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documentation.suse.com / Understanding SELinux Basics

Understanding SELinux Basics

Publication Date: 17 Oct 2024
WHAT?

This article provides basic information about Security-Enhanced Linux.

WHY?

You want to understand SELinux and how to configure it on SUSE Linux Enterprise Micro.

EFFORT

It takes approximately 40 minutes to read this article.

1 About SELinux

 

SELinux was developed as an additional Linux security solution that uses the security framework in the Linux kernel. The purpose was to allow for a more granular security policy that goes beyond the standard Discretionary Access Controls (DAC), the traditional file permissions of owner/group/world, and read/write/execute.

SELinux uses labels attached to objects (for example, files and network sockets) to make access control decisions.

The default action of SELinux is to deny any access. SELinux allows only actions that were specifically allowed in the SELinux policy. Another feature of SELinux that increases security is that SELinux allows strict confinement of processes up to the point where the processes cannot access files of other processes on the same system.

SELinux was designed to enhance existing security solutions, not to replace them. For example, discretionary access control (DAC) is still applied, even if the system is using SELinux. If DAC denies access first, SELinux is then not used as the access was already blocked by another mechanism.

2 Getting SELinux

 

SELinux is installed by default when installing SLE Micro by YaST or is part of the pre-built images.

If SELinux is not set up on your system, run the following command: 

# transactional-update setup-selinux

Reboot your system after the command has finished. The command installs the SELinux policy if it is not installed, sets the enforcing SELinux mode and rebuilds initrd.

3 SELinux modes

 

SELinux can run in one of three modes: disabled, permissive or enforcing.

Using the disabled mode means that no rules from the SELinux policy are applied and your system is not protected. Therefore, we do not recommend using the disabled mode.

In the permissive mode, SELinux is active, the security policy is loaded, the file system is labeled and access denial entries are logged. However, the policy is not enforced and thus no access is actually denied.

In the enforced mode, the security policy is applied. Each access that is not explicitly allowed by the policy is denied.

For information about switching between SELinux modes, refer to Section 3.1, “Changing the SELinux mode”.

3.1 Changing the SELinux mode

 

You can switch the SELinux mode temporarily or permanently.

3.1.1 Changing the SELinux mode temporarily

 

To set SELinux to the permissive or enforcing mode temporarily, use the command setenforce.

The setenforce command has the following syntax: 

# setenforce MODE_ID

where MODE_ID is 0 for the permissive mode or 1 for the enforced mode.

Remember that you cannot disable SELinux using the setenforce command.

3.1.2 Changing the SELinux mode permanently

 

To perform changes to the SELinux mode that persists rebooting of the system, edit the /etc/selinux/config configuration file. In this file, you can also disable SELinux on your system. However, this action is not recommended. If SELinux is possibly causing issues to your system, switch to the permissive mode instead and debug your system.

In the file /etc/selinux/config, change the value of SELINUX to disabled, or permissive, or enforced as follows: 

SELINUX=disabled

The changes in the file are applied after the next reboot.

Note
Note: Relabeling your system after switching from the disabled mode

If you disable SELinux on your system and then enable it later, make sure that you relabel your system. When SELinux is disabled and you perform changes to your file system, the changes are not reflected in the context anymore (for example, new files do not have any context). Therefore, you need to relabel your system by using the restorecon command, using the autorelabel boot parameter, or by creating a file that will trigger relabeling on the next boot. To create the file, run the following command: 

# touch /etc/selinux/.autorelabel

After reboot, the file /etc/selinux/.autorelabel is replaced with another flag file, /etc/selinux/.relabelled, to prevent relabeling on subsequent reboots.

3.1.3 Verifying the active SELinux mode

 

To verify the mode, run the following command: 

# getenforce

The command should return permissive or enforced, depending on the provided MODE_ID.

3.2 Verifying that SELinux is functional

 

If you are performing configuration changes, it may be useful to switch to permissive mode. During this time, users might label files incorrectly, and thus cause problems when switching back to enforcing mode.

To return the system back to its secured state, perform the following steps:

  1. Reset the security context: 

    > sudo restorecon -R /

  2. Switch to enforcing mode by setting SELINUX=enforcing in the /etc/selinux/config.

  3. Reboot the system and log in again.

  4. Run the sestatus -v command. It should give you an output similar to the following one: 

    > sudo sestatus -v
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Memory protection checking:     requested(insecure)
Max kernel policy version:      33

Process contexts:
Current context:                unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
Init context:                   system_u:system_r:init_t:s0
/usr/sbin/sshd                  system_u:system_r:sshd_t:s0-s0:c0.c1023

File contexts:
Controlling terminal:           unconfined_u:object_r:user_tty_device_t:s0
/etc/passwd                     system_u:object_r:passwd_file_t:s0
/etc/shadow                     system_u:object_r:shadow_t:s0
/bin/bash                       system_u:object_r:shell_exec_t:s0 \
                                -> system_u:object_r:shell_exec_t:s0
/bin/login                      system_u:object_r:login_exec_t:s0
/bin/sh                         system_u:object_r:bin_t:s0 \
                                -> system_u:object_r:shell_exec_t:s0
/sbin/agetty                    system_u:object_r:bin_t:s0 \
                                -> system_u:object_r:getty_exec_t:s0
/sbin/init                      system_u:object_r:bin_t:s0 -> \
                                system_u:object_r:init_exec_t:s0
/usr/sbin/sshd                  system_u:object_r:sshd_exec_t:s0

  5. If the system is not working properly, check the log files in /var/log/audit/audit.log. For more details, refer to SELinux troubleshooting.

4 SELinux security context

 

The security context is a set of information assigned to a file or a process. It consists of SELinux user, role, type, level and category. This information is used to make access control decisions.

SELinux context fields
 
SELinux user

An identity defined in the policy that is authorized for a specific set of roles and for a specific level range. Each Linux user is mapped to only one SELinux user. However, one SELinux user can have several roles.

SELinux does not use the list of user accounts maintained by Linux in /etc/passwd, but uses its own database and mapping. By convention, the identity name is suffixed with _u, for example, user_u.

When a new Linux account is created and the SELinux user is not assigned to the account, the default SELinux user is used. Usually, the default value is unconfined_u. For a procedure on how to change the default value, refer to Section 7.5.2, “The semanage login command”.

role

Defines a set of permissions that a user can be granted. A role defines which types a user assigned to this role can access. By convention, the role name is suffixed with _r, for example, system_r.

type

The type conveys information on how particular files and processes can interact. A process consists of files with a concrete SELinux type, and it cannot access files outside of this type. By convention, the type name is suffixed with _t, for example, var_t.

level

An optional attribute that specifies the range of levels of clearance in the multilevel security.

category

An optional attribute that allows you to add categories to processes, files and users. A user can then access files that have the same category.

Here is an example of an SELinux context: 

allow user_t bin_t:file {read execute gettattr};

This example rule states that the user who has the context type user_t (this user is called the source object) is allowed to access objects of the class file with the context type bin_t (the target), using the permissions read, execute and getattr.

5 SELinux policy overview

 

The policy is the key component in SELinux. Your SELinux policy defines rules that specify which objects can access which files, directories, ports and processes on a system. To do this, a security context is defined for all of these.

An SELinux policy contains a huge number of rules. To make it more manageable, policies are often split into modules. This allows the administrator to switch protection on or off for different parts of the system.

When compiling the policy for your system, you will have a choice to either work with a modular policy, or a monolithic policy, where one huge policy is used to protect everything on your system. We strongly recommend using a modular policy and not a monolithic policy. Modular policies are much easier to manage.

SLE Micro is shipped with the targeted SELinux policy.

5.1 Working with SELinux modules

 

As an administrator, you can switch modules on or off. This can be useful if you want to disable only a part of the SELinux policy and you do not want to run a specific service without SELinux protection.

To view all SELinux policy modules in use, run the command: 

semodule -l

After you get the name of the module you want to switch off, run the command: 

> sudo semodule -d MODULENAME

To switch on the policy module, run the command: 

> sudo semodule -e MODULENAME

5.2 Creating policies for containers

 

SLE Micro is delivered with a policy that, by default, does not allow containers to access files outside the container data. On the other hand, all network access is allowed. Typically, containers are created with bind mounts and should be able to access other directories, like /home or /var. You may want a possibility to allow access to these directories or, on the contrary, restrict some ports to the container even if SELinux is used on your system. In this case, you need to create new policy rules that enable or disable the access. SLE Micro provides the Udica tool for this purpose.

The following procedure describes how to create a custom policy for your containers:

  1. Make sure that SELinux is in the enforcing mode. For details, refer to Section 3.1, “Changing the SELinux mode”.

  2. Start a container using the following parameters: 

    # podman run -v /home:/home:rw -v /var/:/var/:rw -p 21:21 -it sle15 bash

    The container runs with the default policy that does not allow access to the mount points but does not restrict other ports.

  3. You can exit the container.

  4. Obtain the container ID: 

    # podman ps -a

CONTAINER ID  IMAGE                                                                              COMMAND     CREATED        STATUS            PORTS               NAMES
e59f9d0f86f2  registry.opensuse.org/devel/bci/tumbleweed/containerfile/opensuse/bci/ruby:latest  /bin/bash   8 minutes ago  Up 8 seconds ago  0.0.0.0:21->21/tcp  zen_ramanujan

  5. Create a JSON file that Udica will use to create a custom policy for the container: 

    # podman inspect e59f9d0f86f2 > OUTPUT_JSON_FILE

    For example, substitute OUTPUT_JSON_FILE with container.json.

  6. Run Udica to generate a policy according to the container parameters: 

    # udica -j OUTPUT_JSON_FILE  CUSTOM_CONTAINER_POLICY

    For example: 

    # udica -j container.json custom_policy

  7. According to the provided instructions, load the policy modules by running: 

    # semodule -i custom_policy.cil /usr/share/udica/templates/{base_container.cil,net_container.cil,home_container.cil}

  8. Run a container with the new policy module by using the --security-opt option as follows: 

    # podman run --security-opt label=type:custom_policy.process -v /home:/home:rw -v /var/:/var/:rw -p 21:21 -it sle15 bash

6 SELinux Booleans

 

SELinux Booleans support a flexible policy management approach. For example, Booleans enable you to disable a particular policy on one server, while keeping the same policy active on another one. In other words, a Boolean can be understood as a switch for a policy rule. Instead of changing a particular policy, you can switch it off. In the policy code, Booleans are called a tunable. Because Booleans are included in the policy, they are available as soon as a policy is loaded.

The changes to the Booleans value may be persistent or temporary, lasting until the end of the session.

SELinux offers tools that enable you to list and view details or change the state of Booleans. See the following sections for details.

6.1 Working with Booleans

 

6.1.1 Listing Booleans

 

You can use the getsebool or semanage command to list currently defined Booleans. To list all currently defined Booleans, along with their state, run the following command: 

# getsebool -a

abrt_anon_write --> off
abrt_handle_event --> off
abrt_upload_watch_anon_write --> on
...

To get more details about particular Booleans, you can use the semanage command as follows: 

# semanage boolean -l

SELinux boolean               State   Default     Description

abrt_anon_write               (off  ,  off)       Allow abrt to anon write
abrt_handle_event             (off  ,  off)       Allow abrt to handle event
abrt_upload_watch_anon_write  (on   ,   on)       Allow abrt to upload watch anon write

To get the status of an individual Boolean, you can use the following command: 

# getsebool BOOLEAN_NAME

Alternatively, you can just use the grep command on the semanage boolean output: 

# semanage boolean -l | grep BOOLEAN_NAME

6.1.2 Toggling Booleans

 

The commands setsebool and semanage can be used to toggle the value of Booleans. You can change the Boolean status persistently or just temporarily until the session ends. To change a Boolean value temporarily, run the following command: 

# setsebool BOOLEAN_NAME BOOLEAN_VALUE

where BOOLEAN_VALUE is either on or off.

To change a Boolean value persistently, run one of the following two commands: 

# setsebool -P BOOLEAN_NAME BOOLEAN_VALUE

Alternatively, using the semanage command: 

# semanage boolean -m --BOOLEAN_VALUE  BOOLEAN_NAME

where BOOLEAN_VALUE is either on or off.

A single Boolean can enable or disable several policy rules. To see which policy rules are enabled or disabled by specific Booleans, use the sedispol tool to analyze the policy file: 

# sedispol /etc/selinux/targeted/policy/policy.32

As the policy rules are usually huge, we recommend setting an output file by selecting the f and specifying a file name. After specifying the file name, press 6. Then you can inspect the file.

7 Tools for managing SELinux

 

SLE Micro provides you with tools to manage SELinux on your system. If the below described tools are not installed on your system, install the tools by running: 

# transactional-update pkg install policycoreutils-python-utils

After successful installation, reboot the system.

7.1 Using the Z option

 

Where SELinux is installed and configured, you can use the -Z to regular commands like ls, id or ps. Using this option, you can display the security context of files or processes. For example, with the ls command: 

> ls -Z /etc/shadow

        system_u:object_r:shadow_t:s0 /etc/shadow

7.2 The chcon command

 

The command name chcon stands for change context. The command can change the full security context of a file to the value provided on the CLI, or it can change parts of the context. Alternatively, you can provide a file that serves as a reference.

To change the full security context of a file, the command syntax looks as follows: 

# chcon SECURITY_CONTEXT FILENAME

where:

  • SECURITY_CONTEXT is in the format: SELinux_USER:ROLE:TYPE:LEVEL:CATEGORY. For example, the context could be: system_u:object_r:httpd_config_t:s0.

  • FILENAME is a path to the file whose context should be changed.

To set a security context according to a provided file that serves as a reference, run chcon as follows: 

# chcon --reference=REFERENCE_FILE FILENAME

where:

  • REFERENCE_FILE is a path to a file that should be used as a reference.

  • FILENAME is a path to the file whose context should be changed.

Alternatively, you can change only one part of the security context. The general syntax of the chcon command is as follows: 

# chcon CONTEXT_OPTION CONTEXT_PART FILENAME

The options and arguments have the following meaning:

  • depending on the context part, CONTEXT_OPTION can be any of the following:

    -u resp --user

    denotes that an SELinux user context will be changed on the provided file: 

    # chcon -u system_u logind.conf
    -r resp --role

    only the role part will be changed in the context of the provided file: 

    # chcon -r object_r logind.conf
    -t resp --type

    only the type part will be changed in the context of the provided file: 

    # chcon -t etc_t logind.conf
    -l resp --range

    only the range part of the security context will be changed: 

    # chcon -l s0 logind.conf

  • CONTEXT_PART is the particular value of the security context to be set.

  • FILENAME is a path to the file whose context will be changed.

Note
Note: Using chcon on symbolic links

By default, when you change the security context on a symbolic link, the context of the link target is changed and the symbolic link context is not changed. To force chcon to change the context of the symbolic link and not the link target, use the --no-dereference option as shown below: 

# chcon --no-dereference -u system_u -t etc_t network.conf

You can change the context of all files in a directory by using the recursive option: 

# chcon --recursive  system_u:object_r:httpd_config_t:s0 conf.d

7.3 getenforce and setenforce commands

 

The getenforce command returns the current SELinux mode: Enforcing, Permissive or Disabled. 

# getenforce

Permissive

The setenforce command temporarily changes the SELinux mode to enforcing or permissive. You cannot use this command to disable SELinux. Remember that the change persists only until the next reboot. To change the state permanently, follow the description in Section 3.1, “Changing the SELinux mode”. 

# setenforce MODE_ID

where MODE_ID is 0 for the permissive mode or 1 for the enforced mode.

7.4 The fixfiles script

 

The script enables you to perform the following tasks with the security context:

  • check if the context is correct

  • change any incorrect file context labels

  • relabel your system if you added a new policy

The script syntax is as follows: 

# fixfiles [OPTIONS] ARGUMENT

where:

  • OPTIONS can be the following:

    -l LOGFILE

    saves the output to the provided file

    -o OUTPUT_FILE

    saves to the provided output file the names of all files whose file context differs from the default

    -F

    forces a reset of context

  • ARGUMENT can be one of the following:

    check

    shows previous and current file context for an incorrect label without performing any changes

    relabel

    relabels incorrect file contexts according to the currently loaded policy

    restore

    restores incorrect file contexts to the default values

    verify

    lists all files with incorrect file context labels without performing any changes

7.5 The semanage command

 

The semanage command can be used to configure parts of the policy without the need to recompile the policy from sources. The command enables you to perform the following tasks:

  • manage Booleans by using the boolean argument. For details about Booleans, refer to Section 6.1, “Working with Booleans”.

  • adjust the context of files by using the fcontext argument

  • manage user mappings using the login argument

  • manage SELinux users using the user argument

  • manage SELinux policy modules using the module argument

The general command syntax looks as follows: 

# semanage ARGUMENT OPTIONS [OBJECT_NAME]

where:

  • ARGUMENT is one of the following: login, user, fcontext, boolean, module.

  • OPTIONS depends on the provided ARGUMENT. Common options are described in Common options.

  • OBJECT_NAME, depending on the provided ARGUMENT, can be a login name, module name, file name or SELinux user.

Common options
 
-a, --add

adds a provided object

-h, --help

prints the command help

--extract

displays commands that were used to change the system (Booleans, file context, and so on)

-l, --list

lists all objects

-m, --modify

modifies the provided object

-n, --noheading

modifies the output of the listing operation by omitting headings

-s, --seuser

specifies the SELinux user

Other options are specific to particular semanage commands and are described in corresponding sections.

7.5.1 The semanage fcontext command

 

Using the semanage fcontext command, you can perform the following tasks:

  • query file context definitions

  • add contexts on files

  • add your own rules

Changes performed to the file context using the semanage fcontext command do not require modifications or recompilation of the policy.

On top of the common options described in Common options, the semanage fcontext command takes the following options:

-e, --equal

The option enables you to use the context of the provided path context to label files in a different directory (the provided target path). For example, you want to assign the same context as /home has to an alternative home directory /export/home. If you use this option, you need to provide the source path and the target path: 

# semanage fcontext -a -e /home /export/home
-f, --ftype

To specify a file type. Use one of the following values:

  • a - all files, which is also the default value

  • b - a block device

  • c - a character device

  • d - a directory

  • f - regular files

  • l - a symbolic link

  • p - a named pipe

  • s - a socket

7.5.2 The semanage login command

 

The semanage login enables you the perform the following tasks:

  • Mapping of Linux users on a particular SELinux user. For example, to map the Linux user tux on sysadm_u, run the command: 

    # semanage login -a -s sysadm_u tux

  • Mapping of a group of Linux users on a particular SELinux user. For example, to map users of the writers group on user_u, run the command: 

    # semanage login -a -s user_u %writers

    The group is then listed in the output of semanage login -l, prefixed with the % character.

    Keep in mind that the user group should be primary because mapping SELinux users on supplementary groups may result in incompatible mappings. 

    # semanage login -m -s staff_u %writers

  • Mapping of Linux users on a particular SELinux MLS/MCS security range.

  • Modifying of the already created mapping. For this purpose, just replace the -a option with -m in the previous commands.

  • Setting the default SELinux user for new Linux users. The usual default SELinux user is unconfined_u. To change the value to staff_u, run the command: 

    # semanage login -m -s staff_u __default__

7.5.3 The semanage boolean command

 

The semanage boolean command is used to control Booleans in the SELinux policy.

The command synopsis looks as follows: 

semanage boolean [-h] [-n]  [ --extract |
       --deleteall | --list [-C] | --modify ( --on | --off | -1 | -0 ) boolean ]

On top of the common options, you can use the following ones specific to the semanage boolean command:

--list -C

To display a list of local modifications to Booleans.

-m --on | -1

To switch the provided Boolean on.

-m --off | -0

To switch the provided Boolean off.

-D, --deleteall

To delete all local modifications to Booleans.

The most common usage of the command is to switch on or off a particular Boolean. For example, to switch on the authlogin_yubikey Boolean, run: 

# semanage boolean -m on authlogin_yubikey

7.5.4 The semanage user command

 

The semanage user command controls the mapping between the SELinux user and the roles and MLS/MCS levels.

On top of the common options described in Common options, the semanage use command takes the following options:

-R [ROLES], --roles [ROLES]

A list of SELinux roles. You can enclose multiple roles within double quotes and separate them by spaces, or you can use the -R several times.

Using this command, you can perform the following tasks:

  • Listing the mapping of SELinux users on roles by running: 

    # semanage user -l

  • Changing the roles assigned to the user_u SELinux user: 

    # semanage user -m -R "system_r unconfined_r user_r"

  • Assigning to admin_u the role staff_r and a category s0: 

    # semanage user -a -R "staff_r -r s0 admin_u

  • Creating a new SELinux user, for example, admin_u with the staff_r role. You also need to define the labeling prefix for this user by using the -P: 

    # semanage user -a -R "staff_r" -P admin admin_u

7.5.5 The semanage module command

 

The semanage module command can install, remove, disable or enable SELinux policy modules.

On top of the common options described in Common options, the semanage fcontext command takes the following options:

-d, --disable

To disable the provided SELinux policy module: 

# semanage module --disable MODULE_NAME
-e, --enable

To enable the provided SELinux policy module: 

# semanage module --enable MODULE_NAME

7.6 The sestatus command

 

The sestatus gets the status of a system where SELinux is running.

The generic syntax of the command looks as follows: 

sestatus [OPTION]

When run without any options and arguments, the command outputs the following information: 

# sestatus

SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Memory protection checking:     requested (insecure)
Max kernel policy version:      33

The command can take the following options:

-b

Displays the status of Booleans on the system.

-v

Displays the security context of files and processes listed in the /etc/sestatus.conf file.