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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:     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
References: <20180829022353.23568-1-dima@arista.com>
 <20180829022353.23568-3-dima@arista.com>
From:   Jiri Slaby <jslaby@suse.cz>
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Message-ID: <914d8184-d5e6-519c-b355-7f1360cfa6a0@suse.cz>
Date:   Wed, 29 Aug 2018 16:40:28 +0200
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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.
> 
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Jiri Slaby <jslaby@suse.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Dmitry Safonov <dima@arista.com>
> ---
>  drivers/tty/tty_io.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 5e5da9acaf0a..3ef8b977b167 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -1267,15 +1267,20 @@ static int tty_reopen(struct tty_struct *tty)
>  	if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN))
>  		return -EBUSY;
>  
> -	tty->count++;
> +	retval = tty_ldisc_lock(tty, 5 * HZ);

Why 5 secs? This would cause random errors on machines under heavy load.

> +	if (retval)
> +		return retval;
>  
> +	tty->count++;
>  	if (tty->ldisc)
> -		return 0;
> +		goto out_unlock;
>  
>  	retval = tty_ldisc_reinit(tty, tty->termios.c_line);
>  	if (retval)
>  		tty->count--;
>  
> +out_unlock:
> +	tty_ldisc_unlock(tty);
>  	return retval;

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;

thanks,
-- 
js
suse labs