bluetooth: Fix missing NULL check

Fortunately this is only exploitable on very unusual hardware.

[Reported a while ago but nothing happened so just fixing it]

Signed-off-by: Alan Cox <[e-mail]>
Cc: [e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/bluetooth/hci_ldisc.c

···
 
 	BT_DBG("tty %p", tty);
 
+	/* FIXME: This btw is bogus, nothing requires the old ldisc to clear
+	   the pointer */
 	if (hu)
 		return -EEXIST;
+
+	/* Error if the tty has no write op instead of leaving an exploitable
+	   hole */
+	if (tty->ops->write == NULL)
+		return -EOPNOTSUPP;
 
 	if (!(hu = kzalloc(sizeof(struct hci_uart), GFP_KERNEL))) {
 		BT_ERR("Can't allocate control structure");