Wednesday, August 7, 2019

A second life for the Sandbox

Hi all,

Anvil is a UK-based company sponsoring one month of work to revive PyPy's "sandbox" mode and upgrade it to PyPy3. Thanks to them, sandboxing will be given a second life!

The sandboxed PyPy is a special version of PyPy that runs fully isolated. It gives a safe way to execute arbitrary Python programs (whole programs, not small bits of code inside your larger Python program). Such scripts can be fully untrusted, and they can try to do anything—there are no syntax-based restrictions, for example—but whatever they do, any communication with the external world is not actually done but delegated to the parent process. This is similar but much more flexible than Linux's Seccomp approach, and it is more lightweight than setting up a full virtual machine. It also works without operating system support.

However, during the course of the years the sandbox mode of PyPy has been mostly unmaintained and unsupported by the core developers, mostly because of a lack of interest by users and because it took too much effort to maintain it.

Now we have found that we have an actual user, Anvil. As far as I can tell they are still using a very old version of PyPy, the last one that supported sandboxing. This is where this contract comes from: the goal is to modernize sandboxing and port it to PyPy3.

Part of my motivation for accepting this work is that I may have found a way to tweak the protocol on the pipe between the sandboxed PyPy and the parent controller process. This should make the sandboxed PyPy more resilient against future developments and easier to maintain; at most, in the future some tweaks will be needed in the controller process but hopefully not deep inside the guts of the sandboxed PyPy. Among the advantages, such a more robust solution should mean that we can actually get a working sandboxed PyPy—or sandboxed PyPy3 or sandboxed version of any other interpreter written in RPython—with just an extra argument when calling rpython to translate this interpreter. If everything works as planned, sandboxing may be given a second life.

Armin Rigo

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