drm/gud: validate TV mode names before creating enum property

The GUD protocol returns TV mode names as fixed-size
GUD_CONNECTOR_TV_MODE_NAME_LEN entries and requires each name to be
NUL-terminated.

gud_connector_add_tv_mode() currently passes each fixed-size entry
directly to drm_mode_create_tv_properties_legacy(), which eventually
reaches drm_property_add_enum() and strlen(). If a device returns an
entry without a terminating NUL byte, strlen() reads past the end of
the slot and can run beyond the allocated buffer, triggering an
out-of-bounds read.

Validate that each returned TV mode name contains a NUL terminator
within its fixed-size slot before passing it to the DRM property code.
If a malformed entry is found, reject the device response with -EIO.

This fixes the out-of-bounds read without changing the handling of
valid devices, and avoids silently truncating malformed protocol data.

Reported-by: [email protected]
Fixes: 40e1a70b4a ("drm: Add GUD USB Display driver")
Signed-off-by: Tao Yu <[email protected]>
Reviewed-by: Ruben Wauters <[email protected]>
Cc: <[email protected]>
Signed-off-by: Ruben Wauters <[email protected]>
Link: https://patch.msgid.link/[email protected]

drivers/gpu/drm/gud/gud_connector.c

@@ -399,8 +399,11 @@ static int gud_connector_add_tv_mode(struct gud_device *gdrm, struct drm_connect
 	for (i = 0; i < num_modes; i++) {
 		char *mode = &buf[i * GUD_CONNECTOR_TV_MODE_NAME_LEN];
 
-		/* The device is not trusted to NUL-terminate the name */
-		mode[GUD_CONNECTOR_TV_MODE_NAME_LEN - 1] = '\0';
+		if (!memchr(mode, '\0', GUD_CONNECTOR_TV_MODE_NAME_LEN)) {
+			ret = -EIO;
+			goto free;
+		}
+
 		modes[i] = mode;
 	}