linux-kernel - Re: [PATCH v2] tty: Fix WARNING in tty_set

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Message-Id: <2EA87A3C-BC0D-4CDA-998D-BA7A6D829356@holtmann.org>
Date:   Fri, 1 Feb 2019 11:00:56 +0100
From:   Marcel Holtmann <marcel@...tmann.org>
To:     Shuah Khan <shuah@...nel.org>
Cc:     Johan Hedberg <johan.hedberg@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Greg KH <gregkh@...uxfoundation.org>,
        Bluez mailing list <linux-bluetooth@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tty: 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
> 
> 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@...kaller.appspotmail.com
> Signed-off-by: Shuah Khan <shuah@...nel.org>
> ---
> 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 wouldn’t even bother with the TTY type. Wouldn’t that be a lot simpler?

Regards

Marcel