Chapter 18. Mandatory Access Control

Virtually all aspects of label policy module configuration will be performed using the base system utilities. These commands provide a simple interface for object or subject configuration or the manipulation and verification of the configuration.

All configuration may be done using setfmac, which is used to set MAC labels on system objects, and setpmac, which is used to set the labels on system subjects. For example, to set the biba MAC label to high on test:

If the configuration is successful, the prompt will be returned without error. A common error is Permission denied which usually occurs when the label is being set or modified on a restricted object. Other conditions may produce different failures. For instance, the file may not be owned by the user attempting to relabel the object, the object may not exist, or the object may be read-only. A mandatory policy will not allow the process to relabel the file, maybe because of a property of the file, a property of the process, or a property of the proposed new label value. For example, if a user running at low integrity tries to change the label of a high integrity file, or a user running at low integrity tries to change the label of a low integrity file to a high integrity label, these operations will fail.

The system administrator may use setpmac to override the policy module’s settings by assigning a different label to the invoked process:

For currently running processes, such as sendmail, getpmac is usually used instead. This command takes a process ID (PID) in place of a command name. If users attempt to manipulate a file not in their access, subject to the rules of the loaded policy modules, the Operation not permitted error will be displayed.

A few FreeBSD policy modules which support the labeling feature offer three predefined labels: low, equal, and high, where:

Such policy modules include mac_biba(4), mac_mls(4) and mac_lomac(4). Each of the predefined labels establishes a different information flow directive. Refer to the manual page of the module to determine the traits of the generic label configurations.

The Biba and MLS policy modules support a numeric label which may be set to indicate the precise level of hierarchical control. This numeric level is used to partition or sort information into different groups of classification, only permitting access to that group or a higher group level. For example:

may be interpreted as "Biba Policy Label/Grade 10:Compartments 2, 3 and 6: (grade 5 …​")

In this example, the first grade would be considered the effective grade with effective compartments, the second grade is the low grade, and the last one is the high grade. In most configurations, such fine-grained settings are not needed as they are considered to be advanced configurations.

System objects only have a current grade and compartment. System subjects reflect the range of available rights in the system, and network interfaces, where they are used for access control.

The grade and compartments in a subject and object pair are used to construct a relationship known as dominance, in which a subject dominates an object, the object dominates the subject, neither dominates the other, or both dominate each other. The "both dominate" case occurs when the two labels are equal. Due to the information flow nature of Biba, a user has rights to a set of compartments that might correspond to projects, but objects also have a set of compartments. Users may have to subset their rights using su or setpmac in order to access objects in a compartment from which they are not restricted.

Users are required to have labels so that their files and processes properly interact with the security policy defined on the system. This is configured in /etc/login.conf using login classes. Every policy module that uses labels will implement the user class setting.

To set the user class default label which will be enforced by MAC, add a label entry. An example label entry containing every policy module is displayed below. Note that in a real configuration, the administrator would never enable every policy module. It is recommended that the rest of this chapter be reviewed before any configuration is implemented.

While users can not modify the default value, they may change their label after they login, subject to the constraints of the policy. The example above tells the Biba policy that a process’s minimum integrity is 5, its maximum is 15, and the default effective label is 10. The process will run at 10 until it chooses to change label, perhaps due to the user using setpmac, which will be constrained by Biba to the configured range.

After any change to login.conf, the login class capability database must be rebuilt using cap_mkdb.

Many sites have a large number of users requiring several different user classes. In depth planning is required as this can become difficult to manage.

Labels may be set on network interfaces to help control the flow of data across the network. Policies using network interface labels function in the same way that policies function with respect to objects. Users at high settings in Biba, for example, will not be permitted to access network interfaces with a label of low.

When setting the MAC label on network interfaces, maclabel may be passed to ifconfig:

This example will set the MAC label of biba/equal on the bge0 interface. When using a setting similar to biba/high(low-high), the entire label should be quoted to prevent an error from being returned.

Each policy module which supports labeling has a tunable which may be used to disable the MAC label on network interfaces. Setting the label to equal will have a similar effect. Review the output of sysctl, the policy manual pages, and the information in the rest of this chapter for more information on those tunables.

Module name: mac_seeotheruids.ko

Kernel configuration line: options MAC_SEEOTHERUIDS

Boot option: mac_seeotheruids_load="YES"

The mac_seeotheruids(4) module extends the security.bsd.see_other_uids and security.bsd.see_other_gids sysctl tunables. This option does not require any labels to be set before configuration and can operate transparently with other modules.

After loading the module, the following sysctl tunables may be used to control its features:

Module name: mac_bsdextended.ko

Kernel configuration line: options MAC_BSDEXTENDED

Boot option: mac_bsdextended_load="YES"

The mac_bsdextended(4) module enforces a file system firewall. It provides an extension to the standard file system permissions model, permitting an administrator to create a firewall-like ruleset to protect files, utilities, and directories in the file system hierarchy. When access to a file system object is attempted, the list of rules is iterated until either a matching rule is located or the end is reached. This behavior may be changed using security.mac.bsdextended.firstmatch_enabled. Similar to other firewall modules in FreeBSD, a file containing the access control rules can be created and read by the system at boot time using an rc.conf(5) variable.

The rule list may be entered using ugidfw(8) which has a syntax similar to ipfw(8). More tools can be written by using the functions in the libugidfw(3) library.

After the mac_bsdextended(4) module has been loaded, the following command may be used to list the current rule configuration:

By default, no rules are defined and everything is completely accessible. To create a rule which blocks all access by users but leaves root unaffected:

While this rule is simple to implement, it is a very bad idea as it blocks all users from issuing any commands. A more realistic example blocks user1 all access, including directory listings, to user2's home directory:

Instead of user1, not uid user2 could be used in order to enforce the same access restrictions for all users. However, the root user is unaffected by these rules.

Module name: mac_ifoff.ko

Kernel configuration line: options MAC_IFOFF

Boot option: mac_ifoff_load="YES"

The mac_ifoff(4) module is used to disable network interfaces on the fly and to keep network interfaces from being brought up during system boot. It does not use labels and does not depend on any other MAC modules.

Most of this module’s control is performed through these sysctl tunables:

One of the most common uses of mac_ifoff(4) is network monitoring in an environment where network traffic should not be permitted during the boot sequence. Another use would be to write a script which uses an application such as security/aide to automatically block network traffic if it finds new or altered files in protected directories.

Module name: mac_portacl.ko

Kernel configuration line: MAC_PORTACL

Boot option: mac_portacl_load="YES"

The mac_portacl(4) module is used to limit binding to local TCP and UDP ports, making it possible to allow non-root users to bind to specified privileged ports below 1024.

Once loaded, this module enables the MAC policy on all sockets. The following tunables are available:

By default, ports below 1024 can only be used by privileged processes which run as root. For mac_portacl(4) to allow non-privileged processes to bind to ports below 1024, set the following tunables as follows:

To prevent the root user from being affected by this policy, set security.mac.portacl.suser_exempt to a non-zero value.

To allow the www user with UID 80 to bind to port 80 without ever needing root privilege:

This next example permits the user with the UID of 1001 to bind to TCP ports 110 (POP3) and 995 (POP3s):

Module name: mac_partition.ko

Kernel configuration line: options MAC_PARTITION

Boot option: mac_partition_load="YES"

The mac_partition(4) policy drops processes into specific "partitions" based on their MAC label. Most configuration for this policy is done using setpmac(8). One sysctl tunable is available for this policy:

When this policy is enabled, users will only be permitted to see their processes, and any others within their partition, but will not be permitted to work with utilities outside the scope of this partition. For instance, a user in the insecure class will not be permitted to access top as well as many other commands that must spawn a process.

This example adds top to the label set on users in the insecure class. All processes spawned by users in the insecure class will stay in the partition/13 label.

This command displays the partition label and the process list:

This command displays another user’s process partition label and that user’s currently running processes:

Module name: mac_mls.ko

Kernel configuration line: options MAC_MLS

Boot option: mac_mls_load="YES"

The mac_mls(4) policy controls access between subjects and objects in the system by enforcing a strict information flow policy.

In MLS environments, a "clearance" level is set in the label of each subject or object, along with compartments. Since these clearance levels can reach numbers greater than several thousand, it would be a daunting task to thoroughly configure every subject or object. To ease this administrative overhead, three labels are included in this policy: mls/low, mls/equal, and mls/high, where:

MLS provides:

The following sysctl tunables are available:

To manipulate MLS labels, use setfmac(8). To assign a label to an object:

To get the MLS label for the file test:

Another approach is to create a master policy file in /etc/ which specifies the MLS policy information and to feed that file to setfmac.

When using the MLS policy module, an administrator plans to control the flow of sensitive information. The default block read up block write down sets everything to a low state. Everything is accessible and an administrator slowly augments the confidentiality of the information.

Beyond the three basic label options, an administrator may group users and groups as required to block the information flow between them. It might be easier to look at the information in clearance levels using descriptive words, such as classifications of Confidential, Secret, and Top Secret. Some administrators instead create different groups based on project levels. Regardless of the classification method, a well thought out plan must exist before implementing a restrictive policy.

Some example situations for the MLS policy module include an e-commerce web server, a file server holding critical company information, and financial institution environments.

Module name: mac_biba.ko

Kernel configuration line: options MAC_BIBA

Boot option: mac_biba_load="YES"

The mac_biba(4) module loads the MAC Biba policy. This policy is similar to the MLS policy with the exception that the rules for information flow are slightly reversed. This is to prevent the downward flow of sensitive information whereas the MLS policy prevents the upward flow of sensitive information.

In Biba environments, an "integrity" label is set on each subject or object. These labels are made up of hierarchical grades and non-hierarchical components. As a grade ascends, so does its integrity.

Supported labels are biba/low, biba/equal, and biba/high, where:

Biba provides:

The following tunables can be used to manipulate the Biba policy:

To access the Biba policy setting on system objects, use setfmac and getfmac:

Integrity, which is different from sensitivity, is used to guarantee that information is not manipulated by untrusted parties. This includes information passed between subjects and objects. It ensures that users will only be able to modify or access information they have been given explicit access to. The mac_biba(4) security policy module permits an administrator to configure which files and programs a user may see and invoke while assuring that the programs and files are trusted by the system for that user.

During the initial planning phase, an administrator must be prepared to partition users into grades, levels, and areas. The system will default to a high label once this policy module is enabled, and it is up to the administrator to configure the different grades and levels for users. Instead of using clearance levels, a good planning method could include topics. For instance, only allow developers modification access to the source code repository, source code compiler, and other development utilities. Other users would be grouped into other categories such as testers, designers, or end users and would only be permitted read access.

A lower integrity subject is unable to write to a higher integrity subject and a higher integrity subject cannot list or read a lower integrity object. Setting a label at the lowest possible grade could make it inaccessible to subjects. Some prospective environments for this security policy module would include a constrained web server, a development and test machine, and a source code repository. A less useful implementation would be a personal workstation, a machine used as a router, or a network firewall.

Module name: mac_lomac.ko

Kernel configuration line: options MAC_LOMAC

Boot option: mac_lomac_load="YES"

Unlike the MAC Biba policy, the mac_lomac(4) policy permits access to lower integrity objects only after decreasing the integrity level to not disrupt any integrity rules.

The Low-watermark integrity policy works almost identically to Biba, with the exception of using floating labels to support subject demotion via an auxiliary grade compartment. This secondary compartment takes the form [auxgrade]. When assigning a policy with an auxiliary grade, use the syntax lomac/10[2], where 2 is the auxiliary grade.

This policy relies on the ubiquitous labeling of all system objects with integrity labels, permitting subjects to read from low integrity objects and then downgrading the label on the subject to prevent future writes to high integrity objects using [auxgrade]. The policy may provide greater compatibility and require less initial configuration than Biba.

Like the Biba and MLS policies, setfmac and setpmac are used to place labels on system objects:

The auxiliary grade low is a feature provided only by the MACLOMAC policy.

Begin the procedure by adding the following user class to /etc/login.conf:

Then, add the following line to the default user class section:

Save the edits and issue the following command to rebuild the database:

Set the root user to the default class using:

All user accounts that are not root will now require a login class. The login class is required, otherwise users will be refused access to common commands. The following sh script should do the trick:

Next, drop the nagios and www accounts into the insecure class:

A contexts file should now be created as /etc/policy.contexts:

This policy enforces security by setting restrictions on the flow of information. In this specific configuration, users, including root, should never be allowed to access Nagios. Configuration files and processes that are a part of Nagios will be completely self contained or jailed.

This file will be read after running setfsmac on every file system. This example sets the policy on the root file system:

Next, add these edits to the main section of /etc/mac.conf:

To finish the configuration, add the following lines to /boot/loader.conf:

And the following line to the network card configuration stored in /etc/rc.conf. If the primary network configuration is done via DHCP, this may need to be configured manually after every system boot:

First, ensure that the web server and Nagios will not be started on system initialization and reboot. Ensure that root cannot access any of the files in the Nagios configuration directory. If root can list the contents of /var/spool/nagios, something is wrong. Instead, a "permission denied" error should be returned.

If all seems well, Nagios, Apache, and Sendmail can now be started:

Double check to ensure that everything is working properly. If not, check the log files for error messages. If needed, use sysctl(8) to disable the mac_biba(4) security policy module and try starting everything again as usual.