Bluetooth: hci_uart: fix null-ptr-deref in hci_uart_write_work

hci_uart_set_proto() sets HCI_UART_PROTO_INIT before calling
hci_uart_register_dev(), which calls proto->open() to initialize
hu->priv. However, if a TTY write wakeup occurs during this window,
hci_uart_tx_wakeup() may schedule write_work before hu->priv is
initialized, leading to a NULL pointer dereference in
hci_uart_write_work() when proto->dequeue() accesses hu->priv.

The race condition is:

CPU0 CPU1
---- ----
hci_uart_set_proto()
set_bit(HCI_UART_PROTO_INIT)
hci_uart_register_dev()
tty write wakeup
hci_uart_tty_wakeup()
hci_uart_tx_wakeup()
schedule_work(&hu->write_work)
proto->open(hu)
// initializes hu->priv
hci_uart_write_work()
hci_uart_dequeue()
proto->dequeue(hu)
// accesses hu->priv (NULL!)

Fix this by moving set_bit(HCI_UART_PROTO_INIT) after proto->open()
succeeds, ensuring hu->priv is initialized before any work can be
scheduled.

Fixes: 5df5dafc171b ("Bluetooth: hci_uart: Fix another race during initialization")
Link: https://lore.kernel.org/linux-bluetooth/[e-mail]/

Signed-off-by: Jia-Hong Su <[e-mail]>
Signed-off-by: Luiz Augusto von Dentz <[e-mail]>

drivers/bluetooth/hci_ldisc.c

···
 		return err;
 	}
 
+	set_bit(HCI_UART_PROTO_INIT, &hu->flags);
+
 	if (test_bit(HCI_UART_INIT_PENDING, &hu->hdev_flags))
 		return 0;
 
···
 		return -EPROTONOSUPPORT;
 
 	hu->proto = p;
-
-	set_bit(HCI_UART_PROTO_INIT, &hu->flags);
 
 	err = hci_uart_register_dev(hu);
 	if (err) {