Hacker News Reader

Hacker News

Comment by phleet | Hacker News Reader

Parent Full thread

phleet

The first thing I always try on sites like these is stuff like this:

__import__('commands').getstatusoutput('ls /')

__import__('subprocess').call(["ls", "-l"])

which gets blocked by the interpreter somehow with

exceptions.OSError - [Errno 11] Resource temporarily unavailable

I'm curious as to how you managed to do this - I've always been interested in how to sandbox something like this.

View on HN

My very favorite Python sandboxing method is the one used by repl.it:

http://repl.it/languages/Python

They compiled CPython with Emscripten, and it gets run on your browser.

This particular behaviour can occur because the process is disallowed to call fork() and can be done with setrlimit() (see RLIMIT_NPROC). There should be other protections, though, because forking a "ls" is not the only way to access the filesystem.

Ah, interesting.

File system access isn't blocked completely:

__import__("os").listdir("/evaluate")

open("/evaluate/test.py").readlines()

Execv'ing processes is OK as long as you don't fork:

    __import__("os").execv("/usr/bin/uname", ["uname", "-a"])
    Linux ip-10-196-3-111 2.6.32-amazon-xen-r3 #1 SMP Mon Jan 16 21:03:16 PST 2012 i686 GNU/Linux

As for the actual files, there are a few clues that a chroot is created for every request : /proc is not mounted, /etc is minimal (root + 1 user in passwd) and "ls -id /" returns a new inode number every time.

Yeah, we are using chroot (along with other things) to sandbox things on a per request basis.

- Tejas from Team PythonMonk (I built the sandboxing stuff)

__import__("os").execv("/usr/bin/env", ["env"])

Gives you a few clues as well

adam.py... :) If only I knew more about bytecode...

  import inspect
  import pprint
  pp = pprint.PrettyPrinter(depth=6)
  f = inspect.currentframe()
  c = 0
  while f is not None:
      c += 1
      if c == 20:
          print f.f_code
          pp.pprint(dict(inspect.getmembers(f.f_code)))
      f = f.f_back

Reply on news.ycombinator.com