nobody has an access to the application which opened the problems originally.

Also, this patch removes tasklist lock/unlock, ->siglock is enough.

Signed-off-by: Oleg Nesterov <[email protected]>

--- t/kernel/signal.c~SQFREE	2007-08-22 20:06:31.000000000 +0400
+++ t/kernel/signal.c	2007-08-23 16:02:57.000000000 +0400
@@ -1297,20 +1297,19 @@ struct sigqueue *sigqueue_alloc(void)
 void sigqueue_free(struct sigqueue *q)
 {
 	unsigned long flags;
+	spinlock_t *lock = &current->sighand->siglock;
+
 	BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
 	/*
 	 * If the signal is still pending remove it from the
-	 * pending queue.
+	 * pending queue. We must hold ->siglock while testing
+	 * q->list to serialize with collect_signal().
 	 */
-	if (unlikely(!list_empty(&q->list))) {
-		spinlock_t *lock = &current->sighand->siglock;
-		read_lock(&tasklist_lock);
-		spin_lock_irqsave(lock, flags);
Hmm, but the existing code _does_ take the siglock here. Is that not sufficient ? Isn't the first list_empty() check without lock only an optimization for the common
case ?

-		if (!list_empty(&q->list))
-			list_del_init(&q->list);
-		spin_unlock_irqrestore(lock, flags);
-		read_unlock(&tasklist_lock);
-	}
+	spin_lock_irqsave(lock, flags);
+	if (!list_empty(&q->list))
+		list_del_init(&q->list);
+	spin_unlock_irqrestore(lock, flags);
+
 	q->flags &= ~SIGQUEUE_PREALLOC;
 	__sigqueue_free(q);
 }

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