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

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Date:   Fri, 16 Feb 2018 09:49:12 +0100
From:   Martijn Coenen <maco@...roid.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        John Stultz <john.stultz@...aro.org>,
        Todd Kjos <tkjos@...gle.com>,
        Arve Hjønnevåg <arve@...roid.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        Martijn Coenen <maco@...gle.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Martijn Coenen <maco@...roid.com>
Subject: Re: [PATCH] ANDROID: binder: synchronize_rcu() when using POLLFREE.

Greg,

This is for 4.14 LTS and 4.16.

Thanks,
Martijn

On Fri, Feb 16, 2018 at 9:47 AM, Martijn Coenen <maco@...roid.com> wrote:
> 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
>