ANDROID: binder: synchronize_rcu() when using POLLFREE.

commit 5eeb2ca02a2f6084fc57ae5c244a38baab07033a upstream.

To prevent races with ep_remove_waitqueue() removing the
waitqueue at the same time.

Reported-by: syzbot+a2a3c4909716e271487e@syzkaller.appspotmail.com
Signed-off-by: Martijn Coenen <maco@android.com>
Cc: stable <stable@vger.kernel.org> # 4.14+
Signed-off-by: Mattias Nissler <mnissler@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index b6f4baa..e12288c 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2641,6 +2641,15 @@
 		wake_up_poll(&thread->wait, POLLHUP | POLLFREE);
 	}
 
+	/*
+	 * This is needed to avoid races between wake_up_poll() above and
+	 * and ep_remove_waitqueue() called for other reasons (eg the epoll file
+	 * descriptor being closed); ep_remove_waitqueue() holds an RCU read
+	 * lock, so we can be sure it's done after calling synchronize_rcu().
+	 */
+	if (thread->looper & BINDER_LOOPER_STATE_POLL)
+		synchronize_rcu();
+
 	if (send_reply)
 		binder_send_failed_reply(send_reply, BR_DEAD_REPLY);
 	binder_release_work(&thread->todo);