drm/modes: Introduce a whitelist for the named modes

The named modes support has introduced a number of glitches that were in
part due to the fact that the parser will take any string as a named mode.

Since we shouldn't have a lot of options there (and they should be pretty
standard), let's introduce a whitelist of the available named modes so that
the kernel can differentiate between a poorly formed command line and a
named mode.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Thomas Graichen <thomas.graichen@gmail.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827115850.25731-3-mripard@kernel.org

drivers/gpu/drm/drm_modes.c

@@ -1677,6 +1677,22 @@ static int drm_mode_parse_cmdline_options(char *str, size_t len,
 	return 0;
 }
 
+static const char *drm_named_modes_whitelist[] = {
+	"NTSC",
+	"PAL",
+};
+
+static bool drm_named_mode_is_in_whitelist(const char *mode, unsigned int size)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(drm_named_modes_whitelist); i++)
+		if (!strncmp(mode, drm_named_modes_whitelist[i], size))
+			return true;
+
+	return false;
+}
+
 /**
  * drm_mode_parse_command_line_for_connector - parse command line modeline for connector
  * @mode_option: optional per connector mode option
@@ -1794,6 +1810,10 @@ bool drm_mode_parse_command_line_for_connector(const char *mode_option,
 	if (named_mode) {
 		if (mode_end + 1 > DRM_DISPLAY_MODE_LEN)
 			return false;
+
+		if (!drm_named_mode_is_in_whitelist(name, mode_end))
+			return false;
+
 		strscpy(mode->name, name, mode_end + 1);
 	} else {
 		ret = drm_mode_parse_cmdline_res_mode(name, mode_end,