tty: moxa: fix TIOCSSERIAL permission check

Changing the port close delay or type are privileged operations so make
sure to return -EPERM if a regular user tries to change them.

Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210407102334.32361-12-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/moxa.c

@@ -2048,6 +2048,7 @@ static int moxa_set_serial_info(struct tty_struct *tty,
 		struct serial_struct *ss)
 {
 	struct moxa_port *info = tty->driver_data;
+	unsigned int close_delay;
 
 	if (tty->index == MAX_PORTS)
 		return -EINVAL;
@@ -2059,19 +2060,24 @@ static int moxa_set_serial_info(struct tty_struct *tty,
 			ss->baud_base != 921600)
 		return -EPERM;
 
+	close_delay = msecs_to_jiffies(ss->close_delay * 10);
+
 	mutex_lock(&info->port.mutex);
 	if (!capable(CAP_SYS_ADMIN)) {
-		if (((ss->flags & ~ASYNC_USR_MASK) !=
+		if (close_delay != info->port.close_delay ||
+		    ss->type != info->type ||
+		    ((ss->flags & ~ASYNC_USR_MASK) !=
 		     (info->port.flags & ~ASYNC_USR_MASK))) {
 			mutex_unlock(&info->port.mutex);
 			return -EPERM;
 		}
+	} else {
+		info->port.close_delay = close_delay;
+
+		MoxaSetFifo(info, ss->type == PORT_16550A);
+
+		info->type = ss->type;
 	}
-	info->port.close_delay = msecs_to_jiffies(ss->close_delay * 10);
-
-	MoxaSetFifo(info, ss->type == PORT_16550A);
-
-	info->type = ss->type;
 	mutex_unlock(&info->port.mutex);
 	return 0;
 }