inconsistency: disallow `new Function()` eval() loophole

[iameli]

Ajv, the JSON-Schema validator doesn't work on Cloudflare Workers.. It relies on eval()'d code generation, and CF workers don't allow eval().

I was surprised to find that it worked perfectly fine in Cloudworker. I think it's because ajv doesn't actually use eval(), it uses the new Function(<string>) constructor, which we patch into the runtime environment here.

For consistency, we should find a way to disable that mechanism without breaking the foo instanceof Function construct.

[iameli]

Hmm, a naive test shows that new Function() doesn't actually work here. Interesting. EIther AJV started working in contexts that disallow code generation or something else is going on... investigating further.

[iameli]

Minimal repro case:

import Ajv from "ajv";

const handleEvent = () => {
  try {
    var schema = {
      type: "string"
    };
    var ajv = new Ajv(); // options can be passed, e.g. {allErrors: true}
    var validate = ajv.compile(schema);
    var valid = validate("foo");
    return new Response(`valid: ${valid}`);
  } catch (e) {
    return new Response(e.stack, { status: 500 });
  }
};

addEventListener("fetch", event => {
  event.respondWith(handleEvent());
});

I'm building that with parcel build -t browser --no-minify worker.js. It works with Cloudworker, but in CF itself I get

EvalError: Code generation from strings disallowed for this context
    at new Function (<anonymous>)
    at Ajv.localCompile (worker.js:2946:26)
    at Ajv.resolve (worker.js:2015:19)
    at Object.resolveRef (worker.js:3015:21)
    at Object.generate_ref [as code] (worker.js:3408:22)
    at Object.generate_validate [as validate] (worker.js:2615:37)
    at Object.generate_properties [as code] (worker.js:5344:26)
    at generate_validate (worker.js:2712:35)
    at localCompile (worker.js:2914:22)
    at Ajv.compile (worker.js:2881:13)

Spooky. How is AJV sneaking past vm.createContext(context, {codeGeneration: {strings: false}})?

[iameli]

Okay, confirmed my original hypothesis - new Function("true"); seems to get around the ban on code generation.

[xtuc]

I added a few cases in your PR that should be disallowed. The rule is any evaluation of any generated code is disallowed (wasm is one non obvious case).