linux-kernel - Re: [PATCHv2 2/4] tty: Hold tty_ldisc_lock() during tty

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Date:   Tue, 4 Sep 2018 11:02:12 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Daniel Axtens <dja@...ens.net>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Nathan March <nathan@...net>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Peter Hurley <peter@...leysoftware.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Pasi Kärkkäinen <pasik@....fi>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        syzbot+3aa9784721dfb90e984d@...kaller.appspotmail.com,
        stable@...r.kernel.org
Subject: Re: [PATCHv2 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()

On 09/03/2018, 06:52 PM, Dmitry Safonov wrote:
> tty_ldisc_reinit() doesn't race with neither tty_ldisc_hangup()
> nor set_ldisc() nor tty_ldisc_release() as they use tty lock.
> But it races with anyone who expects line discipline to be the same
> after hoding read semaphore in tty_ldisc_ref().
> 
> We've seen the following crash on v4.9.108 stable:
> 
> BUG: unable to handle kernel paging request at 0000000000002260
> IP: [..] n_tty_receive_buf_common+0x5f/0x86d
> Workqueue: events_unbound flush_to_ldisc
> Call Trace:
>  [..] n_tty_receive_buf2
>  [..] tty_ldisc_receive_buf
>  [..] flush_to_ldisc
>  [..] process_one_work
>  [..] worker_thread
>  [..] kthread
>  [..] ret_from_fork
> 
> tty_ldisc_reinit() should be called with ldisc_sem hold for writing,
> which will protect any reader against line discipline changes.
> 
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jiri Slaby <jslaby@...e.com>

Reviewed-by: Jiri Slaby <jslaby@...e.cz>

> Cc: stable@...r.kernel.org # depends on commit b027e2298bd5 ("tty: fix
> data race between tty_init_dev and flush of buf")
> Reported-by: syzbot+3aa9784721dfb90e984d@...kaller.appspotmail.com
> Tested-by: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
> Signed-off-by: Dmitry Safonov <dima@...sta.com>

thanks,
-- 
js
suse labs