[PATCH] ANDROID: binder: synchronize_rcu() when using POLLFREE.

From: Martijn Coenen <maco@android.com>
To: gregkh@linuxfoundation.org, john.stultz@linaro.org,
	tkjos@google.com, arve@android.com
Cc: devel@driverdev.osuosl.org, ebiggers@google.com,
	linux-kernel@vger.kernel.org, maco@google.com,
	Martijn Coenen <maco@android.com>,
	dvyukov@google.com
Subject: [PATCH] ANDROID: binder: synchronize_rcu() when using POLLFREE.
Date: Fri, 16 Feb 2018 09:47:15 +0100	[thread overview]
Message-ID: <20180216084715.14969-1-maco@android.com> (raw)

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+
---
 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

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