1. SELinux (Security Enhanced Linux)
By: Corey McClurg
12/1/2018
Corey McClurg SELinux

2. Outline What is SELinux and how do I get it? A History of SELinux
Getting Started
Mandatory Access Control (MAC)
Using Type Enforcement to provide access Control
Creating rules with example
Roles (RBAC)
Limiting CGI Scripts
Conclusion
12/1/2018
Corey McClurg SELinux

3. A Brief History
Originally started by the Information Assurance Research Group of the NSA, working with Secure Computing Corporation.
Based on a strong, flexible mandatory access control architecture with Type Enforcement, a mechanism first developed for the LOCK system
12/1/2018
Corey McClurg SELinux

4. History cont.
Originally started as two prototypes: DTMach and DTOS which were eventually transferred over to the Fluke research operating system
Eventually the architecture was enhanced and renamed Flask. The NSA has now integrated the Flask architecture with Linux (SELinux)
12/1/2018
Corey McClurg SELinux

5. What is the Idea behind SELinux?
An example of how mandatory access controls can be added into Linux (Confining the actions of a process, including a superuser (root) process)
Implemente security mechanisms in the system that provide flexible support for a wide range of security policies.
Make it possible to configure the system to meet a wide range of security requirements.
Documentation and source code is provided.
12/1/2018
Corey McClurg SELinux

6. How do I get SELinux?
It comes as an installation option when you install a number of Linux distributions.
The latest release is also available from the NSA website for download, along with documentation, all of the different libraries and the policy compiler.
12/1/2018
Corey McClurg SELinux

7. How do I get started?
The release includes a general-purpose security policy configuration designed to meet a number of security objectives, this can be used as an example to go off of.
Due to the flexibility of the system, the security policy can be modified and extended to customize for any given installation.
12/1/2018
Corey McClurg SELinux

8. Mandatory Access Control (MAC)
A means of restricting access to objects based on the sensitivity of the information contained in the objects and whether they are authorized to access information of such sensitivity
Authorization is based on prerequisites being met, resulting in an individual gaining access
Enables the ability to deny users full control over the access to resources that they create
access control is based on the compatibility of the security properties of the data and the clearance properties of the individual
12/1/2018
Corey McClurg SELinux

9. Type Enforcement
In order to grant access to something, an allow rule must be created, such as:
allow user_t bin_t : file {read execute getattr};
This means a process with a domain type of user_t can read, execute, or get attributes for a file object with a type of bin_t, there is no significance in the “_t” portion.
This rule might be in a policy to allow users to execute shell programs such as the bash shell (/bin/bash).
12/1/2018
Corey McClurg SELinux

10. Using Type Enforcement
3 Necessary things for a domain transition to occur:
1. The process' new domain type has entrypoint access to an executable file type.
2. The process' current (or old) domain type has execute access to the entry point file type.
3. The process' current domain type has transition access to the new domain type.
12/1/2018
Corey McClurg SELinux

11. Example Execute the command “ls -Z /usr/bin/passwd”
This will produce the output: r-s—x—x root root system_u:object_r:passwd_exec_t /usr/bin/passwd
Using this provided information, we can then create TE rules to have a domain transition.
Three rules are required to give the user the ability to do a domain transition to the password file:
allow user_t passwd_exec_t : file {getattr execute};
Lets user_t execute an execve() system call on passwd_exec_t
allow passwd_t passwd_exec_t : file entrypoint;
This rule provides entrypoint access to the passwd_t domain, entrypoint defines which executable files can “enter” a domain.
allow user_t passwd_t : process transition;
The original type (user_t) must have transition permission to the new type (passwd_t) for the domain transition to be allowed.
12/1/2018
Corey McClurg SELinux

12. Example cont.
This isn't very useful by itself since the user would have to specifically say that they want a domain transition. This is where type transition rules are used.
To create a domain transition by default the following rule is created:
type_transition user_t passwd_exec_t : process passwd_t;
The type_transition rule indicates that by default on an execve() system call, if the calling process' domain type is user_t and the executable file's type is passwd_exec_t a domain transition to a new domain type (passwd_t) will be attempted
A type_transition rule causes a domain transition to be attempted by default, but it does not allow it, that's why the other 3 rules had to be created
12/1/2018
Corey McClurg SELinux

13. Example cont. What does this example accomplish?
It makes it so that the current user can change the password in the shadow(encrypted password) file, without these rules, this would not be possible even for the root user. The user isn't able to access the file directly though, they must access it through the passwd file to restrict what they are allowed to do.
12/1/2018
Corey McClurg SELinux

14. Another Option:
In addition to Type Enforcement, SELinux also provides a form of role-based access control (RBAC).
Roles build on type enforcement to limit the types to which a process may transition. Roles are based on the role identifier in the process' security context.
Using roles is a way to be even more specific about what access rights are given.
12/1/2018
Corey McClurg SELinux

15. Limiting CGI scripts with SELinux
There are types already defined in SELinux with different rights given to them.
httpd_sys_script_ro_t
A CGI script may only read files and directories with this type. Setting all cgi scripts to this other than index.cgi will make it so only index.cgi can do more than read files and directories
httpd_sys_script_exec_t
index.cgi must be set to this since httpd cannot execute any other types.
httpd_sys_script_rw_t
If a cgi script needs to be able to write, say to a database, then it will need this right (read/write access)
12/1/2018
Corey McClurg SELinux

16. CGI cont.
In addition to limiting the access of CGI scripts themselves, unless rules have been specifically declared to give a user access to CGI scripts, no user, including root will be able to access them in the first place.
CGI scripts can be much more safe on an apache server with SELinux implemented
12/1/2018
Corey McClurg SELinux

17. Conclusion
When used properly, SELinux can make a system much more secure
A user is confined to being able to do only what has been defined in the SELinux policy for that user.
There are several different routes that can be used to achieve the level of desired security using SELinux.
12/1/2018
Corey McClurg SELinux

18. References
asdasd
12/1/2018
Corey McClurg SELinux