io_uring/eventfd: ensure io_eventfd_signal() defers another RCU period

Commit c9a4029 upstream.

io_eventfd_do_signal() is invoked from an RCU callback, but when
dropping the reference to the io_ev_fd, it calls io_eventfd_free()
directly if the refcount drops to zero. This isn't correct, as any
potential freeing of the io_ev_fd should be deferred another RCU grace
period.

Just call io_eventfd_put() rather than open-code the dec-and-test and
free, which will correctly defer it another RCU grace period.

Fixes: 21a091b ("io_uring: signal registered eventfd to process deferred task work")
Reported-by: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: axboe

io_uring/io_uring.c

@@ -537,6 +537,13 @@ static __cold void io_queue_deferred(struct io_ring_ctx *ctx)
 	}
 }
 
+static void io_eventfd_free(struct rcu_head *rcu)
+{
+	struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
+
+	eventfd_ctx_put(ev_fd->cq_ev_fd);
+	kfree(ev_fd);
+}
 
 static void io_eventfd_ops(struct rcu_head *rcu)
 {
@@ -550,10 +557,8 @@ static void io_eventfd_ops(struct rcu_head *rcu)
 	 * ordering in a race but if references are 0 we know we have to free
 	 * it regardless.
 	 */
-	if (atomic_dec_and_test(&ev_fd->refs)) {
-		eventfd_ctx_put(ev_fd->cq_ev_fd);
-		kfree(ev_fd);
-	}
+	if (atomic_dec_and_test(&ev_fd->refs))
+		call_rcu(&ev_fd->rcu, io_eventfd_free);
 }
 
 static void io_eventfd_signal(struct io_ring_ctx *ctx)