Managing User Logins

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SELinux System Administration

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by Sven Vermeulen | September 2013 | Networking & Telephony Open Source

In this article, by Sven Vermeulen, the author of SELinux System Administration, has explained how to manage user logins. When we log in to an SELinux enabled system, we are assigned with a default context to work in. This context contains a SELinux user part, a SELinux role, a domain, and optionally a sensitivity range.

In this article, we will define users that have sufficient rights to do their jobs, ranging from unprivileged users to fully privileged users, running almost without SELinux protection.

(For more resources related to this topic, see here.)

So, who am I?

Once logged in to a system, our user will run inside a certain context. This user context defines the rights and privileges that we, as a user, have on the system. The command to obtain current user information, id, also supports SELinux context information. Try it out, and use the -Z switch as follows:

$ id -Z
unconfined_u:unconfined_r:unconfined_t

On SELinux systems with a targeted policy type, chances are very high that all users are logged in as unconfined_u (the first part of the context). On more restricted systems, the user can be user_u (regular restricted users), staff_u (operators), sysadm_u (system administrators), or any other of the SELinux user types.

The SELinux user defines the roles that the user can switch to, which themselves define the domains that the user (or his processes) can run in. By default, a fixed number of SELinux users are available on the system, but administrators can create different SELinux users. It is also the administrator’s task to assign Linux logins to SELinux users.

The rationale behind unconfined

SELinux is able to provide full system confinement: each and every application runs in its own restricted environment from where it cannot break out of. But that requires fine-grained policies that are equally fast developed as the new versions of all the applications that they confine.

The following diagram shows this relation between the policies, the domain applicability towards multiple processes and the development effort. As an example, postfix_cleanup_t is shown as a very fine-grained policy domain (which is used for the cleanup process involved in the Postfix mail infrastructure) whereas the unconfined_t domain is shown as the example for a very broad, almost unlimited access domain:

Most applications do not have a dedicated policy (although most security-sensitive or popular ones do) and policies do not adapt as fast as the applications themselves. For many servers though, this fine-grained mandatory access is not necessary. All that is needed is to confine the services that are exposed to the network. To support this, SELinux policy developers created the “unconfined” concept, that is, although still governed by SELinux, the domain or user is not really restricted.

Not only are those domains very powerful with respect to privilege, the idea is also that these domains do not need to switch to other domains (unless when they need to switch to a confined domain, of course) and that they are not restricted by constraints that are imposed on confined domains (including MLS).

An important asset in this unconfined story is the unconfined_t domain for the unconfined_u SELinux user (with the unconfined_r SELinux role). This is exactly the context that we get when logged in to a default Fedora installation or a Gentoo installation that uses the targeted policy store (or mcs /mls with USE=”unconfined”).

Next to the user domains, we also have unconfined process domains for daemons and other applications. Some of these run in the unconfined_t domain as well, but most of them run in their own domain even though they are still unconfined. The seinfo tool can tell us which domains are unconfined by asking for those domains that have the selinux_unconfined_type attribute set as follows:

# seinfo -aselinux_unconfined_type -x

Defining a domain is unconfined or cannot be toggled by administrators, as this is a pure SELinux policy matter.

SELinux users and roles

Within SELinux systems, the moment a user logs in, the login system checks to which SELinux user his login is mapped. Then, when a SELinux user is found, the system looks up the role and domain that the user should be in.

We all are one SELinux user

When we logged in to the system and checked our context using id -Z, we noticed that the presented context is the same regardless of the username through which we logged in to the system. SELinux does not care which Linux user we are, as long as it knows which context we are in.

When our login process is triggered, a local definition file will be checked to see which SELinux user is mapped to our login. Let us take a look at the existing login mappings using semanage login –l as follows:

# semanage login -l
Login Name SELinux User MLS/MCS Range Service

__default__ unconfined_u s0-s0:c0.c1023 *
root unconfined_u s0-s0:c0.c1023 *
system_u system_u s0-s0:c0.c1023 *

In the output, two login names are special for SELinux: the __default__ and system_u definitions:

  • __default__: It is a catchall rule. If none of the other rules match, then the users are mapped to the SELinux user identified in the second column (SELinux user). On targeted systems, all users are mapped to the unconfined_u SELinux user because the policies within targeted systems are meant to confine daemons rather than users. On policy stores that do not support unconfined domains, administrators usually map regular logins to restricted SELinux users while administrative logins are mapped to the staff_u or sysadm_u SELinux users.
  • The system_u line is meant for system processes (non-interactively logged in Linux accounts). It should never be assigned to end user logins.

The SELinux user is not the only information of a SELinux mapping. In case of an MLS-enabled system, the mapping also contains information about the sensitivity range in which the user is allowed to work (MLS/MCS Range). This way, two users might both be mapped to the user_u restricted SELinux user, but one might only be allowed to access the low sensitivity (s0) whereas another user might also have access to higher sensitivities (for example, s1). Or, in case of MCS, one user might be mapped to a different set of categories different from another user.

As an exercise, let us create a new Linux user called myuser and make sure that, when this user is logged in, the context is that of the unprivileged SELinux user_u user. We accomplish this using the semanage login tool as follows:

# semanage login -a -s user_u myuser

The -s parameter is used to map a login to a SELinux user, whereas sensitivity (and categories) can be handled with the -r parameter. In the next example, we modify the newly created mapping by limiting the user to the sensitivity range s0-s2 and categories c0 to c4:

# semanage login -m -r “s0-s2:c0.c4” myuser

The changes take effect when a new login occurs so we should force a logout for these users. The following command locks our myuser account, kills all the processes of that user, and unlocks the user again as follows:

# passwd -l myuser
Locking password for user myuser.
passwd: Success
# pkill -KILL -u myuser
# passwd -u myuser
Unlocking password for user myuser.
passwd: Success

Also, when an existing user is modified, we should also reset the contexts of that users’ home directory (while he is not logged on). To accomplish this, use restorecon using the -F option as follows:

# restorecon -RF /home/myuser

That is quite easy. Of course, in larger environments, creating mappings this way for individual users is not manageable. In such cases, it might be better to use (primary) group information. Most regular users are part of the user's Linux group, so let us assign those users to the user_u SELinux user. Accounts that are in the admins Linux group are mapped to sysadm_u . To accomplish this, we use the percentage sign to tell the SELinux tools that this mapping is for groups:

# semanage login -a -s user_u “%users”
# semanage login -a -s sysadm_u “%admins”

Now that the group mappings are in place, we can remove the myuser individual mapping we made earlier as follows:

# semanage login -d myuser

Summary

SELinux maps Linux users onto SELinux users and defines the roles that a user is allowed to be in through the SELinux user definitions. We learned how to manage those mappings and the SELinux users with the semanage application and were able to grant the right roles to the right people. We also saw how the same commands are used to grant proper sensitivity to the user and how we can describe these levels in the setrans.conf file. We used the chcat tool to do most of the category-related management activities.

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About the Author :


Sven Vermeulen

Sven Vermeulen is a long term contributor to various free software projects and the author of various online guides and resources. He got his first taste of free software in 1997 and never looked back since then. In 2003, he joined the ranks of the Gentoo Linux project as a documentation developer and has crossed several roles after that, including Gentoo Foundation’s trustee, council member, project leads for documentation, and (his current role) project lead for Gentoo Hardened’s SELinux integration.

In this time frame, he has gained expertise in several technologies, ranging from operating system level knowledge to application servers as he used his interest in security to guide his projects further: security guides using SCAP languages, mandatory access controls through SELinux, authentication with PAM, (application) firewalling, and more.

On SELinux, he has contributed several policies to the reference policy project and participates actively in policy development and user space development projects.

Sven is an IT infrastructure architect working at a European financial institution. Secured implementation of infrastructure (and the surrounding architectural integration) is of course an important part of this. Prior to this, he graduated with an MSc in Computer Engineering at the University of Ghent and then worked as a web application infrastructure engineer with IBM WebSphere AS.

Sven is the main author of Gentoo’s Handbook which covers the installation and configuration of Gentoo Linux on several architectures. He also authored the Linux Sea online publication, which is a gentle introduction to Linux for novice system administrators.

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