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

[Phi Nguyen]

On 8/18/2021 10:03 PM, Greg KH wrote:
> On Sat, Aug 14, 2021 at 02:35:53AM +0800, Phi Nguyen wrote:
> > 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.
>
> As you are fixing this FIXME, please remove it in this patch, otherwise
> we will not know it is resolved :)
>
> Can you add that to the change and submit a new version?
Yes, I will submit a new version.

> Also, this should go to stable kernels, right?  Any idea how far back?
I have no idea about this question, but I see it meets specified 
requirements in stable-kernel-rules.rst

BR,
Phi.