linux-kernel - Re: [PATCH v2] n_tty: Protect tty->disc

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Date:   Wed, 29 Aug 2018 16:48:12 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        gregkh@...uxfoundation.org
Cc:     syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+3aa9784721dfb90e984d@...kaller.appspotmail.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2] n_tty: Protect tty->disc_data using refcount.

On 08/29/2018, 03:53 PM, Jiri Slaby wrote:
> On 07/24/2018, 05:22 PM, Tetsuo Handa wrote:
>> From 118c64e86641a97d44dec39e313a95b12d9bc3b2 Mon Sep 17 00:00:00 2001
>> From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>> Date: Wed, 25 Jul 2018 00:15:18 +0900
>> Subject: [PATCH v2] n_tty: Protect tty->disc_data using refcount.
>>
>> syzbot is reporting NULL pointer dereference at n_tty_set_termios() [1].
>> This is because ioctl(TIOCVHANGUP) versus ioctl(TCSETS) can race.
>>
>> Since we don't want to introduce new locking dependency, this patch
>> converts "struct n_tty_data *ldata = tty->disc_data;" in individual
>> function into a function argument which follows "struct tty *", and
>> holds tty->disc_data at each "struct tty_ldisc_ops" hook using refcount
>> in order to ensure that memory which contains "struct n_tty_data" will
>> not be released while processing individual function.
> 
> This does not look correct and is way too complicated. ioctls should not
> be called while changing/killing/hanging/whatever a ldisc. But there is
> one missing lock in tty_reopen.
> 
> So does the attached patch helps instead?

Which is btw in fact semantically the same as Dmitry's patch:
https://lore.kernel.org/lkml/20180829022353.23568-3-dima@arista.com/

> thanks,-- 
js
suse labs