linux-kernel - Re: [BUG] tiocsti() NULL dereference if ld->ops->receive

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Message-ID: <CAG48ez0=Mcgk15EWBfVw-X7gB2hMJFULRA_w429-uyPZotdqNA@mail.gmail.com>
Date:   Mon, 21 Jan 2019 16:38:33 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jslaby@...e.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-serial@...r.kernel.org
Subject: Re: [BUG] tiocsti() NULL dereference if ld->ops->receive_buf==NULL

On Sun, Jan 20, 2019 at 10:52 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Sat, Jan 19, 2019 at 10:11:08AM +0100, Greg Kroah-Hartman wrote:
> > On Fri, Jan 18, 2019 at 08:09:07PM +0100, Jann Horn wrote:
> > > Hi!
> > >
> > > When a line discipline doesn't have a ->receive_buf handler, tiocsti()
> > > attempts to call a NULL pointer. Both tty_n_tracesink and
> > > spk_ttyio_ldisc_ops don't have such a handler.
> > >
> > > To reproduce, build a kernel with CONFIG_SPEAKUP=y and
> > > CONFIG_SPEAKUP_SYNTH_SOFT=y, set speakup.synth=soft in the kernel
> > > command line, and run the following code as root:
> >
> > <snip>
> >
> > Ugh, thanks for finding this.  I'll look at it later this afternoon...
>
> It looks to be a simple change.  We can't really "fail" this ioctl if
> there's nothing wrong with the structure of the call, so we can just
> quietly "eat" the character, given that the line discipline doesn't care
> about it.
>
> So, any objections to the patch below?

No objection from me.

(spk_ttyio_ldisc_ops has a receive_buf2 handler, but I don't know
whether that should be invoked here or not.)

> -----------------
>
> Subject: [PATCH] tty: Handle problem if line discipline does not have receive_buf
>
> Some tty line disciplines do not have a receive buf callback, so
> properly check for that before calling it.  If they do not have this
> callback, just eat the character quietly, as we can't fail this call.
>
> Reported-by: Jann Horn <jannh@...gle.com>
> Cc: stable <stable@...r.kernel.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  drivers/tty/tty_io.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 23c6fd238422..21ffcce16927 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -2189,7 +2189,8 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
>         ld = tty_ldisc_ref_wait(tty);
>         if (!ld)
>                 return -EIO;
> -       ld->ops->receive_buf(tty, &ch, &mbz, 1);
> +       if (ld->ops->receive_buf)
> +               ld->ops->receive_buf(tty, &ch, &mbz, 1);
>         tty_ldisc_deref(ld);
>         return 0;
>  }
> --
> 2.20.1
>