Oslo Rootwrap

The Oslo Rootwrap allows fine filtering of shell commands to run as root from OpenStack services.

Rootwrap should be used as a separate Python process calling the oslo.rootwrap.cmd:main function. You can set up a specific console_script calling into oslo.rootwrap.cmd:main, called for example nova-rootwrap. To keep things simple, this document will consider that your console_script is called /usr/bin/nova-rootwrap.

The rootwrap command line should be called under sudo. It's first parameter is the configuration file to use, and the remainder of the parameters are the command line to execute:

sudo nova-rootwrap ROOTWRAP_CONFIG COMMAND_LINE

How rootwrap works

OpenStack services generally run under a specific, unprivileged user. However, sometimes they need to run a command as root. Instead of just calling sudo make me a sandwich and have a blanket sudoers permission to always escalate rights from their unprivileged users to root, those services can call sudo nova-rootwrap /etc/nova/rootwrap.conf make me a sandwich.

A sudoers entry lets the unprivileged 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 the OpenStack service matches one of those filters, in which case it executes the command (as root). If no filter matches, it denies the request. This allows for complex filtering of allowed commands, as well as shipping filter definitions together with the OpenStack code that needs them.

Security model

The escalation path is fully controlled by the root user. A sudoers entry (owned by root) allows the unprivileged user to run (as root) a specific rootwrap executable, and only with a specific configuration file (which should be owned by root) as its first parameter.

nova-rootwrap imports the Python modules it needs from a cleaned (and system-default) PYTHONPATH. The configuration file points to root-owned filter definition directories, which contain root-owned filters definition files. This chain ensures that the unprivileged user itself is never in control of the configuration or modules used by the nova-rootwrap executable.

Installation

All nodes wishing to run nova-rootwrap should contain a sudoers entry that lets the unprivileged user run nova-rootwrap as root, pointing to the root-owned rootwrap.conf configuration file and allowing any parameter after that. For example, Nova nodes should have this line in their sudoers file, to allow the nova user to call `sudo nova-rootwrap`:

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

Then the node also should ship the filter definitions corresponding to its usage of nova-rootwrap. You should not install any other filters file on that node, otherwise you would allow extra unneeded commands to be run as root.

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

Rootwrap configuration

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 in the sudoers entry, and must be provided on nova-rootwrap command line as its first argument.

rootwrap.conf uses an INI file format with the following sections and parameters:

[DEFAULT] section

filters_path

Comma-separated list of directories containing filter definition files. All directories listed must be owned and only writeable by root. This is the only mandatory parameter. Example: filters_path=/etc/nova/rootwrap.d,/usr/share/nova/rootwrap

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. Example: exec_dirs=/sbin,/usr/sbin,/bin,/usr/bin

use_syslog

Enable logging to syslog. Default value is False. Example: use_syslog=True

use_syslog_rfc_format

Enable RFC5424 compliant format for syslog (add APP-NAME before MSG part). Default value is False. Example: use_syslog_rfc_format=True

syslog_log_facility

Which syslog facility to use for syslog logging. Valid values include auth, authpriv, syslog, user0, user1... Default value is syslog. Example: syslog_log_facility=syslog

syslog_log_level

Which messages to log. INFO means log all usage, ERROR means only log unsuccessful attempts. Example: syslog_log_level=ERROR

.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 .filters. 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):

[Filters] section

filter_name (different for each filter)

Comma-separated list containing first the Filter class to use, followed by that Filter arguments (which vary depending on the Filter class selected). Example: kpartx: CommandFilter, /sbin/kpartx, root

Available 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:

kpartx: CommandFilter, kpartx, root

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:

tunctl: /usr/sbin/tunctl, root, tunctl, -b, -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:

chown: PathFilter, /bin/chown, root, nova, /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:

dnsmasq: EnvFilter, env, root, CONFIG_FILE=, NETWORK_ID=, dnsmasq

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`:

read_initiator: ReadFileFilter, /etc/iscsi/initiatorname.iscsi

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:

kill_dnsmasq: KillFilter, root, /usr/sbin/dnsmasq, -9, -HUP

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. ip
2. User to run ip under

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

ip: IpFilter, ip, root

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. ip
2. User to run ip under

Example: allow to run ip netns exec <namespace> <command> as long as <command> matches another filter:

ip: IpNetnsExecFilter, ip, root

ChainingRegExpFilter

Filter that allows to run the prefix command, if the beginning of its arguments match to a list of regular expressions, and if remaining arguments are any otherwise-allowed command. 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.

This filter regards the length of the regular expressions list as the number of arguments to be checked, and remaining parts are checked by other filters.

Example: allow to run /usr/bin/nice, but only with first two parameters being -n and integer, and followed by any allowed command by the other filters:

nice: /usr/bin/nice, root, nice, -n, -?\d+

Note: this filter can't be used to impose that the subcommand is always run under the prefix command. In particular, it can't enforce that a particular command is only run under "nice", since the subcommand can explicitly be called directly.

Calling rootwrap from OpenStack services

The oslo.processutils library ships with a convenience execute() function that can be used to call shell commands as root, if you call it with the following parameters:

run_as_root=True

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

NB: Some services ship with a utils.execute() convenience function that automatically sets root_helper based on the value of a rootwrap_config parameter, so only run_as_root=True needs to be set.

If you want to call as root a previously-unauthorized command, you will also need to modify the filters (generally shipped in the source tree under etc/rootwrap.d so that the command you want to run as root will actually be allowed by nova-rootwrap.