CODEWITHSUNDEEP
rubysecurityevalread-eval-print-loopirb
dsp_099
dsp_099

Reputation: 6121

How to provide an online ruby REPL?

On a site like www.codewars.com, one can run ruby in a sort of sandbox, almost identical to IRB.

How does this actually work?

If the submitted code is eval()d, what's preventing me from submitting a system("rm -rf *") or redefining basic functions so that 50% of the time Array.sort actually runs Array.shuffle?

Upvotes: 1

Views: 202

Answers (1)

Jörg W Mittag
Jörg W Mittag

Reputation: 369458

The simplest and safest solution is to run the Ruby code on separate computer, which you wipe and re-install after every run. This is, however, also a pretty heavyweight solution.

More lightweight, but (almost) as safe, would be using a virtual machine or a container instead of a whole separate computer, and e.g. using a read-only filesystem with a ramfs overlay, which you umount after every run. (Or just throw away and recreate the container.)

You could also use JRuby together with the JVM's security features (or IronRuby with the CLI's). The JVM has sandboxing features for JVM programs, and after all, JRuby is just a Java program like any other.

Lastly, you could write your own Ruby implementation with sandboxing in mind, or modify an existing one. The three options above are fairly simple, this one is hard, because most Ruby implementations aren't designed for sandboxing. TryRuby.Com worked this way, for example, and it took a significant amount of time to update it for Ruby 1.9, because it was originally based on a modified version of MRI, but MRI doesn't support Ruby 1.9. So, the implementation had to be switched to YARV, and a lot of the modifications to make it sandboxing-safe had to re-implemented from scratch. (The JRuby/IronRuby option above is similar to this, but you push off the work of making the implementation sandbox-safe to someone else, e.g. Oracle or Microsoft.)

A not-so-safe but also simple solution would be to run the interpreter under a restricted user account.

Of course, you can combine multiple approaches for defense-in-depth, for example, running a sandboxed interpreter under a restricted user account on a separate VM.

What does not work is to statically analyze the code before running it. The pesky Halting Problem bites us here.

Upvotes: 2

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