Using pam_selinux to switch contexts

With SELinux managing the access controls of applications towards the resources on the system, a not-to-be forgotten important component on any Unix/Linux system is the authentication part. Most systems use or support PAM, the Pluggable Authentication Modules, and for SELinux this plays an important role.

Applications that are PAM-enabled use PAM for the authentication of user activities. If this includes setting up an authenticated session, then the “session” part of the PAM configuration is also handled. And for SELinux, this is a nice-to-have, since this means applications that are not SELinux-aware can still enjoy transitions towards specified domains depending on the user that is authenticated.

The “not SELinux-aware” here is important. By default, applications keep running in one security context for their lifetime. If they invoke a execve or similar call (which is used to start another application or command when used in combination with a fork), then the SELinux policy might trigger an automatic transition if the holy grail of fourfold rules is set:

  1. a transition from the current context to the new one is allowed
  2. the label of the executed command/label is marked as an entrypoint for the new context
  3. the current context is allowed to execute that application
  4. an automatic transition rule is made from the current context to the new one over the command label

Or, in SELinux policy terms, assuming the domains are source_t and destination_t with the label of the executed file being file_exec_t:

allow source_t destination_t:process transition;
allow destination_t file_exec_t:file entrypoint;
allow source_t file_exec_t:file execute;
type_transition source_t file_exec_t : process destination_t;

If those four settings are valid, then (and only then) can the automatic transition be active.

Sadly, for applications that run user actions (like cron systems, remote logon services and more) this is not sufficient, since there are two major downsides to this “flexibility”:

  1. The rules to transition are static and do not depend on the identity of the user for which activities are launched. The policy can not deduce this identity from a file context either.
  2. The policy is statically defined: different transitions based on different user identities are not possibel.

To overcome this problem, applications can be made SELinux-aware, linking with the libselinux library and invoking the necessary switches themselves (or running the commands with runcon). Luckily, this is where the PAM system comes to play to aide us in setting up this policy behavior.

When an application is PAM-enabled, it will invoke PAM calls to authenticate and possibly set up the user session. The actions that PAM invokes are defined by the PAM configuration files. For instance, for the at daemon:

## /etc/pam.d/atd
#
# The PAM configuration file for the at daemon
#

auth    required        pam_env.so
auth    include         system-services
account include         system-services
session include         system-services

I am not going to dive into the details of PAM in this blog post, so let’s just jump to the session management part. In the above example file, if PAM sets up (or shuts down) a user session for the service (at in our case), it will go through the PAM services that are listed in the system-services definition, which looks like so:

## /etc/pam.d/system-services
auth            sufficient      pam_permit.so
account         include         system-auth
session         optional        pam_loginuid.so
session         required        pam_limits.so 
session         required        pam_env.so 
session         required        pam_unix.so 
session         optional        pam_permit.so

Until now, nothing SELinux-specific is enabled. But if we change the session section of the at service to the following, then the SELinux pam module will be called as well:

session optional        pam_selinux.so close
session include         system-services
session optional        pam_selinux.so multiple open

Now that the SELinux module is called, pam_selinux will try to switch the context of the process based on the definitions in the /etc/selinux/strict/contexts location (substitute strict with the policy type you use). The outcome of this switching can be checked with the getseuser application:

~# getseuser root system_u:system_r:crond_t
seuser:  root, level (null)
Context 0       root:sysadm_r:cronjob_t
Context 1       root:staff_r:cronjob_t

By providing the contexts in configurable files in /etc/selinux/strict/contexts, a non-SELinux aware application suddenly becomes SELinux-aware (through the PAM support it already has) without needing to patch or even rebuild the application. All that is need is to allow the security context of the application to switch ids and roles (as that is by default not allowed), which I believe is offered through the following statements:

domain_subj_id_change_exemption(atd_t)
domain_role_change_exemption(atd_t)

selinux_validate_context(atd_t)
selinux_compute_access_vector(atd_t)
selinux_compute_create_context(atd_t)
selinux_compute_relabel_context(atd_t)
selinux_compute_user_contexts(atd_t)

seutil_read_config(atd_t)
seutil_read_default_contexts(atd_t)

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