Rootwrap

Purpose
The goal of the root wrapper is to allow a service-specific 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 part of (and its format depending on) specific packaging. It did not allow for complex filtering of parameters (advanced filters). The rootwrap was designed to solve those issues.

An Oslo incubator project
Rootwrap code is now maintained in the oslo-incubator. It is reused in multiple OpenStack projects (Nova, Cinder, Neutron...). To keep it simple, this documentation will talk about the nova copy of the code (nova-rootwrap). To apply this documentation to $PROJECT, just replace 'nova' by $PROJECT in every mention below.

Graduating incubator project. git log. (2. Dec. 2013)

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.

Service configuration
You must provide the location of the rootwrap configuration file to Nova, by setting the following in nova.conf:

rootwrap_config=/etc/nova/rootwrap.conf

The configuration file used here must match the one defined in the sudoers entry (see below), otherwise the commands will be rejected ! There is no need to specify the root_helper parameter anymore.

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.

[DEFAULT] filters_path=/etc/nova/rootwrap.d,/usr/share/nova/rootwrap

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.

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 below, in the Reference section.

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 run by compute nodes, they should modify the /etc/nova/rootwrap.d/compute.filters file.

The format of the filter file is defined below, in the Reference section.

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 the Reference section for details.

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 filters.py code for details.

rootwrap.conf
The rootwrap.conf file is used to influence how nova-rootwrap works. Since it's in the trusted security path, it needs to be owned and writeable only by the root user. Its location is specified both in the sudoers entry and in the Nova configuration file.

It uses an INI file format with the following sections and parameters:

exec_dirs Comma-separated list of directories to search executables in, in case filters do not explicitely specify a full path. If not specified, defaults to the system PATH environment variable. All directories listed must be owned and only writeable by root use_syslog Enable logging to syslog. Default value is False syslog_log_facility Which syslog facility to use for syslog logging. Valid values include auth, authpriv, syslog, user0, user1... Default value is 'syslog' syslog_log_level Which messages to log. INFO means log all usage, ERROR means only log unsuccessful attempts

.filters files
Filters definition files contain lists of filters that nova-rootwrap will use to allow or deny a specific command. They are generally suffixed by. Since they are in the trusted security path, they need to be owned and writeable only by the root user. Their location is specified in the rootwrap.conf file.

It uses an INI file format with a [Filters] section and several lines, each with a unique parameter name (different for each filter you define):

See below for parameters to each Filter classes.

CommandFilter
Basic filter that only checks the executable called. Parameters are:


 * 1) Executable allowed
 * 2) User to run the command under

Example: allow to run kpartx as the root user, with any parameters:

RegExpFilter
Generic filter that checks the executable called, then uses a list of regular expressions to check all subsequent arguments. Parameters are:
 * 1) Executable allowed
 * 2) User to run the command under
 * 3) (and following) Regular expressions to use to match first (and subsequent) command arguments

Example: allow to run /usr/sbin/tunctl, but only with three parameters with the first two being -b and -t:

PathFilter
Generic filter that lets you check that paths provided as parameters fall under a given directory. Parameters are:
 * 1) Executable allowed
 * 2) User to run the command under
 * 3) (and following) Command arguments.

There are three types of command arguments: 'pass' will accept any parameter value, a string will only accept the corresponding string as a parameter, except if the string starts with '/' in which case it will accept any path that resolves under the corresponding directory.

Example: allow to chown to the 'nova' user any file under /var/lib/images:

EnvFilter
Filter allowing extra environment variables to be set by the calling code. Parameters are:
 * 1) 'env'
 * 2) User to run the command under
 * 3) (and following) name of the environment variables that can be set, suffixed by '='
 * 4) Executable allowed

Example: allow to run CONFIG_FILE=foo NETWORK_ID=bar dnsmasq ... as root:

ReadFileFilter
Specific filter that lets you read files as root using cat. Parameters are:
 * 1) Path to the file that you want to read as the root user.

Example: allow to run "cat /etc/iscsi/initiatorname.iscsi" as root:

KillFilter
Kill-specific filter that checks the affected process and the signal sent before allowing the command. Parameters are:
 * 1) User to run kill under
 * 2) Only affect processes running that executable
 * 3) (and following) Signals you're allowed to send

Example: allow to send -9 or -HUP signals to /usr/sbin/dnsmasq processes:

IpFilter
ip-specific filter that allows to run any ip command, except for ip netns (in which case it only allows the list, add and delete subcommands). Parameters are:
 * 1) Executable allowed ('ip')
 * 2) User to run ip under

Example: allow to run any ip command except ip netns exec and ip netns monitor:

IpNetnsExecFilter
ip-specific filter that allows to run any otherwise-allowed command under ip netns exec. The command specified to ip netns exec must match another filter for this filter to accept it. Parameters are:
 * 1) Executable allowed ('ip')
 * 2) User to run ip under

Example: allow to run ip netns exec  as long as   matches another filter: