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Subject: Re: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()
To:     Jiri Slaby <jslaby@suse.com>, 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>,
        stable@vger.kernel.org
References: <20180829022353.23568-1-dima@arista.com>
 <20180829022353.23568-3-dima@arista.com>
 <0cb6bb7c-776b-28d6-65a6-f5b496e32344@I-love.SAKURA.ne.jp>
 <5b5fa362-f5cf-53ab-044b-0577856b872d@suse.com>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <19bf55bc-eca4-672f-8ce7-b72329207074@i-love.sakura.ne.jp>
Date:   Fri, 31 Aug 2018 20:17:45 +0900
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On 2018/08/31 15:51, Jiri Slaby wrote:
> On 08/29/2018, 05:19 PM, Tetsuo Handa wrote:
>> 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.
> 
> Yes, my patch passed, so could you add:
> Reported-by: syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com
> 
> (not adding tested-by as this particular patch was not tested, but
> shoiuld work the same way.)
> 
> thanks,
> 

Tested with all 4 patches applied using syzbot-provided reproducer and
my simplified reproducer. No crashes and no lockdep warnings.
Also, noisy messages like

  pts pts4033: tty_release: tty->count(10529) != (#fd's(7) + #kopen's(0))

are gone. Very nice. Thank you.