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49.6. Multi-Level Security (MLS)

Protecting sensitive or confidential data is paramount in many businesses. In the event such information is made public, businesses may face legal or financial ramifications. At the very least, they will suffer a loss of customer trust. In most cases, however, they can recover from these financial and other losses with appropriate investment or compensation.
The same cannot be said of the defense and related communities, which includes military services, intelligence organizations and some areas of police service. These organizations cannot easily recover should sensitive information be leaked, and may not recover at all. These communities require higher levels of security than those employed by businesses and other organizations.
Having information of different security levels on the same computer systems poses a real threat. It is not a straight-forward matter to isolate different information security levels, even though different users log in using different accounts, with different permissions and different access controls.
Some organizations go as far as to purchase dedicated systems for each security level. This is often prohibitively expensive, however. A mechanism is required to enable users at different security levels to access systems simultaneously, without fear of information contamination.

49.6.1. Why Multi-Level?

The term multi-level arises from the defense community's security classifications: Confidential, Secret, and Top Secret.
Individuals must be granted appropriate clearances before they can see classified information. Those with Confidential clearance are only authorized to view Confidential documents; they are not trusted to look at Secret or Top Secret information. The rules that apply to data flow operate from lower levels to higher levels, and never the reverse. This is illustrated below.
Information Security Levels

Figure 49.2. Information Security Levels The Bell-La Padula Model (BLP)

SELinux, like most other systems that protect multi-level data, uses the BLP model. This model specifies how information can flow within the system based on labels attached to each subject and object. Refer to the following diagram:
Available data flows using an MLS system

Figure 49.3. Available data flows using an MLS system

Under such a system, users, computers, and networks use labels to indicate security levels. Data can flow between like levels, for example between "Secret" and "Secret", or from a lower level to a higher level. This means that users at level "Secret" can share data with one another, and can also retrieve information from Confidential-level (i.e., lower-level), users. However, data cannot flow from a higher level to a lower level. This prevents processes at the "Secret" level from viewing information classified as "Top Secret". It also prevents processes at a higher level from accidentally writing information to a lower level. This is referred to as the "no read up, no write down" model. MLS and System Privileges

MLS access rules are always combined with conventional access permissions (file permissions). For example, if a user with a security level of "Secret" uses Discretionary Access Control (DAC) to block access to a file by other users, this also blocks access by users with a security level of "Top Secret". A higher security clearance does not automatically give permission to arbitrarily browse a file system.
Users with top-level clearances do not automatically acquire administrative rights on multi-level systems. While they may have access to all information on the computer, this is different from having administrative rights.

49.6.2. Security Levels, Objects and Subjects

As discussed above, subjects and objects are labeled with Security Levels (SLs), which are composed of two types of entities:
  1. Sensitivity: — A hierarchical attribute such as "Secret" or "Top Secret".
  2. Categories: — A set of non-hierarchical attributes such as "US Only" or "UFO".
An SL must have one sensitivity, and may have zero or more categories.
Examples of SLs are: { Secret / UFO, Crypto }, { Top Secret / UFO, Crypto, Stargate } and { Unclassified }
Note the hierarchical sensitivity followed by zero or more categories. The reason for having categories as well as sensitivities is so that sensitivities can be further compartmentalized on a need-to-know basis. For example, while a process may be cleared to the "Secret" sensitivity level, it may not need any type of access to the project "Warp Drive" (which could be the name of a category).


  1. Security Levels on objects are called Classifications.
  2. Security Levels on subjects are called Clearances.
Thus, objects are labeled with a Classification, while subjects operate with a specific Clearance. Security Levels can have also Ranges, but these are beyond the scope of this introduction.

49.6.3. MLS Policy

SELinux uses the Bell-La Padula BLP model, with Type Enforcement (TE) for integrity. In simple terms, MLS policy ensures that a Subject has an appropriate clearance to access an Object of a particular classification.
For example, under MLS, the system needs to know how to process a request such as: Can a process running with a clearance of { Top Secret / UFO, Rail gun } write to a file classified as { Top Secret / UFO } ?
The MLS model and the policy implemented for it will determine the answer. (Consider, for example, the problem of information leaking out of the Rail gun category into the file).
MLS meets a very narrow (yet critical) set of security requirements based around the way information and personnel are managed in rigidly controlled environments such as the military. MLS is typically difficult to work with and does not map well to general-case scenarios.
Type Enforcement (TE) under SELinux is a more flexible and expressive security scheme, which is in many cases more suitable than MLS.
There are, however, several scenarios where traditional MLS is still required. For example, a file server where the stored data may be of mixed classification and where clients connect at different clearances. This results in a large number of Security Levels and a need for strong isolation all on a single system.
This type of scenario is the reason that SELinux includes MLS as a security model, as an adjunct to TE.

49.6.4. Enabling MLS in SELinux


It is not recommended to use the MLS policy on a system that is running the X Window System.
Follow these steps to enable the SELinux MLS policy on your system.
  1. Install the selinux-policy-mls package:
    ~]# yum install selinux-policy-mls
  2. Before the MLS policy is enabled, each file on the file system must be relabeled with an MLS label. When the file system is relabeled, confined domains may be denied access, which may prevent your system from booting correctly. To prevent this from happening, configure SELINUX=permissive in the /etc/selinux/config file. Also, enable the MLS policy by configuring SELINUXTYPE=mls. Your configuration file should look like this:
    # This file controls the state of SELinux on the system.
    # SELINUX= can take one of these three values:
    #     enforcing - SELinux security policy is enforced.
    #     permissive - SELinux prints warnings instead of enforcing.
    #     disabled - No SELinux policy is loaded.
    # SELINUXTYPE= can take one of these two values:
    #     targeted - Targeted processes are protected,
    #     minimum - Modification of targeted policy. Only selected processes are protected. 
    #     mls - Multi Level Security protection.
  3. Make sure SELinux is running in the permissive mode:
    ~]# setenforce 0
    ~]# getenforce
  4. Create the .autorelabel file in root's home directory to ensure that files are relabeled upon next reboot:
    ~]# touch /.autorelabel
  5. Reboot your system. During the next boot, all file systems will be relabeled according to the MLS policy. The label process labels all files with an appropriate SELinux context:
    *** Warning -- SELinux mls policy relabel is required.
    *** Relabeling could take a very long time, depending on file
    *** system size and speed of hard drives.
    Each * (asterisk) character on the bottom line represents 1000 files that have been labeled. In the above example, eleven * characters represent 11000 files which have been labeled. The time it takes to label all files depends upon the number of files on the system, and the speed of the hard disk drives. On modern systems, this process can take as little as 10 minutes. Once the labeling process finishes, the system will automatically reboot.
  6. Once the file system is relabeled, execute the following commands to assure that the /root directory and all other home directories are properly labeled:
    ~]# genhomedircon
    ~]# restorecon -R -v /root /home <other_home_directories>
  7. In permissive mode, SELinux policy is not enforced, but denials are still logged for actions that would have been denied if running in enforcing mode. Before changing to enforcing mode, as the Linux root user, run the grep "SELinux is preventing" /var/log/messages command to confirm that SELinux did not deny actions during the last boot. If SELinux did not deny actions during the last boot, this command does not return any output.
  8. If there were no denial messages in /var/log/messages, or you have resolved all existing denials, configure SELINUX=enforcing in the /etc/selinux/config file:
    # This file controls the state of SELinux on the system.
    # SELINUX= can take one of these three values:
    #     enforcing - SELinux security policy is enforced.
    #     permissive - SELinux prints warnings instead of enforcing.
    #     disabled - No SELinux policy is loaded.
    # SELINUXTYPE= can take one of these two values:
    #     targeted - Targeted processes are protected,
    #     minimum - Modification of targeted policy. Only selected processes are protected. 
    #     mls - Multi Level Security protection.
  9. Reboot your system and make sure SELinux is running in permissive mode:
    ~]$ getenforce
    and the MLS policy is enabled:
    ~]# sestatus |grep mls
    Policy from config file:        mls

49.6.5. LSPP Certification

Efforts are being made to have Linux certified as an MLS operating system. The certification is equivalent to the old B1 rating, which has been reworked into the Labeled Security Protection Profile under the Common Criteria scheme.