Κεφάλαιο 17. Υποχρεωτικός Έλεγχος Πρόσβασης

Το FreeBSD 5.X εισήγαγε νέες επεκτάσεις ασφαλείας από το TrustedBSD project, που βασίζονται στο προσχέδιο POSIX®.1e. Δύο από τους πιο σημαντικούς νέους μηχανισμούς ασφαλείας, είναι οι Λίστες Ελέγχου Πρόσβασης (Access Control Lists, ACLs) στο σύστημα αρχείων και ο Υποχρεωτικός Έλεγχος Πρόσβασης (Mandatory Access Control, MAC). Ο Υποχρεωτικός Έλεγχος Πρόσβασης δίνει την δυνατότητας φόρτωσης αρθρωμάτων (modules) ελέγχου τα οποία υλοποιούν νέες πολιτικές ασφαλείας. Μερικά παρέχουν προστασία σε ένα στενό υποσύνολο του συστήματος, ενδυναμώνοντας την ασφάλεια μιας συγκεκριμένης υπηρεσίας. Άλλα παρέχουν συνοπτική ασφάλεια προς όλες τις υπηρεσίες και το σύστημα. Ο έλεγχος ονομάζεται υποχρεωτικός από το γεγονός ότι η επιβολή γίνεται από τους διαχειριστές και το σύστημα, και δεν αφήνεται στη διακριτική ευχέρεια των χρηστών όπως γίνεται με το διακριτικό έλεγχο πρόσβασης (Discretionary Access Control, DAC, τις τυποποιημένες άδειες αρχείων και IPC του System V στο FreeBSD).

Το κεφάλαιο αυτό εστιάζει στο πλαίσιο του Υποχρεωτικού Ελέγχου Πρόσβασης (MAC Framework), και σε ένα σύνολο πρόσθετων αρθρωμάτων για πολιτικές ασφάλειας, που ενεργοποιούν διάφορους μηχανισμούς ασφάλειας.

Αφού διαβάσετε αυτό το κεφάλαιο, θα ξέρετε:

Πριν διαβάσετε αυτό το κεφάλαιο, θα πρέπει:

Before reading this chapter, a few key terms must be explained. This will hopefully clear up any confusion that may occur and avoid the abrupt introduction of new terms and information.

With all of these new terms in mind, consider how the MAC framework augments the security of the system as a whole. The various security policy modules provided by the MAC framework could be used to protect the network and file systems, block users from accessing certain ports and sockets, and more. Perhaps the best use of the policy modules is to blend them together, by loading several security policy modules at a time for a multi-layered security environment. In a multi-layered security environment, multiple policy modules are in effect to keep security in check. This is different to a hardening policy, which typically hardens elements of a system that is used only for specific purposes. The only downside is administrative overhead in cases of multiple file system labels, setting network access control user by user, etc.

These downsides are minimal when compared to the lasting effect of the framework; for instance, the ability to pick and choose which policies are required for a specific configuration keeps performance overhead down. The reduction of support for unneeded policies can increase the overall performance of the system as well as offer flexibility of choice. A good implementation would consider the overall security requirements and effectively implement the various security policy modules offered by the framework.

Thus a system utilizing MAC features should at least guarantee that a user will not be permitted to change security attributes at will; all user utilities, programs and scripts must work within the constraints of the access rules provided by the selected security policy modules; and that total control of the MAC access rules are in the hands of the system administrator.

It is the sole duty of the system administrator to carefully select the correct security policy modules. Some environments may need to limit access control over the network; in these cases, the mac_portacl(4), mac_ifoff(4) and even mac_biba(4) policy modules might make good starting points. In other cases, strict confidentiality of file system objects might be required. Policy modules such as mac_bsdextended(4) and mac_mls(4) exist for this purpose.

Policy decisions could be made based on network configuration. Perhaps only certain users should be permitted access to facilities provided by ssh(1) to access the network or the Internet. The mac_portacl(4) would be the policy module of choice for these situations. But what should be done in the case of file systems? Should all access to certain directories be severed from other groups or specific users? Or should we limit user or utility access to specific files by setting certain objects as classified?

In the file system case, access to objects might be considered confidential to some users, but not to others. For an example, a large development team might be broken off into smaller groups of individuals. Developers in project A might not be permitted to access objects written by developers in project B. Yet they might need to access objects created by developers in project C; that is quite a situation indeed. Using the different security policy modules provided by the MAC framework; users could be divided into these groups and then given access to the appropriate areas without fear of information leakage.

Thus, each security policy module has a unique way of dealing with the overall security of a system. Module selection should be based on a well thought out security policy. In many cases, the overall policy may need to be revised and reimplemented on the system. Understanding the different security policy modules offered by the MAC framework will help administrators choose the best policies for their situations.

The default FreeBSD kernel does not include the option for the MAC framework; thus the following kernel option must be added before trying any of the examples or information in this chapter:

And the kernel will require a rebuild and a reinstall.

A MAC label is a security attribute which may be applied to subjects and objects throughout the system.

When setting a label, the user must be able to comprehend what it is, exactly, that is being done. The attributes available on an object depend on the policy module loaded, and that policy modules interpret their attributes in different ways. If improperly configured due to lack of comprehension, or the inability to understand the implications, the result will be the unexpected and perhaps, undesired, behavior of the system.

The security label on an object is used as a part of a security access control decision by a policy. With some policies, the label by itself contains all information necessary to make a decision; in other models, the labels may be processed as part of a larger rule set, etc.

For instance, setting the label of biba/low on a file will represent a label maintained by the Biba security policy module, with a value of "low".

A few policy modules which support the labeling feature in FreeBSD offer three specific predefined labels. These are the low, high, and equal labels. Although they enforce access control in a different manner with each policy module, you can be sure that the low label will be the lowest setting, the equal label will set the subject or object to be disabled or unaffected, and the high label will enforce the highest setting available in the Biba and MLS policy modules.

Within single label file system environments, only one label may be used on objects. This will enforce one set of access permissions across the entire system and in many environments may be all that is required. There are a few cases where multiple labels may be set on objects or subjects in the file system. For those cases, the multilabel option may be passed to tunefs(8).

In the case of Biba and MLS, a numeric label 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 say, classification only permitting access to that group or a higher group level.

In most cases the administrator will only be setting up a single label to use throughout the file system.

Hey wait, this is similar to DAC! I thought MAC gave control strictly to the administrator. That statement still holds true, to some extent as root is the one in control and who configures the policies so that users are placed in the appropriate categories/access levels. Alas, many policy modules can restrict the root user as well. Basic control over objects will then be released to the group, but root may revoke or modify the settings at any time. This is the hierarchal/clearance model covered by policies such as Biba and MLS.

Whenever a new technology is implemented, a planning phase is always a good idea. During the planning stages, an administrator should in general look at the "big picture", trying to keep in view at least the following:

For MAC installations, these include:

It is always possible to reconfigure and change the system resources and security settings, it is quite often very inconvenient to search through the system and fix existing files and user accounts. Planning helps to ensure a trouble-free and efficient trusted system implementation. A trial run of the trusted system, including the configuration, is often vital and definitely beneficial before a MAC implementation is used on production systems. The idea of just letting loose on a system with MAC is like setting up for failure.

Different environments may have explicit needs and requirements. Establishing an in depth and complete security profile will decrease the need of changes once the system goes live. As such, the future sections will cover the different modules available to administrators; describe their use and configuration; and in some cases provide insight on what situations they would be most suitable for. For instance, a web server might roll out the mac_biba(4) and mac_bsdextended(4) policies. In other cases, a machine with very few local users, the mac_partition(4) might be a good choice.

Every module included with the MAC framework may be either compiled into the kernel as noted above or loaded as a run-time kernel module. The recommended method is to add the module name to the /boot/loader.conf file so that it will load during the initial boot operation.

The following sections will discuss the various MAC modules and cover their features. Implementing them into a specific environment will also be a consideration of this chapter. Some modules support the use of labeling, which is controlling access by enforcing a label such as "this is allowed and this is not". A label configuration file may control how files may be accessed, network communication can be exchanged, and more. The previous section showed how the multilabel flag could be set on file systems to enable per-file or per-partition access control.

A single label configuration would enforce only one label across the system, that is why the tunefs option is called multilabel.

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 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 a utility, ugidfw(8), that has a syntax similar to that of 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; incorrect use could block access to certain parts of the file system.

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 the initial system boot. It does not require any labels to be set up on the system, nor does it have a dependency on other MAC modules.

Most of the control is done through the sysctl tunables listed below.

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.

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. In essence mac_portacl(4) makes it possible to allow non-root users to bind to specified privileged ports, i.e. ports fewer than 1024.

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

The actual mac_portacl policy, as specified in the security.mac.portacl.rules sysctl, 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 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.

By default, on UNIX®-like systems, ports fewer than 1024 can only be used by/bound to privileged processes, i.e. those run as root. For mac_portacl(4) to allow non-privileged processes to bind to ports below 1024 this standard UNIX® restriction has to be disabled. This can be accomplished by setting the sysctl(8) variables net.inet.ip.portrange.reservedlow and net.inet.ip.portrange.reservedhigh to zero.

See the examples below or review the mac_portacl(4) manual page for further information.

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. Think of it as a special type of jail(8), though that is hardly a worthy comparison.

This is one module that should be added to the loader.conf(5) file so that it loads and enables the policy during the boot process.

Most configuration for this policy is done using the setpmac(8) utility which will be explained below. The following 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 above will not be permitted to access the top command as well as many other commands that must spawn a process.

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

This will add the top command to the label set on users in the insecure class. Note that all processes spawned by users in the insecure class will stay in the partition/13 label.

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 each subject or objects label, along with compartments. Since these clearance or sensibility levels can reach numbers greater than six thousand; it would be a daunting task for any system administrator to thoroughly configure each subject or object. Thankfully, three "instant" labels are already 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:

MLS provides for:

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

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

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

This is a summary of the MLS policy’s features. Another approach is to create a master policy file in /etc which specifies the MLS policy information and to feed that file into the setfmac command. This method will be explained after all policies are covered.

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 works much like that of the MLS policy with the exception that the rules for information flow are slightly reversed. This is said to prevent the downward flow of sensitive information whereas the MLS policy prevents the upward flow of sensitive information; thus, 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 hierarchal grades, and non-hierarchal components. As an object’s or subject’s grade ascends, so does its integrity.

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

Biba provides for:

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

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

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, not to be confused with the older lomac(4) implementation, 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 of [auxgrade]. When assigning a lomac policy with an auxiliary grade, it should look a little bit like: 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. This is the [auxgrade] option discussed above, thus the policy may provide for greater compatibility and require less initial configuration than Biba.

The following demonstration will implement a secure environment using various MAC modules with properly configured policies. This is only a test and should not be considered the complete answer to everyone’s security woes. Just implementing a policy and ignoring it never works and could be disastrous in a production environment.

Before beginning this process, the multilabel option must be set on each file system as stated at the beginning of this chapter. Not doing so will result in errors. While at it, ensure that the net-mngt/nagios-plugins, net-mngt/nagios, and www/apache13 ports are all installed, configured, and working correctly.

This example considers a relatively small, fewer than fifty users, storage system. Users would have login capabilities, and be permitted to not only store data but access resources as well.

For this scenario, the mac_bsdextended(4) mixed with mac_seeotheruids(4) could co-exist and block access not only to system objects but to hide user processes as well.

Begin by adding the following lines to /boot/loader.conf:

The mac_bsdextended(4) security policy module may be activated through the use of the following rc.conf variable:

Default rules stored in /etc/rc.bsdextended will be loaded at system initialization; however, the default entries may need modification. Since this machine is expected only to service users, everything may be left commented out except the last two. These will force the loading of user owned system objects by default.

Add the required users to this machine and reboot. For testing purposes, try logging in as a different user across two consoles. Run the ps aux command to see if processes of other users are visible. Try to run ls(1) on another users home directory, it should fail.

Do not try to test with the root user unless the specific sysctls have been modified to block super user access.

During the development stage, a few users reported problems with normal configuration. Some of these problems are listed below: