SELinux/Role-based access control

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To provide segregation of duties and privileges based on the least privilege model, many organizations use a role-based implementation for security and privileges. Some roles are not allowed to perform tasks of another role (segregation of duties), and adding privileges to roles is handled with care to make sure people who are assigned the role do not get too many privileges.

Introduction

In a true RBAC situation, people are assigned one or more roles which grant or deny particular access.

In an RBAC model, there are a couple of important aspects to have in an implementation.

  • Permissions are always granted through roles - no direct assignation to users
  • Users must be explicitly granted roles - no role, no rights

RBAC by itself is not all that hard to implement. On a Linux system, one can make most, if not all of its behavior based on role assignment done through group membership and group privileges.

RBAC in SELinux

The implementation of Role Based Access Control (RBAC) in SELinux is as follows.

SELinux users.png

  1. A user is mapped to a SELinux user, which (amongst some other things) defines the maximum clearance of that user
  2. The SELinux user is allowed one or more roles, effectively restricting which roles a particular user can participate in (segregation of duties)
  3. Roles are allowed certain domains, run-time privileges for one or more applications

It is through the domain permissions that the privileges of a user are controlled. Consider the privileges to administer the web server. A role that isn't allowed a domain with these privileges will not have the ability to run the necessary applications, effectively restricting the ability of the user to administer the web server.

Assigning roles to users

Because a role depends on the SELinux user, and a Linux user is mapped to a SELinux user, assigning roles begins with mapping a Linux user to a SELinux user. As SELinux users are immutable, it is important to differentiate based on role requirements: if two users have different role requirements (user1 needs roleA and roleB, and user2 needs roleB and roleC) then they should not be mapped to the same SELinux user, as this would meant that those users can access a role they are not allowed to.

Assigning permissions to roles and domains

In order to assign a domain (or multiple domains) to a role, the SELinux policy needs to be updated to reflect this. For instance, to allow a role oper_r to manage the Zabbix monitoring infrastructure, the following policy would be defined:

CODE Enabling zabbix administration to the oper_r role
zabbix_admin(oper_t, oper_r)

This simple call allows the oper_t user domain (the default domain that users in the oper_r have as their logon or runtime domain) to administer the Zabbix environment, as well as stop/start/restart the service and edit the Zabbix infrastructure resources.

Permissions or domain transitions

Depending on the purpose and the policy design, either the permissions of a user domain are enhanced (such as with the zabbix_admin example), or the role is allowed to transition to another domain. The latter is in use more when applications are granted (in which case a domain transition is allowed towards the application domain).

Default roles

By default, SELinux offers the following roles:

Role Description
user_r End user role meant to be assigned to users with interactive logon privileges to the system, but without additional role requirements
staff_r End user role meant to be assigned to users who also need to transition to other roles. The staff_r role by itself isn't much more privileged than user_r except that it is allowed to do role transitions
sysadm_r Powerful role for generic system administration
system_r System role meant for daemons and system services
object_r Unassignable role meant for resources rather than domains

Additional roles can be created through policies.