Multi Category Security or MCS is a discretionary implementation of the mandatory Multi Level Security or MLS Model. SELinux Policy MLS is a SELinux Policy model that is used in Department of Defense type environments. In a MLS environment processes are forced to operate on specified Security Levels. The s0 Security Level or SystemLow level is the lower end of the Security Level Range in a MLS environment. The s15 Security Level or SystemHigh level is the upper end of the Security Range in a MLS environment. Between the low and upper end there are fourteen levels to be used.
In MLS there is a rule that says: "no read up and no write down". This means that processes that are forced to operate on for example Security Level s14 can not read processes or files that operate on the s15 Security Level. Processes that are forced to operate on the s5 Security Level can not write to files or interact with processes on the s4 Security Level. The MLS model is used to enforce confidentiality.
MCS basically tries to use the MLS attributes: Security Levels and Security Compartments, in its own model in a way that may be useful in common environments. SELinux Policy Targeted can be build with the MCS functionality. Red Hat distributions have this MCS functionality enabled by default in its Targeted SELinux Policy model. Gentoo Hardened, as far as i know, does not have MCS functionality builtin by default.
MCS pretty much works like the DAC Extended attributes. Users are assigned categories and can apply these categories to content that they own to their discretion. You can easily recognise a MCS enabled system by looking at security contexts. A system that has SELinux Policy Targeted implemented without MCS enabled only has three fields in its Security Context tuples: user_u:role_r:type_t. Systems that have MCS implemented have one or more extra fields in their Security Context tuple: user_u:role_r:type_t:s0:c0. In MCS policy there is only one Security Level. This SystemLow level is s0. MCS does have 1024 categories that can be assigned to processes and files. Categories are the last field in the Security Context tuple. In a MCS environment s0:c0.c1023 is SystemHigh or the upper end of the MCS range. This means that if you are assigned the SystemHigh MCS range that you can access all categories. By default everything in a MCS environment has access to SystemLow or s0.
Create a new SELinux User that is based of off the user_u SELinux User. Call this SELinux User for example "mcsuser_u" and assign the full MCS range to this SELinux User:
sudo semanage user -a -L s0 -r s0-s0:c0.c1023 -R user_u -P user mcsuser_u
sudo cp /etc/selinux/targeted/contexts/users/user_u /etc/selinux/targeted/contexts/users/mcsuser_u
( to list the differences between SELinux User user_u and mcsuser_u simply: sudo semanage user -l | grep user_u )
Now create three Linux Users and map them to the mcsuser_u SELinux user. Give John access to the s0-s0:c122 MCS range. Give Jane access to the s0-s0:c123 MCS range, and give johnjane access to the s0-s0:c122,c123 MCS range.
sudo useradd john
sudo useradd jane
sudo useradd johnjane
sudo semanage login -a -s mcsuser_u -r s0-s0:c122 john
sudo semanage login -a -s mcsuser_u -r s0-s0:c123 jane
sudo semanage login -a -s mcsuser_u -r s0-s0:c122,c123 johnjane
Login as john, touch file with name test and list its attributes:
# touch test; ls -Z test;
The file was created on the s0 SystemLow level which is accessible by everything. Now add the c122 category to the file with the chcat command, and list the SELinux attributes of file test:
# chcat -- +s0:c122 test
# ls -Z test
If you try to add for example category s0:c123 to the file you will be denied access to do so because your assigned MCS range does not include the s0:c123 category.
Try the same procedure for Jane but instead use the s0:c123 category.
Linux User johnjane has access to both s0:c122 and s0:c123 MCS categories. In a shared location where DAC permissions are sufficient johnjane would be able to access both files with s0:c122 as well as s0:c123 categories.
Johnjane can also assign both s0:c122 and s0:c123 to a single file, but then neither John nor Jane woould be able to access it.
If all these MCS category digits make you dizzy then you can install mcstrans. Mcstrans is a daemon that translates the MCS category numbers into strings of letters which are easier to work with. The mcstrans daemon has some problems though.
# sudo yum install mcstrans
# chkconfig mcstrans on
# service mcstrans start
Now one can add translations for the MCS category digits with the semanage command.
Translate s0:c122 to JohnsFriends, s0:c123 to JanesFriends, and s0:c122,c123 to JohnJanesFriends:
# sudo semanage translation -a -T JohnsFriends s0:c122
# sudo semanage translation -a -T JanesFriends s0:c123
# sudo semanage translation -a -T JohnsJanesFriend s0:c122,c123
( use sudo semanage translation -l to list current translation mappings )
Note: After this the mcstrans may have died. If required restart the mcstrans daemon.
# sudo service mcstrans restart
Users can use the chcat command to list which categories they have assigned:
# chcat -L john
User can also use the id command with the -Z option to view their security context. The context displays to categories.
# id -Z
MCS is a neat extra functionality that can be enabled on systems that have SELinux Targeted Policy implemented.
The functionality of MCS is similar to that of Extended Attributes.
man: chcat, man semanage, man id
Attributes make writing SELinux policy easier - Yesterday I received an email from someone who was attempting to write SELinux policy for a daemon process, "abcd", that he was being required to run on ...
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