Re: [PATCH v2] n_tty: Protect tty->disc_data using refcount.

From: Jiri Slaby
Date: Wed Aug 29 2018 - 09:53:56 EST

This is a multi-part message in MIME format.On 07/24/2018, 05:22 PM, Tetsuo Handa wrote:
> From 118c64e86641a97d44dec39e313a95b12d9bc3b2 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> 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?

suse labs