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

[Martijn Coenen]

Greg,

This is for 4.14 LTS and 4.16.

Thanks,
Martijn

On Fri, Feb 16, 2018 at 9:47 AM, Martijn Coenen <maco@xxxxxxxxxxx> wrote:
> To prevent races with ep_remove_waitqueue() removing the
> waitqueue at the same time.
>
> Reported-by: syzbot+a2a3c4909716e271487e@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Martijn Coenen <maco@xxxxxxxxxxx>
> ---
>  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
>