Re: [PATCHv6 2/7] tty: Hold tty_ldisc_lock() during tty_reopen()

[Tycho Andersen <tycho@tycho.ws>]

Hi,

On Thu, Nov 01, 2018 at 12:24:47AM +0000, Dmitry Safonov wrote:
> tty_ldisc_reinit() doesn't race with neither tty_ldisc_hangup()
> nor set_ldisc() nor tty_ldisc_release() as they use tty lock.
> But it races with anyone who expects line discipline to be the same
> after hoding read semaphore in tty_ldisc_ref().
> 
> We've seen the following crash on v4.9.108 stable:
> 
> BUG: unable to handle kernel paging request at 0000000000002260
> IP: [..] n_tty_receive_buf_common+0x5f/0x86d
> Workqueue: events_unbound flush_to_ldisc
> Call Trace:
>  [..] n_tty_receive_buf2
>  [..] tty_ldisc_receive_buf
>  [..] flush_to_ldisc
>  [..] process_one_work
>  [..] worker_thread
>  [..] kthread
>  [..] ret_from_fork
> 
> tty_ldisc_reinit() should be called with ldisc_sem hold for writing,
> which will protect any reader against line discipline changes.
> 
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Jiri Slaby <jslaby@suse.com>
> Cc: stable@vger.kernel.org # b027e2298bd5 ("tty: fix data race between tty_init_dev and flush of buf")
> Reviewed-by: Jiri Slaby <jslaby@suse.cz>
> Reported-by: syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com
> Tested-by: Mark Rutland <mark.rutland@arm.com>
> Tested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>

Feel free to add

Tested-by: Tycho Andersen <tycho@tycho.ws>

to this as well. We've recently seen this bug (well, the one that
syzbot reported), and this patch fixes it.

Tycho