Fri, 25 Mar 2016 12:15:20 GMT

More sandboxing

When working on untrusted code or data it's impossible to predict what happens when one does a:

bundle install --path=vendor

or

npm install

Does this phone out your private SSH and GPG keys? Does a

evince Downloads/justdownloaded.pdf

try to exploit the PDF viewer? While you can run stuff in separate virtual machines this can get cumbersome. libvirt-sandbox to the rescue! It allows to sandbox applications using libvirt's virtualization drivers. It took us a couple of years (The ITP is from 2012) but we finally have it in Debian's NEW queue. When libvirt-sandbox creates a sandbox it uses your root filesystem mounted read only by default so you have access to all installed programs (this can be changed with the --root option though). It can use either libvirt's QEMU or LXC drivers. We're using the later in the examples below:

So in order to make sure the above bundler call has no access to your $HOME you can use:

sudo virt-sandbox \
   -m ram:/tmp=10M \
   -m ram:$HOME=10M \
   -m ram:/var/run/screen=1M \
   -m host-bind:/path/to/your/ruby-stuff=/path/to/your/ruby-stuff \
   -c lxc:/// \
   -S $USER \
   -n rubydev-sandbox \
   -N dhcp,source=default \
   /bin/bash

This will make your $HOME unaccessible by mounting a tmpfs over it and using separate network, ipc, mount, pid and utc namespaces allowing you to invoke bundler with less worries. /path/to/your/ruby-stuff is bind mounted read-write into the sandbox so you can change files there. Bundler can fetch new gems using libvirt's default network connection.

And for the PDF case:

sudo virt-sandbox \
  -m ram:$HOME=10M \
  -m ram:/dev/shm=10M \
  -m host-bind:$HOME/Downloads=$HOME/Downloads \
  -c lxc:/// \
  -S $USER \
  -n evince-sandbox \
  --env="DISPLAY=:0" \
  --env="XAUTHORITY=$XAUTHORITY" \
  /usr/bin/evince Downloads/justdownloaded.pdf

Note that the above example shares /tmp with the sandbox in order to give it access to the X11 socket. A better isolation can probably be achieved using xpra or xvnc but I haven't looked into this yet.

Besides the command line program virt-sandbox there's also the library libvirt-sandbox which makes it simpler to build new sandboxing applications. We're not yet shipping virt-sandbox-service (a tool to provision sandboxed system services) in the Debian packages since it's RPM distro specific. Help on porting this to Debian is greatly appreciated.