New nova-rootwrap landing in folsom-2

This Thursday we will publish our second milestone of the Folsom cycle for Nova. It will include a number of new features, including the one I worked on: a new, more configurable and extensible nova-rootwrap implementation. Here is what you should know about it, depending on whether you're a Nova user, packager or developer !



The goal of the root wrapper is to allow the nova unprivileged user to run a number of actions as the root user, in the safest manner possible. Historically, Nova used a specific sudoers file listing every command that the nova user was allowed to run, and just used sudo to run that command as root. However this was difficult to maintain (the sudoers file was in packaging), and did not allow for complex filtering of parameters (advanced filters). The rootwrap was designed to solve those issues.

How rootwrap works

Instead of just calling sudo make me a sandwich, Nova calls sudo nova-rootwrap /etc/nova/rootwrap.conf make me a sandwich. A generic sudoers entry lets the nova user run nova-rootwrap as root. nova-rootwrap looks for filter definition directories in its configuration file, and loads command filters from them. Then it checks if the command requested by Nova matches one of those filters, in which case it executes the command (as root). If no filter matches, it denies the request.

Security model

The escalation path is fully controlled by the root user. A sudoers entry (owned by root) allows nova to run (as root) a specific rootwrap executable, and only with a specific configuration file (which should be owned by root). nova-rootwrap imports the Python modules it needs from a cleaned (and system-default) PYTHONPATH. The configuration file (also root-owned) points to root-owned filter definition directories, which contain root-owned filters definition files. This chain ensures that the nova user itself is not in control of the configuration or modules used by the nova-rootwrap executable.

Rootwrap for users: Nova configuration

Nova must be configured to use nova-rootwrap as its root_helper. You need to set the following in nova.conf:

root_helper=sudo nova-rootwrap /etc/nova/rootwrap.conf

The configuration file (and executable) used here must match the one defined in the sudoers entry (see below), otherwise the commands will be rejected !

Rootwrap for packagers

Sudoers entry

Packagers need to make sure that Nova nodes contain a sudoers entry that lets the nova user run nova-rootwrap as root, pointing to the root-owned rootwrap.conf configuration file and allowing any parameter after that:

nova ALL = (root) NOPASSWD: /usr/bin/nova-rootwrap /etc/nova/rootwrap.conf *

Filters path

Nova looks for a filters_path in rootwrap.conf, which contains the directories it should load filter definition files from. It is recommended that Nova-provided filters files are loaded from /usr/share/nova/rootwrap and extra user filters files are loaded from /etc/nova/rootwrap.d.


Directories defined on this line should all exist, be owned and writeable only by the root user.

Filter definitions

Finally, packaging needs to install, for each node, the filters definition file that corresponds to it. You should not install any other filters file on that node, otherwise you would allow extra unneeded commands to be run by nova as root.

The filter file corresponding to the node must be installed in one of the filters_path directories (preferably /usr/share/nova/rootwrap). For example, on compute nodes, you should only have /usr/share/nova/rootwrap/compute.filters. The file should be owned and writeable only by the root user.

All filter definition files can be found in Nova source code under etc/nova/rootwrap.d.

Rootwrap for plug-in writers: adding new run-as-root commands

Plug-in writers may need to have the nova user run additional commands as root. They should use nova.utils.execute(run_as_root=True) to achieve that. They should create their own filter definition file and install it (owned and writeable only by the root user !) into one of the filters_path directories (preferably /etc/nova/rootwrap.d). For example the foobar plugin could define its extra filters in a /etc/nova/rootwrap.d/foobar.filters file.

The format of the filter file is defined here.

Rootwrap for core developers

Adding new run-as-root commands

Core developers may need to have the nova user run additional commands as root. They should use nova.utils.execute(run_as_root=True) to achieve that, and add a filter for the command they need in the corresponding etc/nova/rootwrap.d/ .filters file in Nova's source code. For example, to add a command that needs to be tun by network nodes, they should modify the etc/nova/rootwrap.d/network.filters file.

The format of the filter file is defined here.

Adding your own filter types

The default filter type, CommandFilter, is pretty basic. It only checks that the command name matches, it does not perform advanced checks on the command arguments. A number of other more command-specific filter types are available, see here.

That said, you can easily define new filter types to further control what exact command you actually allow the nova user to run as root. See nova/rootwrap/ for details.

This documentation, together with a reference section detailing the file formats, is available on the wiki.