Re: [PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

[Phi Nguyen]

On 8/13/2021 3:33 PM, Greg KH wrote:
> > Signed-off-by: Nguyen Dinh Phi <phind.uet@xxxxxxxxx>
> > Reported-by: syzbot+97388eb9d31b997fe1d0@xxxxxxxxxxxxxxxxxxxxxxxxx
> > ---
> >   drivers/tty/tty_io.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> > index e8532006e960..746fe13a2054 100644
> > --- a/drivers/tty/tty_io.c
> > +++ b/drivers/tty/tty_io.c
> > @@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
> >   	ld = tty_ldisc_ref_wait(tty);
> >   	if (!ld)
> >   		return -EIO;
> > +	tty_buffer_lock_exclusive(tty->port);
> >   	if (ld->ops->receive_buf)
> >   		ld->ops->receive_buf(tty, &ch, &mbz, 1);
> > +	tty_buffer_unlock_exclusive(tty->port);
>
> Did this fix the syzbot reported issue?
>
> thanks,
>
> greg k-h
> Yes, this fixed the syzbot reported issue.

The lock is grabbed in flush_to_ldisc() and paste_selection().
Actually, I follow the document in tty_buffer.c, where it say the 
callers to receive_buff() other than flush_to_ldisc() need to exclude 
the kworker from concurrent use of the line discipline.

And function tiocsti() has the following comment:
/* FIXME: may race normal receive processing */
that why I add lock in this function.

BR,
Phi.