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Getting started with SELinux
Começando com SELinux
The Fedora Docs Team; Peter Boy (pboy)
 
Introduction to SELinux
Introdução ao SELinux
Security Enhanced Linux (SELinux) provides an additional layer of system security. SELinux fundamentally answers the question: _May <subject> do <action> to <object>?_, for example: _May a web server access files in users' home directories?_
 
The standard access policy based on the user, group, and other permissions, known as Discretionary Access Control (DAC), does not enable system administrators to create comprehensive and fine-grained security policies, such as restricting specific applications to only viewing log files, while allowing other applications to append new data to the log files.
 
SELinux implements Mandatory Access Control (MAC). Every process and system resource has a special security label called a _SELinux context_. A SELinux context, sometimes referred to as a _SELinux label_, is an identifier which abstracts away the system-level details and focuses on the security properties of the entity. Not only does this provide a consistent way of referencing objects in the SELinux policy, but it also removes any ambiguity that can be found in other identification methods; for example, a file can have multiple valid path names on a system that makes use of bind mounts.
 
The SELinux policy uses these contexts in a series of rules which define how processes can interact with each other and the various system resources. By default, the policy does not allow any interaction unless a rule explicitly grants access.
 
It is important to remember that SELinux policy rules are checked after DAC rules. SELinux policy rules are not used if DAC rules deny access first, which means that no SELinux denial is logged if the traditional DAC rules prevent the access.
 
SELinux contexts have several fields: user, role, type, and security level. The SELinux type information is perhaps the most important when it comes to the SELinux policy, as the most common policy rule which defines the allowed interactions between processes and system resources uses SELinux types and not the full SELinux context. SELinux types usually end with `_t`. For example, the type name for the web server is `httpd_t`. The type context for files and directories normally found in `/var/www/html/` is `httpd_sys_content_t`. The type contexts for files and directories normally found in `/tmp` and `/var/tmp/` is `tmp_t`. The type context for web server ports is `http_port_t`.
 
For example, there is a policy rule that permits Apache (the web server process running as `httpd_t`) to access files and directories with a context normally found in `/var/www/html/` and other web server directories (`httpd_sys_content_t`). There is no allow rule in the policy for files normally found in `/tmp` and `/var/tmp/`, so access is not permitted. With SELinux, even if Apache is compromised, and a malicious script gains access, it is still not able to access the `/tmp` directory.
 
SELinux allows the Apache process running as httpd_t to access the /var/www/html/ directory and it denies the same process to access the /data/mysql/ directory because there is no allow rule for the httpd_t and mysqld_db_t type contexts). On the other hand, the MariaDB process running as mysqld_t is able to access the /data/mysql/ directory and SELinux also correctly denies the process with the mysqld_t type to access the /var/www/html/ directory labeled as httpd_sys_content_t.
 
SELinux_Apache_MariaDB_example
 
selinux-intro-apache-mariadb.png
 
Additional resources
Recursos Adicionais
To better understand SELinux basic concepts, see the following documentation:
 
link:++https://people.redhat.com/duffy/selinux/selinux-coloring-book_A4-Stapled.pdf++[The SELinux Coloring Book]
 
link:++https://people.redhat.com/tcameron/Summit2012/SELinux/cameron_w_120_selinux_for_mere_mortals.pdf++[SELinux for Mere Mortals]
 
link:++http://selinuxproject.org/page/FAQ++[SELinux Wiki FAQ]
 
link:++http://freecomputerbooks.com/books/The_SELinux_Notebook-4th_Edition.pdf++[The SELinux Notebook]
 
Benefits of running SELinux
 
SELinux provides the following benefits:
 
All processes and files are labeled. SELinux policy rules define how processes interact with files, as well as how processes interact with each other. Access is only allowed if an SELinux policy rule exists that specifically allows it.
 
Fine-grained access control. Stepping beyond traditional UNIX permissions that are controlled at user discretion and based on Linux user and group IDs, SELinux access decisions are based on all available information, such as an SELinux user, role, type, and, optionally, a security level.
 
SELinux policy is administratively-defined and enforced system-wide.
 
Improved mitigation for privilege escalation attacks. Processes run in domains, and are therefore separated from each other. SELinux policy rules define how processes access files and other processes. If a process is compromised, the attacker only has access to the normal functions of that process, and to files the process has been configured to have access to. For example, if the Apache HTTP Server is compromised, an attacker cannot use that process to read files in user home directories, unless a specific SELinux policy rule was added or configured to allow such access.
 
SELinux can be used to enforce data confidentiality and integrity, as well as protecting processes from untrusted inputs.
 
However, SELinux is not:
 
antivirus software,
 
replacement for passwords, firewalls, and other security systems,
 

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