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

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Message-ID: <0cb6bb7c-776b-28d6-65a6-f5b496e32344@I-love.SAKURA.ne.jp>
Date:   Thu, 30 Aug 2018 00:19:04 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Neuling <mikey@...ling.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Pasi Kärkkäinen <pasik@....fi>,
        Peter Hurley <peter@...leysoftware.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tan Xiaojun <tanxiaojun@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, stable@...r.kernel.org
Subject: Re: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()

On 2018/08/29 11:23, 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
> 
> I think, tty_ldisc_reinit() should be called with ldisc_sem hold for
> writing, which will protect any reader against line discipline changes.
> 
> Note: I failed to reproduce the described crash, so obiviously can't
> guarantee that this is the place where line discipline was switched.

This will be same with a report at
https://syzkaller.appspot.com/bug?id=f08670354701fa64cc0dd3c0128a491bdb16adcc .

syzbot is now testing a patch from Jiri Slaby.