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[PATCH v5] bluetooth: Fix WARNING in tty_set_termios()

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 hci_uart_tty_open() to check if tty supports set_termios in addition
to write and error out, if it doesn't.

Reported-by: syzbot+a950165cbb86bdd023a4@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: Johan Hovold <johan@xxxxxxxxxx>
Cc: Marcel Holtmann <marcel@xxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Shuah Khan <shuah@xxxxxxxxxx>
 drivers/bluetooth/hci_ldisc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
index fbf7b4df23ab..087ec2268744 100644
--- a/drivers/bluetooth/hci_ldisc.c
+++ b/drivers/bluetooth/hci_ldisc.c
@@ -474,10 +474,10 @@ 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
+	/* Error if the tty has no write/set_termios ops instead of leaving
+	 * an exploitable hole.
-	if (tty->ops->write == NULL)
+	if (!tty->ops->write || !tty->ops->set_termios)
 		return -EOPNOTSUPP;
 	hu = kzalloc(sizeof(struct hci_uart), GFP_KERNEL);