serial: arc_uart: Fix out-of-bounds access through DT alias

The arc_uart_ports[] array is indexed using a value derived from the
"serialN" alias in DT, which may lead to an out-of-bounds access.

Fix this by adding a range check.

Note that the array size is defined by a Kconfig symbol
(CONFIG_SERIAL_ARC_NR_PORTS), so this can even be triggered using a
legitimate DTB.

Fixes: ea28fd56fcde69af ("serial/arc-uart: switch to devicetree based probing")
Signed-off-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/arc_uart.c

···
 	if (dev_id < 0)
 		dev_id = 0;
 
+	if (dev_id >= ARRAY_SIZE(arc_uart_ports)) {
+		dev_err(&pdev->dev, "serial%d out of range\n", dev_id);
+		return -EINVAL;
+	}
+
 	uart = &arc_uart_ports[dev_id];
 	port = &uart->port;