SELinux/Tutorials/Linux services and the system u SELinux user

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Linux services and the system_u SELinux user

So now we know that the security context of a process contains a part called the SELinux user, one SELinux role and the SELinux domain that the process runs in. We have also talked a bit about SELinux users and that they are somewhat immutable (the user cannot change the SELinux user). Well, in this tutorial we'll talk about an exception to this rule, mainly when managing Linux services.

The system_u SELinux user

The system_u SELinux user is meant to be used for running system services. It should not be assigned to end users. On most servers thus, most daemons will be running as the system_u SELinux user, most likely (or even solely?) in the system_r role.

This is because SELinux is primarily a type enforcement mandatory access control system, thus most access controls are situated around the domain part (i.e. the third field in the context, the one ending in _t, like sshd_t). For these daemons, no additional access controls or constraints should be declared, so they run as the system_u SELinux user and system_r SELinux role.

It is for user domains (local users) that SELinux user and SELinux role constraints are positioned more, as we want to govern the activities that local users can do, as local user access is often considered a risk and thus warrants proper control.

Linux service scripts

Most Linux service scripts are, as you know, located in /etc/init.d (I will not talk about systemd yet as I have no experience with it). These service scripts are launched by the init system upon boot (or runlevel change), but can also be called directly by administrators.

For the standard SELinux policies, these scripts are either labeled initrc_exec_t or <domain>_initrc_exec_t:

user $ls -lZ /etc/init.d/sshd
-rwxr-xr-x. 1 root root system_u:object_r:initrc_exec_t 2057 Nov 21 18:20 /etc/init.d/sshd

The idea is that, when init or the administrator runs those scripts, the script itself runs in the initrc_t domain. However, if you double-check, you will notice that only the system_r role is allowed the initrc_t domain - so a regular user domain should not be able to transition towards the initrc_t domain.

user $seinfo -rsysadm_r -x | grep initrc_t
user $seinfo -rsystem_r -x | grep initrc_t

So what is happening when you do start the script? After all, everything seems to run, right?

root #/etc/init.d/sshd start
Authenticating root.
* Starting SSH daemon...          [ started ]

The process to transition to initrc_t

This is policy-defined, and a small part is Gentoo-specific, so might be a bit different on your system. However, the general idea is the same here (which is the purpose of this tutorial).

What happens is that, upon executing the init script, a transition occurs to the run_init_t domain (Gentoo). Then, OpenRC (Gentoo's init system) calls the run_init command. This command will invoke the script (again) but with a transition, not only to the target initrc_t domain, but also with the system_u:system_r related context (so it switches SELinux user and role).

On non-Gentoo systems, it is very likely that administrators are already asked to use the run_init command upon managing services.

root #run_init /etc/init.d/sshd status
 * sshd: started

The run_init_t domain is one of the few domains that are allowed (by policy) to do an SELinux user- and role change.

Domain-specific init scripts

Some services have a domain-specific init script. For instance, the /etc/init.d/nscd script has the system_u:object_r:nscd_initrc_exec_t context.

In this case, if the system administrator executes this script, no such transition occurs. This is because the nscd_initrc_exec_t script is not defined as an entrypoint for the run_init_t domain. Instead, it is only known as an entrypoint for the initrc_t domain itself. When trying to execute it directly, SELinux attempts to transition to the initrc_t domain but, as there is no switch in user/role, it fails. In the audit logs (or messages file) you then find an error like the following:

type=SELINUX_ERR msg=audit(1363638369.361:144): security_compute_sid:  invalid context staff_u:system_r:initrc_t for 
scontext=staff_u:sysadm_r:sysadm_t tcontext=system_u:object_r:nscd_initrc_exec_t tclass=process

To force the change in user/role, you need to call the run_init command for this (if you are allowed this command), like so:

root #run_init /etc/init.d/nscd status
Authenticating root.
 * nscd: started

Purpose of domain-specific init scripts

The purpose of this distinction is to allow specific users (or roles) to execute the domain-specific init scripts, while not granting privileges to execute the other scripts. In this case, users that are not allowed to execute run_init_t can be allowed to execute the mentioned domain-specific init script. In this case, the policy will automatically (i.e. without using run_init_t) switch the role towards system_r (but mind you, this requires that the user is allowed the system_r role).

When this occurs however, no change in SELinux user happens. As a result, these services will then run under the SELinux user of the user that (re)started the service. As the process runs in the system_r and proper domain however, this has no additional impact on the system.

With systemd, this is no longer the case, as processes are not launched by the user domain; instead, users communicate with the (system_u-running) systemd daemon, which launches the daemons itself

Later in this series, when we know how to create our own policies, we will be creating our own role and type which will be allowed to execute such scripts.

What you need to remember

What you should remember from this tutorial is that

  • calling linux service scripts (init scripts) causes a change in SELinux user, role and domain, partially due to policy and partially due to the run_init command
  • some service scripts are not labeled the regular initrc_exec_t and require a different approach for calling them, and might result in the process to remain running under the users' SELinux user (but that's okay)
  • it is the SELinux policy that governs changes in roles and users

This article is meant to review a bit how SELinux functions (so we use the init scripts here as a common example).