USB: serial: fix null-pointer dereferences on disconnect

Make sure serial-driver dtr_rts is called with disc_mutex held after
checking the disconnected flag.

Due to a bug in the tty layer, dtr_rts may get called after a device has
been disconnected and the tty-device unregistered. Some drivers have had
individual checks for disconnect to make sure the disconnected interface
was not accessed, but this should really be handled in usb-serial core
(at least until the long-standing tty-bug has been fixed).

Note that the problem has been made more acute with commit 0998d06310
("device-core: Ensure drvdata = NULL when no driver is bound") as the
port data is now also NULL when dtr_rts is called resulting in further
oopses.

Reported-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
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/ssu100.c

@@ -506,19 +506,16 @@ static void ssu100_dtr_rts(struct usb_serial_port *port, int on)
 {
 	struct usb_device *dev = port->serial->dev;
 
-	mutex_lock(&port->serial->disc_mutex);
-	if (!port->serial->disconnected) {
-		/* Disable flow control */
-		if (!on &&
-		    ssu100_setregister(dev, 0, UART_MCR, 0) < 0)
+	/* Disable flow control */
+	if (!on) {
+		if (ssu100_setregister(dev, 0, UART_MCR, 0) < 0)
 			dev_err(&port->dev, "error from flowcontrol urb\n");
-		/* drop RTS and DTR */
-		if (on)
-			set_mctrl(dev, TIOCM_DTR | TIOCM_RTS);
-		else
-			clear_mctrl(dev, TIOCM_DTR | TIOCM_RTS);
 	}
-	mutex_unlock(&port->serial->disc_mutex);
+	/* drop RTS and DTR */
+	if (on)
+		set_mctrl(dev, TIOCM_DTR | TIOCM_RTS);
+	else
+		clear_mctrl(dev, TIOCM_DTR | TIOCM_RTS);
 }
 
 static void ssu100_update_msr(struct usb_serial_port *port, u8 msr)