Minetest server-sent CSM proof-of-concept

Attempts to run server-sent CSMs locally in a sandbox.

How it works

Any client with the CSM installed will automatically attempt to request SSCSMs from the server via a mod channel. If the server has this mod installed, it will reply with a few messages containing the mod name and partially minified mod code. The CSM will then create a separate environment so SSCSMs cannot mess with existing CSMs (and so CSMs do not accidentally interfere with SSCSMs), and execute the SSCSMs inside this environment. Note that it is trivial for users to modify this environment. The server-side mod sends two "built-in" SSCSMs before and after all other SSCSMs to add extra helper functions (in the sscsm namespace), to execute register_on_mods_loaded callbacks and attempt to leave the mod channel.

Instructions

To create a SSCSM:

• Install this mod onto a server.
• Enable mod channels on the server (add enable_mod_channels = true to minetest.conf).
• Create SSCSMs with the API.
• Install the CSM (in the csm/ directory) onto clients and enable it.

Server-side mod facing API

This API is subject to change.

sscsm.register(def)

Registers a server-provided CSM with the following definition table.

• name (string): The name of the server-provided CSM. Please use the modname:sscsmname convention. Cannot start with a colon or contain newlines.
• code (string): The code to be sent to clients.
• file (string): The file to read the code from, read during the register() call.
• depends (list): A list of SSCSMs that must be loaded before this one.

This definition table must have name and either code or file.

Maximum SSCSM size

Because of Minetest network protocol limitations, the amount of data that can be sent over mod channels is limited, and therefore the maximum SSCSM size is 65300 (to leave room for the player name and future expansion). The name of the SSCSM also counts towards this total.

Because of this size limitation, SSCSMs are passed through a primitive code minifier that removes some whitespace and comments, so even if your code is above this size limit it could still work.

Server-sent CSM facing API

SSCSMs can access most functions on client_lua_api.txt, as well as a separate sscsm namespace:

• sscsm.global_exists(name): The same as minetest.global_exists.
• sscsm.register_on_mods_loaded(callback): Runs the callback once all SSCSMs are loaded.
• sscsm.register_chatcommand(...): Similar to minetest.register_chatcommand, however overrides commands starting in / instead. This can be used to make some commands have instantaneous responses. The command handler is only added once register_chatcommand has been called.
• sscsm.unregister_chatcommand(name): Unregisters a chatcommand.
• sscsm.get_player_control(): Returns a table similar to the server-side player:get_player_control().
• sscsm.restriction_flags: The csm_restriction_flags setting set in the server's minetest.conf.
• sscsm.restrictions: A table based on csm_restriction_flags:
  • chat_messages: When true, SSCSMs can't send chat messages or run server chatcommands.
  • read_itemdefs: When true, SSCSMs can't read item definitions.
  • read_nodedefs: When true, SSCSMs can't read node definitions.
  • lookup_nodes_limit: When true, any get_node calls are restricted.
  • read_playerinfo: When true, minetest.get_player_names() will return nil.

To communicate with the server-side mods, it is possible to open a mod channel.

CSM restriction flags example

minetest.register_chatcommand('m', {
    description = 'Alias for /msg',
    func = function(param)
        if sscsm.restrictions.chat_messages then
            return false, 'Sorry, csm_restriction_flags prevents chat messages'
                .. ' from being sent.'
        end
        minetest.run_server_chatcommand('msg', param)
    end,
})

Note that modifying sscsm.restrictions or sscsm.restriction_flags will not add or remove restrictions and is not recommended.

Security considerations

Do not trust any input sent to the server via SSCSMs (and do not store sensitive data in SSCSM code), as malicious users can and will inspect code and modify the output from SSCSMs.

I repeat, do not trust the client and/or SSCSMs with any sensitive information and do not trust any output from the client and/or SSCSMs. Make sure to rerun any privilege checks on the server.

Other recommendations

Although it is possible to kick clients that do not support SSCSMs, this has not been implemented. Some users may not want to allow servers to automatically download and run code locally for security reasons. Please try and make sure clients without SSCSMs do not suffer from major functionality loss.