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    Subject[PATCH 4.9 31/47] ANDROID: binder: synchronize_rcu() when using POLLFREE.
    Date
    From: Martijn Coenen <maco@android.com>

    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>
    ---
    drivers/android/binder.c | 9 +++++++++
    1 file changed, 9 insertions(+)

    --- a/drivers/android/binder.c
    +++ b/drivers/android/binder.c
    @@ -2641,6 +2641,15 @@ static int binder_free_thread(struct bin
    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);

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