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SELinux - Redhat / CentOS 7
Introduction
Security-Enhanced Linux (SELinux) is a mandatory access control (MAC) security mechanism implemented in the kernel. SELinux was first introduced in CentOS 4 and significantly enhanced in later CentOS releases. These enhancements mean that content varies as to how to approach SELinux over time to solve problems.
Some of the Problems
In order to better understand why SELinux is important and what it can do for you, it is easiest to look at some examples. Without SELinux enabled, only traditional discretionary access control (DAC) methods such as file permissions or access control lists (ACLs) are used to control the file access of users. Users and programs alike are allowed to grant insecure file permissions to others or, conversely, to gain access to parts of the system that should not otherwise be necessary for normal operation. For example:
- Administrators have no way to control users: A user could set world readable permissions on sensitive files such as ssh keys and the directory containing such keys, customarily: ~/.ssh/
- Processes can change security properties: A user's mail files should be readable only by that user, but the mail client software has the ability to change them to be world readable
- Processes inherit user's rights: Firefox, if compromised by a trojaned version, could read a user's private ssh keys even though it has no reason to do so.
Essentially under the traditional DAC model, there are two privilege levels, root and user, and no easy way to enforce a model of least-privilege. Many processes that are launched by root later drop their rights to run as a restricted user and some processes may be run in a chroot jail but all of these security methods are discretionary.
The Solution
SELinux follows the model of least-privilege more closely. By default under a strict enforcing setting, everything is denied and then a series of exceptions policies are written that give each element of the system (a service, program or user) only the access required to function. If a service, program or user subsequently tries to access or modify a file or resource not necessary for it to function, then access is denied and the action is logged.
Because SELinux is implemented within the kernel, individual applications do not need to be especially written or modified to work under SELinux although, of course, if written to watch for the error codes which SELinux returns, vide infra, might work better afterwards. If SELinux blocks an action, this is reported to the underlying application as a normal (or, at least, conventional) “access denied” type error to the application. Many applications, however, do not test all return codes on system calls and may return no message explaining the issue or may return in a misleading fashion.
Please note, however, that the hypothetical examples posed to provide possible greater safety of e.g., constraining programs authorized to a limited set of programs permitted to read a user's ~/.ssh/ directory, preventing a Mail Delivery Agent from tampering with group owernship or setting on group or other file read permissions, or a web browser being constrained from reading the user's home directory have not been implemented in SELinux policies accompanying any version of CentOS up to version 6. CentOS 6 and 7 have limited support for confining user programs as described above, but doesn't have as much coverage over user programs as targeted system daemons. If an admin wishes to change from the default unconfined login configuration, they can see the section below on Role-Based Access Control.
SELinux Modes
SELinux has three basic modes of operation, of which Enforcing is set as the installation default mode. There is, however, an additional qualifier of targeted or mls which control how pervasive SELinux rules are applied, with targeted being the less stringent level.
- Enforcing: The default mode which will enable and enforce the SELinux security policy on the system, denying access and logging actions
- Permissive: In Permissive mode, SELinux is enabled but will not enforce the security policy, only warn and log actions. Permissive mode is useful for troubleshooting SELinux issues
- Disabled: SELinux is turned off
The SELinux mode can be viewed and changed by using the SELinux Management GUI tool available on the Administration menu or from the command line by running 'system-config-selinux' (the SELinux Management GUI tool is part of the policycoreutils-gui package and is not installed by default).
Users who prefer the command line may use the “sestatus” command to view the current SELinux status:
# sestatus
[root@vbulli ~]# sestatus SELinux status: enabled SELinuxfs mount: /sys/fs/selinux SELinux root directory: /etc/selinux Loaded policy name: targeted Current mode: permissive Mode from config file: enforcing Policy MLS status: enabled Policy deny_unknown status: allowed Max kernel policy version: 28
The setenforce command may be used to switch between 1 →Enforcing and 0 → Permissive modes on the fly but note that these changes do not persist through a system reboot.
To make changes persistent through a system reboot, edit the 'SELINUX=' line in /etc/selinux/config for either 'enforcing', 'permissive', or 'disabled'. For example: 'SELINUX=permissive'
Note: When switching from Disabled to either Permissive or Enforcing mode, it is highly recommended that the system be rebooted and the filesystem relabeled.
SELinux Policy
As noted, SELinux follows the model of least-privilege; by default everything is denied and then a policy is written that gives each element of the system only the access required to function. This description best describes the strict policy. However, such a policy is difficult to write that would be suitable in the wide range of circumstances that a product such as Enterprise Linux is likely to be used. The end result is that SELinux is likely to cause problems for system administrators and end users and rather than resolve these issues, system administrators may just disable SELinux thereby defeating the built-in protections.
By design, SELinux allows different policies to be written that are interchangeable. The default policy in CentOS is the targeted policy which “targets” and confines selected system processes. In CentOS 4 only 15 defined targets existed (including httpd, named, dhcpd, mysqld). Later, in CentOS 5 this number had risen to over 200 targets.
All other system processes and all remaining userspace programs, as well as any in-house applications, that is everything else on the system, runs in an unconfined domain and is not covered by the SELinux protection model.
One goal might be for every process that is installed and, by default, running at boot should be run in a confined domain. The targeted policy is designed to protect as many key processes as possible without adversely affecting the end user experience and most users should be totally unaware that SELinux is even running.
SELinux Access Control
The targeted SELinux policy on Redhat/CentOS ships with 4 forms of access control:
- Type Enforcement (TE): Type Enforcement is the primary mechanism of access control used in the targeted policy
- Role-Based Access Control (RBAC): Based around SELinux users (not necessarily the same as the Linux user), but not used in the default configuration of the targeted policy
- Multi-Level Security (MLS): Not commonly used and often hidden in the default targeted policy.
- Multi-Category Security(MCS): An extension of Multi-Level Security, used in the targeted policy to implement compartmentalization of virtual machines and containers through sVirt.
All processes and files have an SELinux security context! Let's see these in action by looking at the SELinux security context of the Apache homepage: '/var/www/html/index.html'
# ls -Z /var/www/html/index.html
-rw-r--r-- username username system_u:object_r:httpd_sys_content_t /var/www/html/index.html
In addition to the standard file permissions and ownership, we can see the SELinux security context fields: system_u:object_r:httpd_sys_content_t.
This is based upon user:role:type:mls. In our example above, user:role:type fields are displayed and mls is hidden. Within the default targeted policy, type is the important field used to implement Type Enforcement, in this case httpd_sys_content_t.
Now consider the SELinux security context of the Apache web server process: 'httpd'
# ps axZ | grep httpd
system_u:system_r:httpd_t 3234 ? Ss 0:00 /usr/sbin/httpd
Here we see the from the type field that Apache is running under the httpd_t type domain.
Finally, let's look at the SELinux security context of a file in our home directory:
# ls -Z /home/username/myfile.txt
-rw-r--r-- username username user_u:object_r:user_home_t /home/username/myfile.txt
here we see the type is user_home_t, the default type for files in a user's home directory.
Access is only allowed between similar types, so Apache running as httpd_t can read /var/www/html/index.html of type httpd_sys_content_t. Because Apache runs in the httpd_t domain and does not have the userid:username, it can not access /home/username/myfile.txt even though this file is world readable because /home/username/myfile.txt SELinux security context is not of type httpd_t. If Apache were to be exploited, assuming for the sake of this example that the root account right needed to effect a SELinux re-labeling into another context were not obtained, it would not be able to start any process not in the httpd_t domain (which prevents escalation of privileges) or access any file not in an httpd_t related domain.
Role-Based Access Control (RBAC)
Although the default configuration of the targeted policy is to use unconfined logins, the administrator can quite easily switch to the Role-Based Access Control model. This model also switches to 'strict' mode for user domains, to allow targeting each program individually. To enable this, use semanage-login to add a login mapping for your user.
