3.16.50-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Oleg Nesterov <oleg@xxxxxxxxxx> commit 138e4ad67afd5c6c318b056b4d17c17f2c0ca5c0 upstream. The race was introduced by me in commit 971316f0503a ("epoll: ep_unregister_pollwait() can use the freed pwq->whead"). I did not realize that nothing can protect eventpoll after ep_poll_callback() sets ->whead = NULL, only whead->lock can save us from the race with ep_free() or ep_remove(). Move ->whead = NULL to the end of ep_poll_callback() and add the necessary barriers. TODO: cleanup the ewake/EPOLLEXCLUSIVE logic, it was confusing even before this patch. Hopefully this explains use-after-free reported by syzcaller: BUG: KASAN: use-after-free in debug_spin_lock_before ... _raw_spin_lock_irqsave+0x4a/0x60 kernel/locking/spinlock.c:159 ep_poll_callback+0x29f/0xff0 fs/eventpoll.c:1148 this is spin_lock(eventpoll->lock), ... Freed by task 17774: ... kfree+0xe8/0x2c0 mm/slub.c:3883 ep_free+0x22c/0x2a0 fs/eventpoll.c:865 Fixes: 971316f0503a ("epoll: ep_unregister_pollwait() can use the freed pwq->whead") Reported-by: 范龙飞 <long7573@xxxxxxx> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> [bwh: Backported to 3.16: EPOLLEXCLUSIVE is not supported] Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> --- --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -518,8 +518,13 @@ static void ep_remove_wait_queue(struct wait_queue_head_t *whead; rcu_read_lock(); - /* If it is cleared by POLLFREE, it should be rcu-safe */ - whead = rcu_dereference(pwq->whead); + /* + * If it is cleared by POLLFREE, it should be rcu-safe. + * If we read NULL we need a barrier paired with + * smp_store_release() in ep_poll_callback(), otherwise + * we rely on whead->lock. + */ + whead = smp_load_acquire(&pwq->whead); if (whead) remove_wait_queue(whead, &pwq->wait); rcu_read_unlock(); @@ -1006,17 +1011,6 @@ static int ep_poll_callback(wait_queue_t struct epitem *epi = ep_item_from_wait(wait); struct eventpoll *ep = epi->ep; - if ((unsigned long)key & POLLFREE) { - ep_pwq_from_wait(wait)->whead = NULL; - /* - * whead = NULL above can race with ep_remove_wait_queue() - * which can do another remove_wait_queue() after us, so we - * can't use __remove_wait_queue(). whead->lock is held by - * the caller. - */ - list_del_init(&wait->task_list); - } - spin_lock_irqsave(&ep->lock, flags); /* @@ -1081,6 +1075,22 @@ out_unlock: if (pwake) ep_poll_safewake(&ep->poll_wait); + if ((unsigned long)key & POLLFREE) { + /* + * If we race with ep_remove_wait_queue() it can miss + * ->whead = NULL and do another remove_wait_queue() after + * us, so we can't use __remove_wait_queue(). + */ + list_del_init(&wait->task_list); + /* + * ->whead != NULL protects us from the race with ep_free() + * or ep_remove(), ep_remove_wait_queue() takes whead->lock + * held by the caller. Once we nullify it, nothing protects + * ep/epi or even wait. + */ + smp_store_release(&ep_pwq_from_wait(wait)->whead, NULL); + } + return 1; }