linux-kernel - Re: [PATCH v3] bluetooth: Fix WARNING in tty_set

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Message-Id: <0CC28468-8E2F-4DEE-8307-DA0CEA59A8D0@holtmann.org>
Date:   Sun, 3 Feb 2019 18:31:38 +0100
From:   Marcel Holtmann <marcel@...tmann.org>
To:     Shuah Khan <shuah@...nel.org>
Cc:     Johan Hedberg <johan.hedberg@...il.com>, johan@...nel.org,
        viro@...iv.linux.org.uk, linux-bluetooth@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] bluetooth: Fix WARNING in tty_set_termios()

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
> 
> Johan Hovold said: "The problemm 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 it by by preventing setting the HCI line discipline for PTYs in
> hci_uart_tty_open(). Looked into keying off of tty and ldisc ops, and
> couldn't find any that would be conclusive. Checking tty as such clearly
> tags the reason for rejecting the request to set ldisc.
> 
> Reported-by: syzbot+a950165cbb86bdd023a4@...kaller.appspotmail.com
> Cc: Johan Hovold <johan@...nel.org>
> Cc: Marcel Holtmann <marcel@...tmann.org>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Signed-off-by: Shuah Khan <shuah@...nel.org>
> ---
> drivers/bluetooth/hci_ldisc.c | 5 +++++
> 1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
> index fbf7b4df23ab..a3d313fcc0f2 100644
> --- a/drivers/bluetooth/hci_ldisc.c
> +++ b/drivers/bluetooth/hci_ldisc.c
> @@ -480,6 +480,11 @@ static int hci_uart_tty_open(struct tty_struct *tty)
> 	if (tty->ops->write == NULL)
> 		return -EOPNOTSUPP;
> 
> +	/* don't set HCI line discipline on PTYs */
> +	if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
> +	    tty->driver->subtype == PTY_TYPE_MASTER)
> +		return -EINVAL;
> +

this is turning in circles. What is wrong with checking !tty->ops->set_termios here?

Regards

Marcel