ANDROID: binder: synchronize_rcu() when using POLLFREE.

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index a85f9033b57e..764b63a5aade 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -4382,6 +4382,15 @@ static int binder_thread_release(struct binder_proc *proc,
 
         binder_inner_proc_unlock(thread->proc);
 
+        /*
+         * 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(proc, &thread->todo);