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FreeIPAv2:Integration with SELinux

FreeIPAv2 Development Documentation

Please note that this content was made during FreeIPAv2 development and it may no longer be relevant for current versions of FreeIPA. We left the content here for study and archaeological purposes.

Please check our Documentation for a recent list of topics.

Overview

To support SELinux as an application the IPA shall have several different kinds of plug-ins implemented. This article talks about these plug-ins from the prospective of FreeIPA v2 plans.

Design

We agreed that SELinux application would heavily leverage the policy engine, role engine and pluggable PAM responder. The fact that PAM responder is not planned to be pluggable in v2 complicates things a bit. We agreed that we will hard code SELinux related functionality in PAM responder in v2 and refactor it into a sepaFreeIPAv2:rate plug-in in v3 time frame.

Policy Engine

So far we have identified three different areas where the IPA policy engine can help to manage a large number of hosts running SELinux.

  • download, install or remove SELinux policy modules
  • configure SELinux policy modules via SELinux policy booleans
  • manage the relation of Linux users to SELinux users (role management?)

These three areas map well to the policy section, action, configuration and role respectively, we have identified. See FreeIPAv2:Overall Design of Policy Related Components for more details.

Download, install or remove SELinux policy modules with IPA action policy

An IPA action policy has three parts, <condition>, <file> and <run> (see FreeIPAv2:Overall Design of Policy Related Components for more details). In the context of SELinux we can for example check with the <condition> if SELinux is enabled at all and if yes download a SELinux policy module and install it. The main section of a corresponding IPA policy file might look like

  <ipaaction>
    <condition>
     <command>/bin/cat /etc/redhat-release</command>
     <expected_output>Fedora release 9 (Sulphur)</expected_output>
    </condition>
    <file>
      <url>http://my.server.org/my_selinux_policy.pp</url>
      <path>/tmp/my_selinux_policy.pp</path>
      <owner>root</owner>
      <group>root</group>
      <access>0400</access>
      <cleanup>yes</cleanup>
    </file>
    <run>
      <command>/usr/sbin/semodule -i /tmp/my_selinux_policy.pp</command>
      <user>root</user>
    </run> 
  </ipaaction>

This example shows how to download the binary SELinux policy module and calling semodule to install it.

XSLT Processing

This IPA action policy will be processed as any other IPA action policy.

Configure SELinux policy modules via SELinux policy booleans with IPA config policy

SELinux policies can include conditional section to allow runtime modification without having to load a new policy. These conditional sections can be enable or disabled with the help of SELinux policy booleans.

SELinux policy booleans are simple boolean variable which can be manipulated with semanage or get/set/togglesebool. Call 'semanage boolean -l' to get a list and a short description of all SELinux policy booleans currently know to the system.

To make the SELinux policy booleans accessible to IPA a configuration policy, selinux_booleans, is used. The main section of an IPA XML policy file may look like this:

  <ipaconfig>
    <selinux_boolean>
      <name>webadm_manage_user_files</name>
      <value>true</value>
    </selinux_boolean>
    <selinux_boolean>
      <name>ssh_sysadm_login</name>
      <value>false</value>
    </selinux_boolean>
  </ipaconfig>

XSLT Processing

With the help of a XSL template the IPA config policy will be transformed to a format suitable as a command line argument for setseboot. The policy downloader will than call setsebool with the result as defined by XSL metadata

  <md:output_handler>
    <exec_with_args command_name="/usr/sbin/setsebool" user="root"/>
  </md:output_handler>

Manage the relation of Linux users to SELinux users with IPA role policy

To give or take certain privileges to/from a user with the help of SELinux we have to connect three SELinux element, namely

  • the Linux user, e.g. luser
  • the SELinux user, e.g. seuser_u
  • the SELinux role, e.g. serole_r

(It is a widespread convention to use suffix _u for SELinux users and _r for SELinux roles.)

SELinux roles are defined by the SELinux policy or SELinux policy modules. SELinux users can be connected to SELinux role with semanage:

/usr/sbin/semanage user -a -R "serole_r guest_r" seuser_u

Linux user can mapped to SELinux user with semanage, too.

/usr/sbin/semanage login -s seuser_u luser

While the relation of SELinux users to roles should be created when the corresponding SELinux policy modules is installed (see above), the management of Linux users and SELinux users can be handled by IPA policies. The simple mapping of a Linux user to a SELinux user can be done with the help of an IPA action policy or using role defintion policy. We will concentrate here an a role based approach where a Linux user can be mapped with the help of pam_selinux to different SELinux users depending if he logs in via an insecure, e.g. telnet, or secure, e.g. console login, channel. The following IPA role definition policy specifies three roles: "guest", "user" and "admin", with different settings. Users which have the "guest" role on a specific host will always associated with guest_u, independent of the channel they are using to access the host. If a user logs in via ssh and is associated to the admin role he will be mapped to staff_u.

  <iparole>
    <role>
      <name>guest</name>
      <default_context>
        <selinux_user>guest_u</selinux_user>
        <mls>S0</mls>
      </default_context>
    </role>

    <role>
      <name>user</name>
      <default_context>
        <selinux_user>guest_u</selinux_user>
        <mls>S0</mls>
      </default_context>
      <context>
        <service>ssh</service>
        <service>console</service>
        <selinux_user>user_u</selinux_user>
        <mls>S0</mls>
      </context>
    </role>

    <role>
      <name>admin</name>
      <default_context>
        <selinux_user>guest_u</selinux_user>
        <mls>S0</mls>
      </default_context>
      <context>
        <service>ssh</service>
        <selinux_user>staff_u</selinux_user>
        <mls>S0</mls>
      </context>
      <context>
        <service>login</service>
        <selinux_user>staff_u</selinux_user>
        <mls>S0-S15</mls>
      </context>
    </role>
  </iparole>

XSLT Processing

The IPA role policy will be transformed to an LDIF format and written into the LDB of the IPA client. Further processing has to be done elsewhere, because the XSLT engine has no knowledge of the user-host-role association.

pam_ipa and PAM Responder

All the work for the pam_ipa is actually done in the PAM responder. At the login moment the PAM responder will implement a specific SELinux logic. Later in v3 it will be replaced with a proper plug-in. The IPA's PAM module will be higher in the stack than pam_selinux and will prepare information for it to consume.

This logic will follow the following algorithm:

  • If the user logging in is not known to IPA, e.g. a local account, return PAM_USER_UNKNOWN and let the the following PAM modules decide
  • Get IPA pam_selinux role for user logging into the machine (this is done via same internal calls as used to satisfy requests from EXT library).
  • If there is no role for the user return PAM_USER_UNKNOWN and let the the following PAM modules decide (maybe it would make sense to have an configurable switch to force role association on certain host, in this case PAM_AUTH_ERR/PAM_ABORT can be returned).
  • Get IPA pam_selinux role definition from the LDB (or other file if we decide it is better to use a different storage)
  • Create a file for this user in the following format:
   <value of SELinux user from IPA role>:<service from IPA role>:<MLS value from IPA role>

with the example above it would look like this:

   guest_u:*:S0
   staff_u:ssh:S0
   staff_u:login:S0-S15
  • Put this file into
   /etc/SELinux/targeted/login/<user name>/seusers

pam_selinux

The current version of pam_selinux will only use /etc/selinux/targeted/seusers but coming version of SELinux PAM will be modified to look for

   /etc/selinux/targeted/logins/<user name>/seusers

file before falling back to default

   /etc/selinux/targeted/seusers

If there is no seusers file under specific user name directory or directory does not exist at all then the default seusers files from /etc/selinux/targeted/seusers will be used. If the file exists and pam_selinux cannot decide what to do based content of the file the authentication should fail and not use the default from /etc/selinux/targeted/seusers.