16.5. Available MAC Policies

Beginning with FreeBSD 8.0, the default FreeBSD kernel includes options MAC. This means that every module included with the MAC framework can be loaded with kldload as a run-time kernel module. After testing the module, add the module name to /boot/loader.conf so that it will load during boot. Each module also provides a kernel option for those administrators who choose to compile their own custom kernel.

FreeBSD includes a group of policies that will cover most security requirements. Each policy is summarized below. The last three policies support integer settings in place of the three default labels.

16.5.1. 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 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:

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

  • security.mac.seeotheruids.specificgid_enabled allows specified groups to be exempt from this policy. To exempt specific groups, use the security.mac.seeotheruids.specificgid=XXX sysctl tunable, replacing 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.

16.5.2. 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 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:

# ugidfw list
0 slots, 0 rules

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:

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

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:

# 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 in order to enforce the same access restrictions for all users. However, the root user is unaffected by these rules.

Note:

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

16.5.3. 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 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:

  • 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 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.

16.5.4. 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, 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:

  • 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 policy as a text string of the form rule[,rule,...], with as many rules as needed, and where each rule is of the form idtype:id:protocol:port. The idtype is either uid or gid. The protocol parameter can be tcp or udp. The port parameter is the port number to allow the specified user or group to bind to. Only numeric values can be used for the user ID, group ID, and port parameters.

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:

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

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

# sysctl security.mac.portacl.suser_exempt=1

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

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

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

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

16.5.5. 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 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:

  • 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.

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.

# setpmac partition/13 top

This command displays the partition label and the process list:

# ps Zax

This command displays 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.

16.5.6. 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 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:

  • 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 to a lower level.

  • mls/equal should be placed on objects which should be exempt from the policy.

  • mls/high 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:

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

# setfmac mls/5 test

To get the MLS label for the file test:

# 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.

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.

16.5.7. 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.

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/low is considered the lowest integrity an object or subject may have. Setting this on objects or subjects blocks their write access to objects or subjects marked as biba/high, but will not prevent read access.

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

  • biba/high permits writing to objects set at a lower label, but does 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 levels 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 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

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.

16.5.8. The MAC Low-watermark 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 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:

# 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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