Luacontroller: Restrict digiline messages

Restrict maximum length of messages to 50.000 characters and disable sending functions or table references over the wire. Restrict types of channel variable to string, number or boolean.

The missing length restriction made DoS-like attacks possible by overflowing memory using string concatenation. Thanks to gamemanj for disclosing this issue.

mesecons_luacontroller/init.lua

@@ -273,9 +273,23 @@ end
 local function get_digiline_send(pos)
 	if not digiline then return end
 	return function(channel, msg)
+		-- Make sure channel is string, number or boolean
+		if (type(channel) ~= "string" and type(channel) ~= "number" and type(channel) ~= "boolean") then
+			return false
+		end
+
+		-- No sending functions over the wire and make sure serialized version
+		-- of the data is not insanely long to prevent DoS-like attacks
+		msg = mesecon.tablecopy_stripfunctions(msg)
+		local msg_ser = minetest.serialize(msg)
+		if #msg_ser > mesecon.setting("luacontroller_digiline_maxlen", 50000) then
+			return false
+		end
+
 		minetest.after(0, function()
 			digiline:receptor_send(pos, digiline.rules.default, channel, msg)
 		end)
+		return true
 	end
 end
 
@@ -284,6 +298,7 @@ local safe_globals = {
 	"assert", "error", "ipairs", "next", "pairs", "select",
 	"tonumber", "tostring", "type", "unpack", "_VERSION"
 }
+
 local function create_environment(pos, mem, event)
 	-- Gather variables for the environment
 	local vports = minetest.registered_nodes[minetest.get_node(pos).name].virtual_portstates