Capsicum

A simple FFI wrapper around the Capsicum OS capability and sandbox framework.

Installation

A Capsicum-enabled OS is, of course, required. FreeBSD 10+ (or derivative), possibly capsicum-linux.

Add this line to your application's Gemfile:

gem 'capsicum'

And then execute:

$ bundle

Or install it yourself as:

$ gem install capsicum

Usage

Basic synopsis:

Capsicum.sandboxed?    # => false
Capsicum.enter!        # => true
Capsicum.sandboxed?    # => true

File.new("/dev/null")  # => Errno::ECAPMODE: Not permitted in capability mode @ rb_sysopen - /dev/null
TCPSocket.new("0", 80) # => Errno::ECAPMODE: Not permitted in capability mode - connect(2) for "0" port 80
`rm -rf /`             # => Errno::ECAPMODE: Not permitted in capability mode - rm
system "rm -rf /"      # => nil
require 'time'         # => LoadError: cannot load such file -- time

i.e. anything that involves opening a file, connecting a socket, or executing a program is verboten. Kinda.

On fork-capable Rubies, you can also do this:

Capsicum.sandboxed?   # => false

status = Capsicum.within_sandbox do
  Capsicum.sandboxed? # => true
  exit 42
end

Capsicum.sandboxed?   # => false
status.exitstatus     # => 42

The result is a Process::Status object.

But How Can I get Anything Done?

Open your files and sockets before entering the sandbox. If you have a TCPServer open, for example, you can still call #accept on it, so a useful server could conceivably run within it.

You can open new files, but this requires access to *at() syscalls. If Ruby supported them, it might look something like this:

dir = Dir.open("/path/to/my/files")

Capsicum.enter!

file = File.openat(dir, "mylovelyfile")
File.renameat(dir, "foo", dir, "bar")
File.unlinkat(dir, "moo")

Unfortunately, it doesn't. See https://bugs.ruby-lang.org/issues/10181

You may consider spawning off workers, maintaining a privileged master process, and using IPC to communicate with them.

Todo

Wrap Casper to provide DNS services, additional rights controls, etc.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/Freaky/ruby-capsicum.

License

The gem is available as open source under the terms of the MIT License.