patch "ANDROID: binder: synchronize_rcu() when using POLLFREE." added to char-misc-linus

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    ANDROID: binder: synchronize_rcu() when using POLLFREE.

to my char-misc git tree which can be found at
    git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git
in the char-misc-linus branch.

The patch will show up in the next release of the linux-next tree
(usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the
next -rc kernel release.

If you have any questions about this process, please let me know.


>From 5eeb2ca02a2f6084fc57ae5c244a38baab07033a Mon Sep 17 00:00:00 2001
From: Martijn Coenen <maco@android.com>
Date: Fri, 16 Feb 2018 09:47:15 +0100
Subject: 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>
---
 drivers/android/binder.c | 9 +++++++++
 1 file changed, 9 insertions(+)

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);
-- 
2.16.1