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The whole point of sandboxing is that the entire global state has read-only entries and read-only metatables. You have to maintain this yourself. All of the built-in libraries have this property (once Luau can't possibly expect to cover all edge cases where sandboxing might be broken; for example, In my own implementation of Luau i've set it to set everything as read only after all values to a table have been set but before it has been added to the global state: lua_createtable(L, 0, 8);
// ....
lua_pushlstring(L, classMeta._name.data(), classMeta._name.length());
lua_rawsetfield(L, -2, "__type");
// Hide metatable behind a read-only empty table.
// Luau's `setmetatable` will hide the real metatable if the `__metatable` field is set.
lua_createtable(L, 0, 0);
lua_setreadonly(L, -1, true);
lua_rawsetfield(L, -2, "__metatable");
// Metatables should be read-only to prevent hijacking :)
lua_setreadonly(L, -1, true);
lua_setuserdatametatable(L, tag, -1);
lua_setuserdatadtor(L, tag, classMeta._destructor); |
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Thanks for your reply! This is insightful and I'll keep your steps in mind when I'm implementing my sandboxing.
I agree that they absolutely can't expect to do so given foolish C code, but I would at least hope they could fully sandbox edge cases where Lua code could break sandboxing. But it's possible that my usecase is too far removed from the Luau team's usecase. My goal is to execute user-sourced Lua code, sandbox the resulting global state, and then execute user-sourced functions. |
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If I sandbox an environment with the following code,
a.b.xis successfully incremented, and then an error is thrown at the point whena.xwould be incremented. Is this intentional on the part of the Luau team?(For my use case -- user scripting with savestates that don't include any Luau state -- I'll need to implement deep sandboxing downstream, but I could do that in my client application, or I could do it by modifying the
luaL_sandboxfunctions and submitting a PR. This should be trivial to implement with a depth-first algorithm, usinggetreadonlyto prevent cycles.)(The same question applies for closures.)
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