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A Secured Sandboxing Gem for Ruby

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README.md

Secure is the ruby sandboxing gem that powers http://rubymonk.com

Copyright (c) Tejas Dinkar and C42 Engineering

How To Install:

$ gem install secure

or add the following to your Gemfile

gem 'secure'

API Documentation:

Secure.ly do
  File.read("some file")
end

You can pass options to tweak what security checks are put in place. If the option is not there, then the security check is not put in place by default

Secure.ly
  :timeout => 0.15
  :limit_memory => 10000000
  :limit_cpu => 2
  :pipe_stdout => File.open("foo", "w") do
  # Some secure operation here
end

Options:

  • :timeout => Guard thread that monitors the child process. If this elapses, this raises a Secure::TimeoutError
  • :limit_memory => This is an absolute value of how much memory your block can take in bytes. Remember, absolute. I'll be getting relative support in soon
  • :limit_cpu => This is the limit of how many cpu-seconds your process can use. MUST be an integer. This should be used as a fallback in case :timeout is not honored
  • :run_before => A block, or array of blocks that is run before your code is sandboxed. Be careful. Remember how lambdas are bound in ruby. Refer to this for more details: http://blog.sidu.in/2007/11/ruby-blocks-gotchas.html
  • :pipe_stdin, :pipe_stdout, :pipe_stderr => A File to pipe the stdin, out ond stderr to
  • :safe => An integer that represents the new safe mode (default 3)
  • :limit_files => Maximum file descriptor the block can open. If you want to say no files, set this to 0
  • :limit_procs => Maximum number of processes that the user can create. Set this to 0 if you want to ensure no one forks

Errors:

  • Secure::TimeoutError => This is thrown if the :timeout limit is reached. The stack trace will be whatever line of code the app was running at the time
  • Secure::ChildKilledError => This is thrown if one of the kernel level checks cause the child to die. The stack trace for this exception will be junk
  • SecurityError => This is thrown if ruby tries to execute some code which is not allowed. The stack trace will help you figure out what was in violation
  • Any other Error will be thrown as if it had happened in the parent process. We do our best to preserve the stack trace.

How Does it work:

  • Secure runs your ruby code in SAFE mode 3, which prevents evaluation of tainted strings and opening of new files.
  • It also puts in kernel level RLIMIT checks, to make sure that your ruby process behaves itself
  • It also spawns a monitoring thread, to make sure the thread doesn't take too long
  • Secure runs in a new process, so people can screw up the Ruby tree as much as they like :-)

Known Issues:

  • :limit_memory and :limit_procs does not work on OSX (at least whatever version I use), but it does work on linux
  • :pipe_stdout is not tested because of some rspec weirdness
  • A block bound before $SAFE is set sees the old safe value. Refer to this for some clue about the reason why this happens: http://blog.sidu.in/2007/11/ruby-blocks-gotchas.html
  • Stdout cannot be piped to a StringIO. You need to open a unix PIPE. There are two reasons for this. The code runs in a child process, so you need to use and IPC mechanism, and a string IO is not recognized as a file at the C level

Soon:

  • Getting rid of SAFE level 3, and moving everything into the kernel space. cgroups sounds hopeful here. As does more rlimit stuff

Performance:

RubyMonk is backed by an code evaluation server that uses secure gem in the backend. A single small (EC2) instance was able to consistently handle 150 code evaluation requests per minute, and we were able to horizontally scale when load went above this. YMMV

Contributing:

Feel free to file bugs. However, if it is a security issue, we appreciate it if you shoot me a mail at tejas@c42.in before you file a bug.

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