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Date:   Thu, 30 Aug 2018 00:02:30 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Jiri Slaby <jslaby@...e.cz>, gregkh@...uxfoundation.org,
        jslaby@...e.com
Cc:     syzbot <syzbot+3aa9784721dfb90e984d@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2] n_tty: Protect tty->disc_data using refcount.

On 2018/08/29 22:53, 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?
> 
> thanks,
> 

That patch seems to help avoiding crashes. (You can use #syz test: command.)
But I think you need to check tty_ldisc_lock() return value...

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