From 60187d2708caa870f0825d753df1612ea688eb9e Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Thu, 30 Aug 2007 23:56:35 -0700 Subject: [PATCH] sigqueue_free: fix the race with collect_signal() 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 Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/signal.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-) diff --git a/kernel/signal.c b/kernel/signal.c index ad63109e41..3169bed0b4 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 = ¤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); } -- 2.39.5