serial: return -EINVAL for non-legacy RS485 flags

In order to be add new flags more cleanly and safely, return -EINVAL
from TIOCSRS485 ioctl for the flags bits which are not among the
current legacy ones.

This might cause a regression for userspace as those non-flag bits do
not currently trigger -EINVAL. However, it would only occur if the
userspace is sending garbage bits so perhaps we'll get away with this
change.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220606100433.13793-25-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1276,6 +1276,27 @@ static int uart_get_icount(struct tty_struct *tty,
 	return 0;
 }
 
+#define SER_RS485_LEGACY_FLAGS	(SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND | \
+				 SER_RS485_RTS_AFTER_SEND | SER_RS485_RX_DURING_TX | \
+				 SER_RS485_TERMINATE_BUS)
+
+static int uart_check_rs485_flags(struct uart_port *port, struct serial_rs485 *rs485)
+{
+	u32 flags = rs485->flags;
+
+	/* Don't return -EINVAL for unsupported legacy flags */
+	flags &= ~SER_RS485_LEGACY_FLAGS;
+
+	/*
+	 * For any bit outside of the legacy ones that is not supported by
+	 * the driver, return -EINVAL.
+	 */
+	if (flags & ~port->rs485_supported->flags)
+		return -EINVAL;
+
+	return 0;
+}
+
 static void uart_sanitize_serial_rs485(struct uart_port *port, struct serial_rs485 *rs485)
 {
 	u32 supported_flags = port->rs485_supported->flags;
@@ -1375,6 +1396,9 @@ static int uart_set_rs485_config(struct uart_port *port,
 	if (copy_from_user(&rs485, rs485_user, sizeof(*rs485_user)))
 		return -EFAULT;
 
+	ret = uart_check_rs485_flags(port, &rs485);
+	if (ret)
+		return ret;
 	uart_sanitize_serial_rs485(port, &rs485);
 
 	spin_lock_irqsave(&port->lock, flags);