From 4b6f210cb63eeceb5ea009ecd7a69c88d9e1b7c3 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Thu, 30 Aug 2007 23:56:35 -0700 Subject: [PATCH] [PATCH] sigqueue_free: fix the race with collect_signal() commit 60187d2708caa870f0825d753df1612ea688eb9e in mainline. Spotted by taoyue and Jeremy Katz . 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 Cc: taoyue Cc: Jeremy Katz Cc: Sukadev Bhattiprolu Cc: Alexey Dobriyan Cc: Ingo Molnar Cc: Thomas Gleixner Cc: Roland McGrath Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- kernel/signal.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-) diff --git a/kernel/signal.c b/kernel/signal.c index 5630255d2e2a9..4975f4ccc6789 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1345,20 +1345,19 @@ struct sigqueue *sigqueue_alloc(void) void sigqueue_free(struct sigqueue *q) { unsigned long flags; + spinlock_t *lock = ¤t->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 = ¤t->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); } -- 2.47.2