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

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Message-ID: <021343cd-dc28-9889-7656-e624861bc770@suse.cz>
Date:   Wed, 29 Aug 2018 15:53:49 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        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 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,
-- 
js
suse labs

View attachment "0001-tty-fix-NULL-ptr-dereference.patch" of type "text/x-patch" (1111 bytes)