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Subject: Re: [PATCHv2 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()
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>,
        Nathan March <nathan@gt.net>,
        Tan Xiaojun <tanxiaojun@huawei.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        =?UTF-8?B?UGFzaSBLw6Rya2vDpGluZW4=?= <pasik@iki.fi>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Michael Neuling <mikey@neuling.org>,
        Mikulas Patocka <mpatocka@redhat.com>,
        syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com,
        stable@vger.kernel.org
References: <20180903165257.29227-1-dima@arista.com>
 <20180903165257.29227-3-dima@arista.com>
From:   Jiri Slaby <jslaby@suse.cz>
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Message-ID: <6a2acb53-7dfc-9426-b277-4368a6d2a2e8@suse.cz>
Date:   Tue, 4 Sep 2018 11:02:12 +0200
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On 09/03/2018, 06:52 PM, 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
> 
> tty_ldisc_reinit() should be called with ldisc_sem hold for writing,
> which will protect any reader against line discipline changes.
> 
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Jiri Slaby <jslaby@suse.com>

Reviewed-by: Jiri Slaby <jslaby@suse.cz>

> Cc: stable@vger.kernel.org # depends on commit b027e2298bd5 ("tty: fix
> data race between tty_init_dev and flush of buf")
> Reported-by: syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com
> Tested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
> Signed-off-by: Dmitry Safonov <dima@arista.com>

thanks,
-- 
js
suse labs