From 60187d2708caa870f0825d753df1612ea688eb9e Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@tv-sign.ru>
Date: Thu, 30 Aug 2007 23:56:35 -0700
Subject: [PATCH] sigqueue_free: fix the race with collect_signal()

Spotted by taoyue <yue.tao@windriver.com> and Jeremy Katz <jeremy.katz@windriver.com>.

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 <oleg@tv-sign.ru>
Cc: taoyue <yue.tao@windriver.com>
Cc: Jeremy Katz <jeremy.katz@windriver.com>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Cc: Alexey Dobriyan <adobriyan@sw.ru>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Roland McGrath <roland@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 kernel/signal.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index ad63109e413..3169bed0b4d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1300,20 +1300,19 @@ 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);
-		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);
 }