[PATCH v3] bluetooth: Fix WARNING in tty_set_termios()

[Shuah Khan <shuah@kernel.org>]

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@syzkaller.appspotmail.com
Cc: Johan Hovold <johan@kernel.org>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Shuah Khan <shuah@kernel.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;
+
 	hu = kzalloc(sizeof(struct hci_uart), GFP_KERNEL);
 	if (!hu) {
 		BT_ERR("Can't allocate control structure");
-- 
2.17.1