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Date:   Wed, 29 Aug 2018 13:34:30 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To:     Dmitry Safonov <dima@arista.com>
Cc:     linux-kernel@vger.kernel.org,
        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>,
        Pasi =?iso-8859-1?Q?K=E4rkk=E4inen?= <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>,
        Jiri Slaby <jslaby@suse.com>, stable@vger.kernel.org,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()
Message-ID: <20180829043430.GB13049@jagdpanzerIV>
References: <20180829022353.23568-1-dima@arista.com>
 <20180829022353.23568-3-dima@arista.com>
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Hi,

Cc-ing Benjamin on this.

On (08/29/18 03:23), Dmitry Safonov wrote:
> 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

Seems that you are not the first one to hit this NULL deref.

> I think, tty_ldisc_reinit() should be called with ldisc_sem hold for
> writing, which will protect any reader against line discipline changes.

Per https://lore.kernel.org/patchwork/patch/777220/

: Note that we noticed one path that called reinit without the ldisc lock
: held for writing, we added that, but it didn't fix the problem.

And I guess that Ben meant the same reinit path which you patched:

> @@ -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);
> +	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;
>  }

	-ss