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Subject: Re: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()
To:     Dmitry Safonov <dima@arista.com>, linux-kernel@vger.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@gmail.com>,
        Daniel Axtens <dja@axtens.net>,
        Dmitry Vyukov <dvyukov@google.com>,
        Michael Neuling <mikey@neuling.org>,
        Mikulas Patocka <mpatocka@redhat.com>,
        =?UTF-8?B?UGFzaSBLw6Rya2vDpGluZW4=?= <pasik@iki.fi>,
        Peter Hurley <peter@hurleysoftware.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Tan Xiaojun <tanxiaojun@huawei.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>, stable@vger.kernel.org
References: <20180829022353.23568-1-dima@arista.com>
 <20180829022353.23568-3-dima@arista.com>
From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Message-ID: <0cb6bb7c-776b-28d6-65a6-f5b496e32344@I-love.SAKURA.ne.jp>
Date:   Thu, 30 Aug 2018 00:19:04 +0900
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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.