Improving Nova privilege escalation model, part 3
In the previous two posts of this series, we explored the deficiencies of the current model and the features of an alternative implementation. In this last post, we'll discuss the advantages of a Python implementation and open discussion on how to secure it properly.
It's quite easy to implement the features that were mentioned in the previous post in Python. The main advantage of doing so is that the code can happily live inside Nova code, in particular the filters definition files can be implemented as Python modules that are loaded if present. That solves the issue of shipping definitions within Nova and also the separation of allowed commands based on locally-deployed nodes. The code is simple and easy to review. The trick is to make sure that no malicious code can be injected in the elevated rights process. This is why I'd like to present a model and open it for comments in the community.
Proposed security model
The idea would be to have Nova code optionally use "sudo nova-rootwrap" instead of "sudo" as the root_helper. A generic sudoers file would allow the nova user to run /usr/bin/nova-rootwrap as root, while stripping environment variables like PYTHONPATH. To load its filters definitions, nova-rootwrap would try to import a set of predefined modules (like nova.rootwrap.compute), but if those aren't present, it should ignore them. Can this model be abused ?
The obvious issue is to make sure sys.path (the set of directories from which Python imports its modules) is secure, so that nobody can insert their own modules in the process. I've given some thoughts to various checks, but actually there is no way around trusting the default sys.path you're given when you start python as root from a cleaned env. If that's compromised, you're toasted the moment you "import sys" anyway. So using sudo to only allow /usr/bin/nova-rootwrap and cleaning the environment should be enough. Or am I missing something ?
Insecure mode ?
One thing we could do is check that sys.path all belongs to root and refuse to run in the case it's not. That would tell the user that his setup is insecure (potentially allowing him to bypass that by running "sudo nova-rootwrap --insecure" as the root_helper). But that's a convenience to detect insecure setups, not a security addition (the fact that it doesn't complain doesn't mean you're safe, it could mean you're already compromised).
Test mode ?
For tests, it's convenient to allow to run code from branches. To allow this (unsafe) mode, you would tweak sudoers to allow it to run \$BRANCH/bin/nova-rootwrap as root, and prepend ".." to sys.path in order to allow modules to be loaded from \$BRANCH (maybe requiring --insecure mode for good measure). It sounds harmless, since if you run from /usr/bin/nova-rootwrap you can assume that /usr is safe... Or should that idea be abandoned altogether ?
Nothing beats peer review when it comes to secure design. I call all Python module-loading experts and security white-hats out there: would this work ? Are those safe assumptions ? How much do you like insecure and test modes ? Would you suggest something else ? If you're one of those that can't think in words but require code, you can get a glimpse of work in progress here. It will all be optional (and not used by default), so it can be added to Nova without much damage, but I'd rather do it right from the beginning :) Please comment !