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 <mbizon@freebox.fr>
Cc: <stable@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 1 deletions

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;