How to execute an untrusted Lua file in its own environment from the C API

Question

I want to execute an untrusted .lua file in its own environment by calling lua_setfenv() so that it cannot affect any of my code.

The documentation for that function though only explains how to call a function, not how to execute a file.

Currently to run the file I use:

int error = luaL_loadfile(mState, path.c_str()) || lua_pcall(mState, 0, 0, 0);

Do I have to call the "dofile" lua function from the C API with lua_setfenv, or is there a more elegant way to do it?

Yonah

Answer 1

See the discussion at the Lua User's Wiki of sandboxing, and the more general topic of script security. There are a number of subtle and not so subtle issues with this kind of thing. It can be done, but protecting against code such as for i=1,1e39 do end requires more than just restricting what functions are available to a sandbox.

The general technique is to create a function environment for the sandbox that has a whitelist of permitted functions in it. In some cases, that list might even be empty, but letting the user have access to pairs(), for example, is almost certainly harmless. The sandbox page has a list of the system functions broken down by their safety as a handy reference for constructing such a whitelist.

You then use lua_setfenv() to apply the function environment to the user's script which you loaded (but haven't yet executed) with lua_loadfile() or lua_loadstring() as appropriate. With the environment attached, you could execute it with lua_pcall() and friends. Before execution, some people have actually scanned the loaded bytecode for operations that they don't want to permit. That can be used to absolutely forbid loops or writing to global variables.

One other note is that the load functions will generally load either precompiled bytecode or Lua text. It turns out to be a lot safer if you never permit precompiled bytecode, as a number of ways to make the VM misbehave have been identified that all depend on handcrafting invalid bytecode. Since bytecode files begin with a well-defined byte sequence that is not plain ASCII text, all you need to do is read the script into a string buffer, test for the absense of the marker, and only pass it to lua_loadstring() if it is not bytecode.

There has been a fair amount of discussion at the Lua-L mailing list over the years of this kind of thing, so searching there is also likely to be helpful.

Guanase

Thank you; I already took care of infinite loops by setting a debug hook that terminates the script after 10 million instructions. As for the functions, the file provided by the user contains only global variables and no functions whatsoever, so I don't have to whitelist any function. – Yonah

Answer 2

By the way, this is what I ended up doing:

/* Loads, compiles and executes an unstrusted file. */
bool Lua::RunUntrustedFile(const string& path)
{
    if(luaL_loadfile(mState, path.c_str()))
    {
        ErrorLog(lua_tostring(mState, 1));
        Pop(1);
        return false;
    }

    Lua::SetMaximumInstructions(100000000);
    lua_newtable(mState);
    lua_setglobal(mState, "upload");
    ASSERT(Lua::GetStackSize() == 1);
    lua_getglobal(mState, "upload");
    ASSERT_ALWAYS(lua_setfenv(mState, 1) != 0);
    ASSERT(Lua::GetStackSize() == 1);

    if(lua_pcall(mState, 0, 0, 0))
    {
        Lua::ClearMaximumInstructions();
        ErrorLog(lua_tostring(mState, -1));
        Pop(1);
        return false;
    }

    ASSERT(Lua::GetStackSize() == 0);
    Lua::ClearMaximumInstructions();

    return true;
}

"Support" functions:

static void Pop(int elements = 1) { lua_pop(mState, elements); }

/* Sets a maximum number of instructions before throwing an error */
static void SetMaximumInstructions(int count) {
    lua_sethook(mState, &Lua::MaximumInstructionsReached, LUA_MASKCOUNT, count);
}
static void ClearMaximumInstructions() {
    lua_sethook(mState, &Lua::MaximumInstructionsReached, 0, 0);
}

static void MaximumInstructionsReached(lua_State *, lua_Debug *)
{
    Error("The maximum number of instructions has been reached");
}

static int GetStackSize() { return lua_gettop(mState); }

Yonah

Answer 3

luaL_loadfile() will load the chunk, then call lua_setfenv() to set the environment table, then call lua_pcall() to execute the chunk. See recent answer given by Judge Maygarden at Calling lua functions from .lua's using handles?

Weakwilled