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

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jul 25, 2018 at 12:22:16AM +0900, Tetsuo Handa wrote:
> >From 118c64e86641a97d44dec39e313a95b12d9bc3b2 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@I-love.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.
> 
> [1] https://syzkaller.appspot.com/bug?id=1e850009fca0b64ce49dc16499bda4f7de0ab1a5
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  drivers/tty/n_tty.c | 511 ++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 314 insertions(+), 197 deletions(-)

What changed from v1?  I haven't had the chance to review your first
patch, sorry, it's still in my queue.  I was hoping that someone else
would help out with that as well :)

thanks,

greg k-h