From 971316f0503a5c50633d07b83b6db2f15a3a5b00 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Fri, 24 Feb 2012 20:07:29 +0100 Subject: epoll: ep_unregister_pollwait() can use the freed pwq->whead signalfd_cleanup() ensures that ->signalfd_wqh is not used, but this is not enough. eppoll_entry->whead still points to the memory we are going to free, ep_unregister_pollwait()->remove_wait_queue() is obviously unsafe. Change ep_poll_callback(POLLFREE) to set eppoll_entry->whead = NULL, change ep_unregister_pollwait() to check pwq->whead != NULL under rcu_read_lock() before remove_wait_queue(). We add the new helper, ep_remove_wait_queue(), for this. This works because sighand_cachep is SLAB_DESTROY_BY_RCU and because ->signalfd_wqh is initialized in sighand_ctor(), not in copy_sighand. ep_unregister_pollwait()->remove_wait_queue() can play with already freed and potentially reused ->sighand, but this is fine. This memory must have the valid ->signalfd_wqh until rcu_read_unlock(). Reported-by: Maxime Bizon Cc: Signed-off-by: Oleg Nesterov Signed-off-by: Linus Torvalds --- fs/signalfd.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) (limited to 'fs/signalfd.c') diff --git a/fs/signalfd.c b/fs/signalfd.c index 79c1eea98a3a..7ae2a574cb25 100644 --- a/fs/signalfd.c +++ b/fs/signalfd.c @@ -33,7 +33,11 @@ void signalfd_cleanup(struct sighand_struct *sighand) { wait_queue_head_t *wqh = &sighand->signalfd_wqh; - + /* + * The lockless check can race with remove_wait_queue() in progress, + * but in this case its caller should run under rcu_read_lock() and + * sighand_cachep is SLAB_DESTROY_BY_RCU, we can safely return. + */ if (likely(!waitqueue_active(wqh))) return; -- cgit v1.2.3-59-g8ed1b