linux-kernel - [PATCH] ANDROID: binder: synchronize

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Date:   Fri, 16 Feb 2018 09:47:15 +0100
From:   Martijn Coenen <maco@...roid.com>
To:     gregkh@...uxfoundation.org, john.stultz@...aro.org,
        tkjos@...gle.com, arve@...roid.com
Cc:     linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
        maco@...gle.com, ebiggers@...gle.com, dvyukov@...gle.com,
        Martijn Coenen <maco@...roid.com>
Subject: [PATCH] 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@...kaller.appspotmail.com
Signed-off-by: Martijn Coenen <maco@...roid.com>
---
 drivers/android/binder.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 15e3d3c2260d..359220f87e1d 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -4376,6 +4376,15 @@ 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);
-- 
2.16.1.291.g4437f3f132-goog