gavid
Can you break this "safe" interpreter?
I wanted to create a way to “safely” evaluate a string containing Elixir code. The code in the string should not be allowed to use any functions or modules (not even those from the Kernel e.g. defmodule/2).
Have I succeeded, or are there still ways to create a string that will allow the function to succeed while violating my criteria above?
def safe_eval(code) when is_binary(code) do
quoted_form = Code.string_to_quoted!(code, existing_atoms_only: true)
Macro.postwalk(quoted_form, fn node ->
if is_tuple(node) and (Kernel.elem(node, 0) in [:__aliases__]) do
raise "Cannot use aliases or references to other modules"
end
end)
{evaluated, _bindings} = Code.eval_string(code, [], %Macro.Env{})
{"ok", evaluated}
end
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jhogberg
Try "<<0::size(123456789123456789)>>" 
Why does it have to be Elixir code? If you can’t call any functions or modules then it sounds restricted enough that you could probably do what you want with a simple DSL instead, and writing an interpreter for a (say) small Lisp-like DSL doesn’t take very long.
adamu
Have you seen Dune - Sandbox for Elixir?
jhogberg
I guess it depends on the threat model, the underlying :erl_eval module wasn’t designed to be safe against a determined attacker: at the very least it’ll be possible for them to exhaust system resources.
If it’s just about guarding against accidental calls to other modules and the like, then I think a better approach would be to use the local/non-local function handler functionality in :erl_eval to filter calls, as nothing can sneak through that (as @hst337 pointed out, dynamic calls still sneak through yours). Code.eval_string/3 doesn’t expose this functionality (yet?) so it’ll be a bit of a slog to copy and modify Code.eval_string/3 + :elixir.eval_forms/4, but it can be done.
… but you might not have to if Dune is good enough, as pointed out by @adamu
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