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: 5df5daf ("Bluetooth: hci_uart: Fix another race during initialization")
Link: https://lore.kernel.org/linux-bluetooth/6969764f.170a0220.2b9fc4.35a7@mx.google.com/

Signed-off-by: Jia-Hong Su <s11242586@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

Author: AxelHowe

drivers/bluetooth/hci_ldisc.c

@@ -685,6 +685,8 @@ static int hci_uart_register_dev(struct hci_uart *hu)
 		return err;
 	}
 
+	set_bit(HCI_UART_PROTO_INIT, &hu->flags);
+
 	if (test_bit(HCI_UART_INIT_PENDING, &hu->hdev_flags))
 		return 0;
 
@@ -712,8 +714,6 @@ static int hci_uart_set_proto(struct hci_uart *hu, int id)
 
 	hu->proto = p;
 
-	set_bit(HCI_UART_PROTO_INIT, &hu->flags);
-
 	err = hci_uart_register_dev(hu);
 	if (err) {
 		return err;