USB: console: fix potential use after free

commit 32a4bf2e upstream.

Use tty kref to release the fake tty in usb_console_setup to avoid use
after free if the underlying serial driver has acquired a reference.

Note that using the tty destructor release_one_tty requires some more
state to be initialised.

Fixes: 4a90f09b ("tty: usb-serial krefs")
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/serial/console.c

@@ -47,6 +47,8 @@ static struct console usbcons;
  * ------------------------------------------------------------
  */
 
+static const struct tty_operations usb_console_fake_tty_ops = {
+};
 
 /*
  * The parsing of the command line works exactly like the
@@ -141,14 +143,17 @@ static int usb_console_setup(struct console *co, char *options)
 				goto reset_open_count;
 			}
 			kref_init(&tty->kref);
-			tty_port_tty_set(&port->port, tty);
 			tty->driver = usb_serial_tty_driver;
 			tty->index = co->index;
+			INIT_LIST_HEAD(&tty->tty_files);
+			kref_get(&tty->driver->kref);
+			tty->ops = &usb_console_fake_tty_ops;
 			if (tty_init_termios(tty)) {
 				retval = -ENOMEM;
 				err("no more memory");
-				goto free_tty;
+				goto put_tty;
 			}
+			tty_port_tty_set(&port->port, tty);
 		}
 
 		/* only call the device specific open if this
@@ -170,7 +175,7 @@ static int usb_console_setup(struct console *co, char *options)
 			serial->type->set_termios(tty, port, &dummy);
 
 			tty_port_tty_set(&port->port, NULL);
-			kfree(tty);
+			tty_kref_put(tty);
 		}
 		set_bit(ASYNCB_INITIALIZED, &port->port.flags);
 	}
@@ -186,8 +191,8 @@ static int usb_console_setup(struct console *co, char *options)
 
  fail:
 	tty_port_tty_set(&port->port, NULL);
- free_tty:
-	kfree(tty);
+ put_tty:
+	tty_kref_put(tty);
  reset_open_count:
 	port->port.count = 0;
 	usb_autopm_put_interface(serial->interface);