Re: [PATCH 2/4] tty: Hold tty_ldisc_lock() during tty_reopen()

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Wed, 2018-08-29 at 13:34 +0900, Sergey Senozhatsky wrote:
> Hi,
> 
> Cc-ing Benjamin on this.
> 
> On (08/29/18 03:23), Dmitry Safonov wrote:
> > 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
> 
> Seems that you are not the first one to hit this NULL deref.
> 
> > I think, tty_ldisc_reinit() should be called with ldisc_sem hold for
> > writing, which will protect any reader against line discipline changes.
> 
> Per https://lore.kernel.org/patchwork/patch/777220/
> 
> : Note that we noticed one path that called reinit without the ldisc lock
> : held for writing, we added that, but it didn't fix the problem.
> 
> And I guess that Ben meant the same reinit path which you patched:

This is too old for me to remember buit yes, there definitely was a bug
there...

> > @@ -1267,15 +1267,20 @@ static int tty_reopen(struct tty_struct *tty)
> >  	if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN))
> >  		return -EBUSY;
> >  
> > -	tty->count++;
> > +	retval = tty_ldisc_lock(tty, 5 * HZ);
> > +	if (retval)
> > +		return retval;
> >  
> > +	tty->count++;
> >  	if (tty->ldisc)
> > -		return 0;
> > +		goto out_unlock;
> >  
> >  	retval = tty_ldisc_reinit(tty, tty->termios.c_line);
> >  	if (retval)
> >  		tty->count--;
> >  
> > +out_unlock:
> > +	tty_ldisc_unlock(tty);
> >  	return retval;
> >  }
> 
> 	-ss