linux-kernel - Re: [PATCH] tty: Fix data race between tiocsti() and flush_to

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Message-ID: <YR0TRf+283ujBxbv@kroah.com>
Date:   Wed, 18 Aug 2021 16:03:49 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Phi Nguyen <phind.uet@...il.com>
Cc:     jirislaby@...nel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel@...r.kernel.org,
        syzbot+97388eb9d31b997fe1d0@...kaller.appspotmail.com
Subject: Re: [PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

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@...il.com>
> > > Reported-by: syzbot+97388eb9d31b997fe1d0@...kaller.appspotmail.com
> > > ---
> > >   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?

Also, this should go to stable kernels, right?  Any idea how far back?

thanks,

greg k-h