Re: [PATCH v2] tty: Fix WARNING in tty_set_termios()

[shuah]

On 2/1/19 3:00 AM, Marcel Holtmann wrote:
> Hi Shuah,
>
> > tty_set_termios() has the following WARN_ON which can be triggered with a
> > syscall to invoke TIOCSETD __NR_ioctl.
> >
> > WARN_ON(tty->driver->type == TTY_DRIVER_TYPE_PTY &&
> >                 tty->driver->subtype == PTY_TYPE_MASTER);
> > Reference: https://syzkaller.appspot.com/bug?id=2410d22f1d8e5984217329dd0884b01d99e3e48d
> >
> > The problem started with commit 7721383f4199 ("Bluetooth: hci_uart: Support
> > operational speed during setup") which introduced a new way for how
> > tty_set_termios() could end up being called for a master pty.
> >
> > Fix the problem by preventing setting the HCI line discipline for PTYs
> > from hci_uart_setup() and hci_uart_set_flow_control().
> >
> > The reproducer is used to reproduce the problem and verify the fix.
> >
> > Reported-by: syzbot+a950165cbb86bdd023a4@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Shuah Khan <shuah@xxxxxxxxxx>
> > ---
> > drivers/bluetooth/hci_ldisc.c | 11 +++++++++++
> > 1 file changed, 11 insertions(+)
> >
> > diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
> > index fbf7b4df23ab..ce84ca91ca70 100644
> > --- a/drivers/bluetooth/hci_ldisc.c
> > +++ b/drivers/bluetooth/hci_ldisc.c
> > @@ -314,6 +314,11 @@ void hci_uart_set_flow_control(struct hci_uart *hu, bool enable)
> > 		return;
> > 	}
> >
> > +	/* don't set HCI line discipline on PTYs */
> > +	if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
> > +	    tty->driver->subtype == PTY_TYPE_MASTER)
> > +		return;
> > +
>
> just do it once in hci_uart_tty_open. Actually we already check for ops->write and so lets just ensure all ops we ever call are present. If they are not, then bail out. I wouldnât even bother with the TTY type. Wouldnât that be a lot simpler?

As you said, hci_uart_tty_open()is a good place to check it. I think
checking tty type is necessary. I couldn't find any missing ops I could
base the check on to prevent the ldisc set.

I have the patch that does the tty check in hci_uart_tty_open() tested
and ready for sending.

thanks,
-- Shuah