linux-kernel - Re: [PATCH v4] bluetooth: Fix WARNING in tty_set

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Message-Id: <EBD36B7C-0B26-4483-B7C8-736F27B9D267@holtmann.org>
Date:   Mon, 4 Feb 2019 06:39:36 +0100
From:   Marcel Holtmann <marcel@...tmann.org>
To:     Shuah Khan <shuah@...nel.org>
Cc:     Johan Hedberg <johan.hedberg@...il.com>,
        Johan Hovold <johan@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Bluez mailing list <linux-bluetooth@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] 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
> 
> "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 it by by preventing setting the HCI line discipline for PTYs in
> hci_uart_tty_open().
> 
> 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 | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
> index fbf7b4df23ab..c8faa4759cb7 100644
> --- a/drivers/bluetooth/hci_ldisc.c
> +++ b/drivers/bluetooth/hci_ldisc.c
> @@ -475,9 +475,9 @@ static int hci_uart_tty_open(struct tty_struct *tty)
> 	BT_DBG("tty %p", tty);
> 
> 	/* Error if the tty has no write op instead of leaving an exploitable
> -	 * hole
> +	 * hole. In addition check if setting HCI line discipline is allowed.
> 	 */

this comment is wrong. The result of setting the HCI line discipline is a call into hci_uart_tty_open. You have to check ops->write and ops->set_termios since both are required. That is it.

> -	if (tty->ops->write == NULL)
> +	if (tty->ops->write == NULL || tty->ops->set_termios == NULL)
> 		return -EOPNOTSUPP;

And while at it, change this to (!tty->ops->write).

Regards

Marcel