USB: cypress_m8: fix use-after-free in TIOCMIWAIT

Use the port wait queue and make sure to check the serial disconnected
flag before accessing private port data after waking up.

This is is needed as the private port data (including the wait queue
itself) can be gone when waking up after a disconnect.

Also remove bogus test for private data pointer being NULL as it is
never assigned in the loop.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/cypress_m8.c

@@ -111,7 +111,6 @@ struct cypress_private {
 	int baud_rate;			   /* stores current baud rate in
 					      integer form */
 	int isthrottled;		   /* if throttled, discard reads */
-	wait_queue_head_t delta_msr_wait;  /* used for TIOCMIWAIT */
 	char prev_status, diff_status;	   /* used for TIOCMIWAIT */
 	/* we pass a pointer to this as the argument sent to
 	   cypress_set_termios old_termios */
@@ -449,7 +448,6 @@ static int cypress_generic_port_probe(struct usb_serial_port *port)
 		kfree(priv);
 		return -ENOMEM;
 	}
-	init_waitqueue_head(&priv->delta_msr_wait);
 
 	usb_reset_configuration(serial->dev);
 
@@ -868,12 +866,16 @@ static int cypress_ioctl(struct tty_struct *tty,
 	switch (cmd) {
 	/* This code comes from drivers/char/serial.c and ftdi_sio.c */
 	case TIOCMIWAIT:
-		while (priv != NULL) {
-			interruptible_sleep_on(&priv->delta_msr_wait);
+		for (;;) {
+			interruptible_sleep_on(&port->delta_msr_wait);
 			/* see if a signal did it */
 			if (signal_pending(current))
 				return -ERESTARTSYS;
-			else {
+
+			if (port->serial->disconnected)
+				return -EIO;
+
+			{
 				char diff = priv->diff_status;
 				if (diff == 0)
 					return -EIO; /* no change => error */
@@ -1187,7 +1189,7 @@ static void cypress_read_int_callback(struct urb *urb)
 	if (priv->current_status != priv->prev_status) {
 		priv->diff_status |= priv->current_status ^
 			priv->prev_status;
-		wake_up_interruptible(&priv->delta_msr_wait);
+		wake_up_interruptible(&port->delta_msr_wait);
 		priv->prev_status = priv->current_status;
 	}
 	spin_unlock_irqrestore(&priv->lock, flags);