Re: Using Lua for config files

[Roberto Ierusalimschy <roberto@...>]

> Rather than controlling what users can do, I would prefer to control the effect they can have. Why should they not have the benefit of a full featured language to configure your program?
> 
> So, for example, assuming you are using Lua as an embedded language, run the configuration script in its own Lua state and then have a (protected) procedure to copy the resultant data structures over into your main Lua state, with verification and filtering as necessary.
> 

Probably I am being too naive, but I have written a very simple sandbox
that seems to be enough to ensure a "safe" execution of a script. It
simply uses debug hooks to control CPU usage, finalizers to control
memory usage, and a restricted environment (emtpy by default) to control
what the script can call. It would be nice if other people could
check it. (It assumes Lua 5.2.)

-- Roberto


local debug = require'debug'

assert(arg[1], "usage: lua protect.lua script")

-- maximum memory (in KB) that can be used
local memlimit = 10000

-- maximum "steps" that can be performed
local steplimit = 1000

-- what the script can call (beware what you put here!!!)
local basiclib = {}

do
  -- track memory use
  local mt = {__gc = function (u)
    if collectgarbage("count") > memlimit then
      error("script uses too much memory")
    else
      setmetatable({}, getmetatable(u))
    end
  end}
  setmetatable({}, mt)
end

local count = 0
local function step ()
  count = count + 1
  if count > steplimit then
    error("script uses too much CPU")
  end
end

local f = assert(loadfile(arg[1], "t", basiclib))

debug.sethook(step, "c", 100)

f()