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Subject: Re: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()
From:   Jiri Slaby <jslaby@suse.cz>
To:     Dmitry Safonov <dima@arista.com>, linux-kernel@vger.kernel.org
Cc:     Daniel Axtens <dja@axtens.net>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Tan Xiaojun <tanxiaojun@huawei.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        =?UTF-8?B?UGFzaSBLw6Rya2vDpGluZW4=?= <pasik@iki.fi>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Michael Neuling <mikey@neuling.org>,
        Mikulas Patocka <mpatocka@redhat.com>, stable@vger.kernel.org
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Message-ID: <1a3acb7c-5523-7b72-1468-682c429b56e6@suse.cz>
Date:   Wed, 29 Aug 2018 16:45:39 +0200
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On 08/29/2018, 04:40 PM, Jiri Slaby wrote:
> On 08/29/2018, 04:23 AM, 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.

...

> So what about:
>         tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
>         if (!tty->ldisc)
>                 ret = tty_ldisc_reinit(tty, tty->termios.c_line);
>         tty_ldisc_unlock(tty);
> 
>         if (!ret)
>                 tty->count++;
> 
>         return ret;

I forgot to add that I debugged a different NULL ptr deref to this very
same root cause today (set_termios called with NULL tty->disc_data). So
really, tty_reinit's ldisc change must be protected by the ldisc_sem,
otherwise other threads will see tty->ldisc, but not tty->disc_data.

thanks,
-- 
js
suse labs