SELinux/Quick introduction

SELinux is an access control mechanism, supported through the Linux kernel, that works alongside the Linux regular access controls. But unlike the regular access controls, it is a mandatory access control system, ensuring that users cannot work around the rules set by the administrator. But before we get into the gritty details of SELinux, let's first see how the regular access controls on a Linux system work. Then, we continue with the SELinux features one by one, explaining most of the models supported by SELinux.

Regular Linux access control
Assume a user wants to execute a script, then the Linux discretionary access control system will check the information of the process that the user is running (which is most likely a shell process) and that of the script itself to decide if the execution can go through.



In the example above, Linux will allow the execution, because the permissions on the file (which are ) allow other users to execute it.

The discretionary part comes from the fact that the owner of the file can decide on the access rights to the file. Even if the file wasn't executable at first, the user could easily assign the proper permission set to the file:

On a multi-user system, a user will be able to share whatever resources he has access to with the other users, as well as enable any action (such as writing to or executing the files). All he needs to do is to open up the rights to these resources for other users (or if they all share the same group, to the group owner). And although the example of "sharing" files is a simple one, it can go further than that. In some enterprises, access to particular software on a system is governed through the group membership. Developers might be assigned a developer group, allowing them access to the system compiler. But once a developer can access the compiler, he can easily also copy all compiler files into his own directory and share that directory with other, non-developer users who can then call the compiler as well.

The discretionary part of the Linux permission system goes further than just users. Services too, such as the Apache web server, can decide how they open up their resources. And although it might not be by design, a malfunctioning service might expose sensitive data to users through this aspect.

Mandatory access control
With a mandatory access control, the permissions are not governed by the owner of the resource, nor can they be worked around by users. Instead, they are set in stone by the security administrator. In SELinux, this is done through the SELinux policy that is loaded at the start of the system.



The interaction shown is still the same, but an additional set of components is added: the SELinux subsystem (running in the Linux kernel) and the policy.

SELinux security subsystem
SELinux is a security subsystem that works inside the Linux kernel. It uses Linux Security Modules (LSM) hooks provided by the Linux kernel developers to intercept any call and check if the call is allowed or not. If it is, then the activity can go through. Otherwise, a Permission Denied error is returned to the system and a denial message is sent to the audit subsystem. This way, administrators can investigate denials (as they usually reflect unwanted behavior being exerted on the system) and rest assured that security rules cannot be worked around.

It is important to understand that the SELinux permission check happens after the Linux DAC check. In other words, SELinux cannot be used to override access controls on the system. If the regular permission system disallows an activity, then SELinux is not even consulted.

Context based approach
SELinux uses contexts to identify resources. In the example, two contexts take part in the activity:
 * 1) the context of the user process (such as a shell), which is
 * 2) the context of the target file, which is

A context in SELinux consists out of 3, sometimes 4 parts:
 * 1) it starts with a SELinux user (which is not the same as a Linux user),
 * 2) followed by the SELinux role,
 * 3) followed by the SELinux type,
 * 4) and then optionally followed by a sensitivity

Or, in a regular schematic representation:



Each field is used by SELinux for deciding access controls. Most of the rules however are made for the SELinux type of a context, which is why contexts are often reduced to just the type.

SELinux policy
The SELinux security subsystem checks any access attempt through the policy. SELinux uses a deny-by-default setup, so it looks for explicit allow statements on the SELinux type. In our case, it looks for an allow rule that allows    rights on a   resource with the   type assigned to it:

Small set of SELinux policy rules

In our example, it doesn't find such a rule, so it denies the execution attempt.

The SELinux policy is what defines the behavior of the SELinux subsystem. Widely different policies will result in different behavior on the system. And the term behavior here is well chosen: the policy is a write-down of what acceptable, normalized behavior is. It is a full description of how processes on the system are allowed to behave.

The policy is separate from the SELinux subsystem; an update to the policy does not require kernel rebuilds or application updates. Instead, policies are built into binary policy packages that can be dynamically loaded, similar to how kernel modules can be loaded.

Access controls
The SELinux subsystem has various access controls in place. One of them we roughly saw, which is called the type enforcement. Next to this, SELinux also supports role-based access control and user-based access control. The last access control system we'll briefly cover is the multi-level security model.

Type enforcement
The type enforcement access control system focuses on the SELinux type within a SELinux context. It is also what most of the SELinux rules are written for and covers the vast majority of SELinux rules in a SELinux policy.

The access check that SELinux performs as part of the type enforcement is based on the access vector built up from the attempt. Such an access vector contains - the source context (such as ) - the target context (such as ) - the class of the target (such as ) - the activity that is invoked (such as )

If an allow statement exists for the entire access vector, then the activity is allowed. If not, it is denied. The allow statement for our example would look like so:

Allow executing lib_t for user_t

The source context type and target context type are defined by the policy developer (or security administrator). On an average SELinux system several hundreds, if not thousands of types exist. These types are declared every time differentiation is needed on the access. For instance, policy developers noticed that there were cases where a shell script only needed to call the shell interpreter itself (such as ) but no other binaries. If the shell interpreter was labeled as  then these shell scripts had access to all binaries labeled. So the policy developers created a second type,, so that shell scripts only needed access to that type and were prevented from executing other, generic binaries.

Next to the types, SELinux also supports a large set of classes. This allows SELinux to differentiate accesses based on the class of a resource. Privileges on a  file versus those of a   socket file or directory are completely separate for SELinux. This is unlike the discretionary access controls on a Linux system.

The set of classes can be seen on a SELinux enabled system like so:

Each class has its own set of supported privileges. The supported permissions of a regular file for instance:

The supported permissions for a TCP socket:

With such a vast set of possible access vectors, policy development is a large undertaking. Luckily, distributions such as Gentoo Linux come with a complete set of default policies to use, with thanks to the reference policy project which manages a base policy for distributions to work with.

Role-based access control
From the SELinux type, we now get into the realm of the SELinux role. Remember the context in our example? It had  as the SELinux role in the context of the process that was trying to execute a file.

Roles are like caps that a user can put on. A user is always assigned to a role, but can decide to switch roles (of course if allowed to switch to a role). An unprivileged user might only have access to one role whereas an administrator might have access to several roles (  for regular operations, and   for system administrative tasks). This is also how roles in organizations are used. There are developer roles, system engineer roles, system administrator roles, it architect roles, database administrator roles, etc.

In SELinux, roles decide which types a process context can be in. The  role has the right to have processes run in the   type. Types for processes are also called domains. So the role-based access control decides which domains a role is allowed to have.

Consider a regular user role versus database administrator role. The regular user will not have access to the backup infrastructure, so the  domain is not allowed for the   role. Even if the user would somehow be able to run backup software (say from a USB stick) this software will be running in the user domain and not the backup domain. And as the user domain is not allowed to access database files directly, the user will not be able to backup or restore data.

Unlike the database administrator, whose  role is allowed to access. The database administrator can therefor execute backup software, which (assuming the policies are all correct) run in the  domain, which is allowed to manage database files. As a result, the database administrator can backup and restore databases.

The set of domains that a role has access to can be requested on a SELinux system through the  command:

Security administrators will usually design least privilege access through the role-based access controls, and then assign the roles to the users on a need-to-have basis. So privileges are not assigned to users individually (as that doesn't scale well) but to the domains, which are assigned to the roles, and users are assigned one or more roles to perform their duties.

Used-based access control
The first part of a context is the SELinux user. The idea behind the SELinux user is that it has to remain immutable, unlike roles which can be switched on user request.

The SELinux user decides which roles someone is allowed to go to. For instance, the  SELinux user specifies that only the   role is allowed, whereas the   user specifies that the allowed roles are   and. This can be seen by either  or  :

On SELinux systems, Linux users are mapped to a SELinux user. This mapping is what decides what a user is allowed to do on a system.

Mappings can be seen (and managed) using :

This is still part of the role-based access control, but important to know as we can now go to the used-based access control.

On Gentoo, if  is set, User Based Access Control (UBAC) is enabled. This ensures that unprivileged users with a different SELinux user cannot access each others' resources even if the types would allow this.

Remember the example of one user sharing access to his files to other users? Well, if all users run with the  type (which is normal for unprivileged users) and have the files labeled as   (which is normal for the contents in the user  locations) then type enforcement would still allow one user to access another users' resources if that user opened up his home directory. After all, SELinux has the following rule to allow users to manage their own files:

SELinux rule allowing regular users to manage their files

However, we can create SELinux users for each user individually, and map each Linux user to a specific SELinux user. With that in place, the contexts of userA's and userB's files differ:

In the example, userB has opened up access to his file (making it world writable). But userA will not be able to access it, as userA will run with the  SELinux user, which is not allowed to access   labeled resources of type   (UBAC works on a per type basis, not all types are what is called ubac-constrained).

Multi-level security
The last access control to discuss is the multi-level security.

In the security world, the sensitivity can be seen as the classification of data. For instance, you can have strictly confidential data, confidential data, internal data and public data. There usually is a strict order in this classification. Someone who has clearance to read confidential data can read internal and public data as well, but not strictly confidential data. Similarly, someone working on confidential data should not at the same time be allowed to write to internal or public data (as that would lead to information leakage).

This gives an information flow model that is often mentioned as "no read up, no write down".

In SELinux, this model is supported through the sensitivity of a context. In that case, a user context could look like  or. Note however that the added part is just a single set, so  is one added field to the context.

This added field is the sensitivity, and contains one, sometimes two parts.
 * 1) The first part is the sensitivity level, which is an integer representation
 * 2) The second part is the category set, which are integers as well

The sensitivity level represents the classification. For instance,  is public data,   internal,   confidential and   strictly confidential. It is up to the security administrator to decide how many layers he wants.

The category represents a set of integers which can be assigned to resources. Each integer is some sort of "tag" on a resource, and can be used to differentiate classes of data. One could use a category for every department in an organization, or a category for every company on a multi-tenant system. A few examples of category sets are: -  meaning category 0 -  meaning categories 0 and 4 -  meaning categories 0 up to 4 (so 0, 1, 2, 3 and 4)

The clearance shows what a domain is allowed to access. A clearance of  means that the process runs in the sensitivity level   and is allowed to access resources with sensitivity up to   and categories 0 and 4 to 8. A resource with a subset of categories assigned is also accessible, but a resource that has another category that is not part of the clearance will not be accessible by that domain.

SELinux users can be given a default clearance set, and the login mappings can reduce the clearance of users even further.

Multi-level security is supported in Gentoo through the mls policy store. The mcs policy store also supports MLS, but only with a single sensitivity.

Integration
So far for all the features, but how is SELinux integrated in Gentoo then?

Enabling SELinux support in packages
First of all, the SELinux support in the applications is enabled through the selinux USE flag. This flag is not configurable by end users - for that a SELinux profile needs to be enabled.

But just enabling this profile isn't sufficient as a set of actions need to be taken to convert a system to become a SELinux system. These instructions are available in the SELinux handbook for now, and will be in the Installation page on the wiki later.

Policy
The SELinux policy is provided through the various  packages, with a  package that contains the base policy (a minimal set of policies that cannot be segregated from each other) and a  package that contains the minimal set of SELinux policy modules for a Gentoo system.

The SELinux policy itself is compiled in a binary format, whose resulting files are available at and all have an   suffix. These binary policy packages are loaded by the  command, similar to how Linux kernel modules are loaded by the   command.

Thanks to the USE flag integration, depending policies are automatically loaded when a user installs software that has a policy module for. Applications that have specific SELinux support (such as the core utilities to display SELinux labels) will also build in this support when  is set.

Administration
Once a system is SELinux-enabled, some additional administration is needed. It is not our intention to cover all administrative commands, but instead give the necessary pointers to the right resources.

SELinux state and mode
The  command tells us what the current SELinux state is.

Possible states are disabled or enabled. If SELinux is enabled, then its mode can be permissive or enforcing.

In permissive mode, SELinux will be consulted but will not enforce its decision. Instead, it will only log denials and still allow the action to continue. This is great for troubleshooting issues, as we can temporarily disable SELinux to see if it is really to blame, but also to convert a system to an SELinux system: we can verify that everything would work (no specific denials) after which we can enable SELinux.

In enforcing mode, SELinux will enforce its policy on the system. Denials are of course still logged, but now the applications will actually be prevented from doing any action that isn't allowed by the policy. It is possible to mark a specific domain as permissive though, disabling SELinux enforcement for just those actions taken by that particular domain.

Next to the mode, SELinux has also a concept called unconfined domains. This is not really a feature within SELinux, but rather a part of the SELinux policy that just allows almost all possible permissions to the domain. As a result, even though SELinux is in enforcing mode, it will allow (almost) all actions taken by the domain as the policy says that that is ok.

Listing contexts
Many Linux utilities support showing the SELinux context. As everything SELinux handles must have a context, viewing the context associated with every resource is very important. Contexts can be easily viewed for many resource types.

File, directory and other file-represented resource contexts can be seen through  or  :

Process contexts can be seen through :

Port contexts can be queried with :

Label management
The most common administrative set of commands are related to labeling of resources. Remember that for SELinux, everything is managed through the contexts. When problems arise, administrators will need to check and, if necessary, update the context of the resources in order to have everything working again.

Contexts can be set explicitly using :

However, SELinux utilities have a specific database in which the "correct" labels for all files are stored. This database uses path expressions to match a file path against and to decide what the right context should be. If this database is correctly defined, then a relabeling operation can help to reset the context of a file. This can be done using :

If all files of a Gentoo package need to be relabeled,  can be used. For instance, to relabel all files provided by the package:

Administrators can query the current file label list using :

The file label list can be updated as well. For instance, to have also marked as  :

After updating the database, the file needs to be relabeled to reset its context to the newly defined value:

User management
SELinux users are managed through  whereas Linux user account mappings are managed with.

For instance, to map the Linux account 'john' to the  SELinux user:

Additional SELinux users can be created using, like so:

Sensitivities and categories
Security clearance for users is set during the definition of the user mapping (with ) or on the SELinux user itself (with  ). Once a user has a clearance set, he can reduce his own clearance through the  command.

Categories on files and other resources can be set using, like so:

To make category and sensitivity management simpler, it is possible to create translations for this. By editing and launching the   daemon, instead of showing something like   this sensitivity could be named.

Troubleshooting denials
When SELinux prevents an access, it will log this in the audit subsystem (or to the system logger if auditing isn't enabled).

To query the audit logs,  can be used:

In this example, the  domain wanted to call mmap_zero for the memprotect class but was prevented to do so by SELinux.

Adding SELinux policy rules
It is not possible to easily remove rules from an active policy, but it is possible to add them.

For simple policy enhancements, Gentoo offers a  script to do just that. For instance, to allow the above mentioned access:

When the enhancements are frequent, it is better to create a SELinux policy module ourselves that contains the rule(s) and load it:

Summary
SELinux is a very flexible, mandatory access control system. In this article we just scratched the surface of the SELinux subsystem and gave a quick introduction to the various areas. For a complete overview of SELinux and how to manage SELinux on Gentoo, please refer to the SELinux wiki page.