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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:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        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>
From:   Jiri Slaby <jslaby@suse.com>
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Message-ID: <5b5fa362-f5cf-53ab-044b-0577856b872d@suse.com>
Date:   Fri, 31 Aug 2018 08:51:18 +0200
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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,
-- 
js
suse labs