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.
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.
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