16.7. Available MAC Policies

FreeBSD includes a group of policies that will cover most security requirements. Each policy is discussed below.

16.7.1. The MAC See Other UIDs Policy

Module name: mac_seeotheruids.ko

Kernel configuration line: options MAC_SEEOTHERUIDS

Boot option: mac_seeotheruids_load="YES"

The mac_seeotheruids(4) module mimics and 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 the other modules.

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

  • security.mac.seeotheruids.enabled enables the module and uses the default settings which deny users the ability to view processes and sockets owned by other users.

  • security.mac.seeotheruids.specificgid_enabled allows certain groups to be exempt from this policy. To exempt specific groups from this policy, use the security.mac.seeotheruids.specificgid=XXX sysctl tunable. Replace XXX with the numeric group ID to be exempted.

  • security.mac.seeotheruids.primarygroup_enabled is used to exempt specific primary groups from this policy. When using this tunable, security.mac.seeotheruids.specificgid_enabled may not be set.

16.7.2. The MAC BSD Extended Policy

Module name: mac_bsdextended.ko

Kernel configuration line: options MAC_BSDEXTENDED

Boot option: mac_bsdextended_load="YES"

The mac_bsdextended(4) module enforces the file system firewall. This module's policy 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 by the use of a sysctl(8) parameter, 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.

Extreme caution should be taken when working with this module as incorrect use could block access to certain parts of the file system.

16.7.2.1. Examples

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

# ugidfw list
0 slots, 0 rules

By default, no rules are defined and everything is completely accessible. To create a rule which will block all access by users but leave root unaffected, run the following command:

# ugidfw add subject not uid root new object not uid root mode n

This is a very bad idea as it will block all users from issuing even the most simple commands, such as ls. The next example will block user1 any and all access, including directory listings, to user2's home directory:

# ugidfw set 2 subject uid user1 object uid user2 mode n
# ugidfw set 3 subject uid user1 object gid user2 mode n

Instead of user1, not uid user2 could be used. This enforces the same access restrictions for all users instead of just one user.

Note:

The root user is unaffected by these changes.

For more information, refer to mac_bsdextended(4) and ugidfw(8)

16.7.3. The MAC Interface Silencing Policy

Module name: mac_ifoff.ko

Kernel configuration line: options MAC_IFOFF

Boot option: mac_ifoff_load="YES"

The mac_ifoff(4) module exists solely to disable network interfaces on the fly and keep network interfaces from being brought up during system boot. It does not require any labels to be set up on the system, nor does it depend on other MAC modules.

Most of this module's control is performed through the sysctl tunables listed below.

  • security.mac.ifoff.lo_enabled enables or disables all traffic on the loopback (lo(4)) interface.

  • security.mac.ifoff.bpfrecv_enabled enables or disables all traffic on the Berkeley Packet Filter interface (bpf(4))

  • security.mac.ifoff.other_enabled enables or disables traffic on all other interfaces.

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 suggested use would be to write a script which uses security/aide to automatically block network traffic if it finds new or altered files in protected directories.

16.7.4. The MAC Port Access Control List Policy

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 using a variety of sysctl variables. mac_portacl(4) makes 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:

  • security.mac.portacl.enabled enables or disables the policy completely.

  • security.mac.portacl.port_high sets the highest port number that mac_portacl(4) protects.

  • security.mac.portacl.suser_exempt, when set to a non-zero value, exempts the root user from this policy.

  • security.mac.portacl.rules specifies the mac_portacl policy, which is a text string of the form: rule[,rule,...] with as many rules as needed. Each rule is of the form: idtype:id:protocol:port. The idtype parameter can be uid or gid and is used to interpret the id parameter as either a user id or group id, respectively. The protocol parameter is used to determine if the rule should apply to TCP or UDP by setting the parameter to tcp or udp. The final port parameter is the port number to allow the specified user or group to bind to.

Note:

Since the ruleset is interpreted directly by the kernel, only numeric values can be used for the user ID, group ID, and port parameters. Names cannot be used for users, groups, or services.

By default, ports below 1024 can only be used by or bound to privileged processes, which run as root. For mac_portacl(4) to allow non-privileged processes to bind to ports below 1024, this restriction has to be disabled by setting the sysctl(8) variables net.inet.ip.portrange.reservedlow and net.inet.ip.portrange.reservedhigh to zero:

# sysctl security.mac.portacl.port_high=1023
# sysctl net.inet.ip.portrange.reservedlow=0
net.inet.ip.portrange.reservedhigh=0

See the examples below or refer to mac_portacl(4) for further information.

16.7.4.1. Examples

Since the root user should not be crippled by this policy, this example starts by setting the security.mac.portacl.suser_exempt to a non-zero value.

# sysctl security.mac.portacl.suser_exempt=1

Next, allow the user with UID 80 to bind to port 80. This allows the www user to run a web server without ever having root privilege.

# sysctl security.mac.portacl.rules=uid:80:tcp:80

The next example permits the user with the UID of 1001 to bind to the TCP ports 110 (pop3) and 995 (pop3s). This permits this user to start a server that accepts connections on ports 110 and 995.

# sysctl security.mac.portacl.rules=uid:1001:tcp:110,uid:1001:tcp:995

16.7.5. The MAC Partition Policy

Module name: mac_partition.ko

Kernel configuration line: options MAC_PARTITION

Boot option: mac_partition_load="YES"

The mac_partition(4) policy will drop processes into specific partitions based on their MAC label. This module should be added to loader.conf(5) so that it loads and enables the policy at system boot.

Most configuration for this policy is done using setpmac(8). One sysctl tunable is available for this policy:

  • security.mac.partition.enabled enables the enforcement of MAC process partitions.

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.

To set or drop utilities into a partition label, use the setpmac utility:

# setpmac partition/13 top

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.

16.7.5.1. Examples

The following command will display the partition label and the process list:

# ps Zax

This command will display another user's process partition label and that user's currently running processes:

# ps -ZU trhodes

Note:

Users can see processes in root's label unless the mac_seeotheruids(4) policy is loaded.

A really crafty implementation could have all of the services disabled in /etc/rc.conf and started by a script that starts them with the proper labeling set.

Note:

The following policies support integer settings in place of the three default labels offered. These options, including their limitations, are further explained in the module manual pages.

16.7.6. The MAC Multi-Level Security Module

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 or sensibility levels can reach numbers greater than several thousand; it would be a daunting task for any system administrator to thoroughly configure each subject or object. Thankfully, three instant labels are included in this policy.

These labels are mls/low, mls/equal and mls/high. Since these labels are described in depth in the manual page, they will only get a brief description here:

  • The mls/low label contains a low configuration which permits it to be dominated by all other objects. Anything labeled with mls/low will have a low clearance level and not be permitted to access information of a higher level. This label also prevents objects of a higher clearance level from writing or passing information on to them.

  • The mls/equal label should be placed on objects considered to be exempt from the policy.

  • The mls/high label is the highest level of clearance possible. Objects assigned this label will hold dominance over all other objects in the system; however, they will not permit the leaking of information to objects of a lower class.

MLS provides:

  • A hierarchical security level with a set of non hierarchical categories.

  • Fixed rules of no read up, no write down. This means that a subject can have read access to objects on its own level or below, but not above. Similarly, a subject can have write access to objects on its own level or above but not beneath.

  • Secrecy, or the prevention of inappropriate disclosure of data.

  • A basis for the design of systems that concurrently handle data at multiple sensitivity levels without leaking information between secret and confidential.

The following sysctl tunables are available for the configuration of special services and interfaces:

  • security.mac.mls.enabled is used to enable or disable the MLS policy.

  • security.mac.mls.ptys_equal labels all pty(4) devices as mls/equal during creation.

  • security.mac.mls.revocation_enabled revokes access to objects after their label changes to a label of a lower grade.

  • security.mac.mls.max_compartments sets the maximum number of compartment levels allowed on a system.

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

# setfmac mls/5 test

To get the MLS label for the file test, issue the following command:

# getfmac 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. This method will be explained after all policies are covered.

16.7.6.1. Planning Mandatory Sensitivity

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 during the configuration stage;.

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 such 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.

16.7.7. The MAC Biba Module

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. Much of this section can apply to both policies.

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 an grade ascends, so does its integrity.

Supported labels are biba/low, biba/equal, and biba/high; as explained below:

  • The biba/low label is considered the lowest integrity an object or subject may have. Setting this on objects or subjects will block their write access to objects or subjects marked high. They still have read access though.

  • The biba/equal label should only be placed on objects considered to be exempt from the policy.

  • The biba/high label will permit writing to objects set at a lower label, but not permit reading that object. It is recommended that this label be placed on objects that affect the integrity of the entire system.

Biba provides:

  • Hierarchical integrity level with a set of non hierarchical integrity categories.

  • Fixed rules are no write up, no read down, the opposite of MLS. A subject can have write access to objects on its own level or below, but not above. Similarly, a subject can have read access to objects on its own level or above, but not below.

  • Integrity by preventing inappropriate modification of data.

  • Integrity levels instead of MLS sensitivity levels.

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

  • security.mac.biba.enabled is used to enable or disable enforcement of the Biba policy on the target machine.

  • security.mac.biba.ptys_equal is used to disable the Biba policy on pty(4) devices.

  • security.mac.biba.revocation_enabled forces the revocation of access to objects if the label is changed to dominate the subject.

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

# setfmac biba/low test
# getfmac test
test: biba/low

16.7.7.1. Planning Mandatory Integrity

Integrity, which is different from sensitivity, guarantees that the information will never be manipulated by untrusted parties. This includes information passed between subjects, objects, and both. It ensures that users will only be able to modify or access information they explicitly need to.

The mac_biba(4) security policy module permits an administrator to address which files and programs a user may see and invoke while assuring that the programs and files are free from threats and 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. Users will be blocked access not only to data but to programs and utilities both before and after they start. 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 observe 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.

16.7.8. The MAC LOMAC Module

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 MAC version of the Low-watermark integrity policy works almost identically to Biba, but 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 LOMAC policy with an auxiliary grade, use the syntax lomac/10[2] where the number two (2) is the auxiliary grade.

The MAC LOMAC 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 for greater compatibility and require less initial configuration than Biba.

16.7.8.1. Examples

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

# setfmac /usr/home/trhodes lomac/high[low]
# getfmac /usr/home/trhodes lomac/high[low]

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

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