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      SELinux

      A Beginner's Guide to SELinux on CentOS 8


      Updated by Linode

      Contributed by
      Linode

      SELinux is a Mandatory Access Control (MAC) system, developed by the NSA. SELinux was developed as a replacement for Discretionary Access Control (DAC) that ships with most Linux distributions.

      The difference between DAC and MAC is how users and applications gain access to machines. Traditionally, the command sudo gives a user the ability to heighten permissions to root-level. Root access on a DAC system gives the person or program access to all programs and files on a system.

      A person with root access should be a trusted party. But if security has been compromised, so too has the system. SELinux and MACs resolve this issue by both confining privileged processes and automating security policy creation.

      SELinux defaults to denying anything that is not explicitly allowed. SELinux has two global modes, permissive and enforcing. Permissive mode allows the system to function like a DAC system, while logging every violation to SELinux. The enforcing mode applies a strict denial of access to anything that isn’t explicitly allowed. To explicitly allow certain behavior on a machine, you, as the system administrator, have to write policies that allow it. This guide provides a brief and basic introduction to commonly used commands and practices for SELinux system administration.

      Before You Begin

      1. Ensure that you have followed the Getting Started and Securing Your Server guides.

        Note

        This guide is written for a non-root user. Commands that require elevated privileges are prefixed with sudo. If you’re not familiar with the sudo command, you can check our Users and Groups guide.
      2. Update your system:

        sudo yum update
        

        Note

        The Linode kernel does not support SELinux by default. However, all new Linodes running CentOS 8 use the distribution provided kernel, which has SELinux enabled by default.

        If your system is running a Linode kernel, you will need to change to an upstream kernel in order to use SELinux. See the How to Change Your Linode’s Kernel for more steps. Once you’re kernel is set to the upstream kernel, continue on with the steps in this guide.

      Install Supporting SELinux Packages

      In this section, you will install various SELinux packages that will help you when creating, managing, and analyzing SELinux policies.

      1. Verify which SELinux packages are installed on your system:

        sudo rpm -aq | grep selinux
        

        A newly deployed CentOS 8 Linode should have the following packages installed:

          
        libselinux-2.5-14.1.el7.x86_64
        selinux-policy-3.13.1-252.el7_7.6.noarch
        selinux-policy-targeted-3.13.1-252.el7_7.6.noarch
        libselinux-utils-2.5-14.1.el7.x86_64
        libselinux-python-2.5-14.1.el7.x86_64
            
        
      2. Install the following packages and their associated dependencies:

        sudo yum install policycoreutils policycoreutils-python setools setools-console setroubleshoot
        
        • policycoreuitls and policyoreutils-python contain several management tools to administer your SELinux environment and policies.
        • setools provides command line tools for working with SELinux policies. Some of these tools include, sediff which you can use to view differences between policies, seinfo a tool to view information about the components that make up SELinux policies, and sesearch used to search through your SELinux policies. setools-console consists of sediff, seinfo, and sesearch. You can issue the --help option after any of the listed tools in order to view more information about each one.
        • setroubleshoot suite of tools help you determine why a script or file may be blocked by SELinux.

        Optionally, install setroubleshoot-server and mctrans. The setroubleshoot-server allows, among many other things, for email notifications to be sent from the server to notify you of any policy violations. The mctrans daemon translates SELinux’s output to human readable text.

      SELinux States and Modes

      SELinux States

      When SELinux is installed on your system, it can be either enabled or disabled. By default, the CentOS 8 image provided by Linode has SELinux in an enabled state.

      • To disable SELinux, update your SELinux configuration file using the text editor of your choice. Set the SELINUX directive to disabled as shown in the example.

        /etc/selinux/config
         1
         2
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         4
         5
         6
         7
         8
         9
        10
        11
        12
        
        # This file controls the state of SELinux on the system.
        # SELINUX= can take one of these three values:
        #     enforcing - SELinux security policy is enforced.
        #     permissive - SELinux prints warnings instead of enforcing.
        #     disabled - No SELinux policy is loaded.
        SELINUX=disabled
        # SELINUXTYPE= can take one of three values:
        #     targeted - Targeted processes are protected,
        #     minimum - Modification of targeted policy. Only selected processes are protected.
        #     mls - Multi Level Security protection.
        SELINUXTYPE=targeted
              

        Note

        You can update the SELINUX directive with any of the available SELinux states or modes.
      • Reboot your Linode for the changes to take effect:

        sudo reboot
        
      • Connect to your Linode via SSH (replace 192.0.2.0 with your own Linode’s IP address) and verify your SELinux installation’s status:

        ssh [email protected]
        sudo sestatus
        

        Its output should display disabled

          
        SELinux status:                 disabled
            
        

      SELinux Modes

      When SELinux is enabled, it can run in either enforcing or permissive modes.

      Note

      If SELinux is currently disabled, update your SELinux configuration file with the SELINUX directive set to enabled, then reboot your system, and SSH back into your Linode. These steps are outlined in the SELinux States section of the guide.
      • In enforcing mode, SELinux enforces its policies on your system and denies access based on those policies. Use the following command to view SELinux policy modules currently loaded into memory:

        sudo semodule -l
        
      • Permissive mode does not enforce any of your SELinux policies, instead, it logs any actions that would have been denied to your /var/log/audit/audit.log file.

      • You can check which mode your system is running by issuing the following command:

        sudo getenforce
        
      • To place SELinux in permissive mode, use the following command:

        sudo setenforce 0
        

        Permissive mode is useful when configuring your system, because you and your system’s components can interact with your files, scripts, and programs without restriction. However, you can use audit logs and system messages to understand what would be restricted in enforcing mode. This will help you better construct the necessary policies for your system’s user’s and programs.

      • Use the sealert utility to generate a report from your audit log. The log will include information about what SELinux is preventing and how to allow the action, if desired.

        sudo sealert -a /var/log/audit/audit.log
        

        The output will resemble the example, however, it varies depending on the programs and configurations on your system. The example was generated using a Linode running the Apache webserver with a virtual hosts configuration.

          
        SELinux is preventing /usr/sbin/httpd from write access on the directory logs.
        
        *****  Plugin httpd_write_content (92.2 confidence) suggests   ***************
        
        If you want to allow httpd to have write access on the logs directory
        Then you need to change the label on 'logs'
        Do
        # semanage fcontext -a -t httpd_sys_rw_content_t 'logs'
        # restorecon -v 'logs'
            
        
      • To allow /usr/sbin/httpd write access to the directory logs, as shown by the output, you can execute the suggested commands, semanage fcontext -a -t httpd_sys_rw_content_t 'logs' and restorecon -v 'logs'.

      SELinux Context

      SELinux marks every single object on a machine with a context. Every file, user, and process has a context. The context is broken into three parts: user, role, and type. An SELinux policy controls which users can get which roles. Each specific role places a constraint on what type of files that user can access. When a user logs in to a system, a role is assigned to the user as seen in the ls -Z example, the output unconfined_u is a user role.

      1. Create a directory in your home folder:

        mkdir ~/example_dir
        
      2. Print the SELinux security context of your home folder’s directories and files :

        ls -Z ~/
        

        The output is similar to:

          
        drwxrwxr-x. example_user example_user unconfined_u:object_r:user_home_t:s0 example_dir
            
        

        The SELinux specific information is contained in the unconfined_u:object_r:user_home_t:s0 portion, which follows the following syntax: user:role:type:level. To learn more about users, roles, and related access control, see the CentOS SELinux documentation.

      SELinux Boolean

      An SELinux Boolean is a variable that can be toggled on and off without needing to reload or recompile an SELinux policy.

      1. You can view the list of boolean variables using the getsebool -a command. Pipe the command through grep to narrow down your results.

        sudo getsebool -a | grep "httpd_can"
        

        You will see a similar output:

          
        httpd_can_check_spam --> off
        httpd_can_connect_ftp --> off
        httpd_can_connect_ldap --> off
        httpd_can_connect_mythtv --> off
        httpd_can_connect_zabbix --> off
        httpd_can_network_connect --> off
        httpd_can_network_connect_cobbler --> off
        httpd_can_network_connect_db --> off
        httpd_can_network_memcache --> off
        httpd_can_network_relay --> off
        httpd_can_sendmail --> off
            
        

        You can change the value of any variable using the setsebool command. If you set the -P flag, the setting will persist through reboots. If, for example, you want to allow HTTPD scripts and modules to connect to the network, update the corresponding boolean variable

        sudo setsebool -P httpd_can_network_connect ON
        

        When viewing a list of your boolean variables, you should now see that it is set to ON.

        sudo getsebool -a | grep "httpd_can"
        
          
        httpd_can_check_spam --> off
        httpd_can_connect_ftp --> off
        httpd_can_connect_ldap --> off
        httpd_can_connect_mythtv --> off
        httpd_can_connect_zabbix --> off
        httpd_can_network_connect --> on
        httpd_can_network_connect_cobbler --> off
        httpd_can_network_connect_db --> off
        httpd_can_network_memcache --> off
        httpd_can_network_relay --> off
        httpd_can_sendmail --> off
              
        

      Next Steps

      This guide provides a brief and basic introduction to SELinux administration. You can now take a deeper dive into SELinux by consulting some of the resources include in the More Information section of this guide.

      More Information

      You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

      This guide is published under a CC BY-ND 4.0 license.



      Source link

      A Beginner's Guide to SELinux on CentOS 7


      Updated by Linode

      Written by Angel Guarisma

      Getting Started with SELinux

      SELinux is a Mandatory Access Control (MAC) system, developed by the NSA. SELinux was developed as a replacement for Discretionary Access Control (DAC) that ships with most Linux distributions.

      The difference between DAC and MAC is how users and applications gain access to machines. Traditionally, the command sudo gives a user the ability to heighten permissions to root-level. Root access on a DAC system gives the person or program access to all programs and files on a system.

      A person with root access should be a trusted party. But if security has been compromised, so too has the system. SELinux and MACs resolve this issue by both confining privileged processes and automating security policy creation.

      SELinux defaults to denying anything that is not explicitly allowed. SELinux has two global modes, permissive and enforcing. Permissive mode allows the system to function like a DAC system, while logging every violation to SELinux. The enforcing mode applies a strict denial of access to anything that isn’t explicitly allowed. To explicitly allow certain behavior on a machine, you, as the system administrator, have to write policies that allow it. This guide provides a brief and basic introduction to commonly used commands and practices for SELinux system administration.

      Before You Begin

      1. Ensure that you have followed the Getting Started and Securing Your Server guides.

        Note

        This guide is written for a non-root user. Commands that require elevated privileges are prefixed with sudo. If you’re not familiar with the sudo command, you can check our Users and Groups guide.
      2. Update your system:

        sudo yum update
        

        Note

        The Linode kernel does not support SELinux by default. However, all new Linodes running CentOS 7 use the distribution provided kernel, which has SELinux enabled by default.

        If your system is running a Linode kernel, you will need to change to an upstream kernel in order to use SELinux. See the How to Change Your Linode’s Kernel for more steps. Once you’re kernel is set to the upstream kernel, continue on with the steps in this guide.

      Install Supporting SELinux Packages

      In this section, you will install various SELinux packages that will help you when creating, managing, and analyzing SELinux policies.

      1. Verify which SELinux packages are installed on your system:

        sudo rpm -aq | grep selinux
        

        A newly deployed CentOS 7 Linode should have the following packages installed:

          
        libselinux-2.5-14.1.el7.x86_64
        selinux-policy-3.13.1-252.el7_7.6.noarch
        selinux-policy-targeted-3.13.1-252.el7_7.6.noarch
        libselinux-utils-2.5-14.1.el7.x86_64
        libselinux-python-2.5-14.1.el7.x86_64
            
        
      2. Install the following packages and their associated dependencies:

        sudo yum install policycoreutils policycoreutils-python setools setools-console setroubleshoot
        
        • policycoreuitls and policyoreutils-python contain several management tools to administer your SELinux environment and policies.
        • setools provides command line tools for working with SELinux policies. Some of these tools include, sediff which you can use to view differences between policies, seinfo a tool to view information about the components that make up SELinux policies, and sesearch used to search through your SELinux policies. setools-console consists of sediff, seinfo, and sesearch. You can issue the --help option after any of the listed tools in order to view more information about each one.
        • setroubleshoot suite of tools help you determine why a script or file may be blocked by SELinux.

        Optionally, install setroubleshoot-server and mctrans. The setroubleshoot-server allows, among many other things, for email notifications to be sent from the server to notify you of any policy violations. The mctrans daemon translates SELinux’s output to human readable text.

      SELinux States and Modes

      SELinux States

      When SELinux is installed on your system, it can be either enabled or disabled. By default, the CentOS 7 image provided by Linode has SELinux in an enabled state.

      • To disable SELinux, update your SELinux configuration file using the text editor of your choice. Set the SELINUX directive to disabled as shown in the example.

        /etc/selinux/config
         1
         2
         3
         4
         5
         6
         7
         8
         9
        10
        11
        12
        
        # This file controls the state of SELinux on the system.
        # SELINUX= can take one of these three values:
        #     enforcing - SELinux security policy is enforced.
        #     permissive - SELinux prints warnings instead of enforcing.
        #     disabled - No SELinux policy is loaded.
        SELINUX=disabled
        # SELINUXTYPE= can take one of three values:
        #     targeted - Targeted processes are protected,
        #     minimum - Modification of targeted policy. Only selected processes are protected.
        #     mls - Multi Level Security protection.
        SELINUXTYPE=targeted
              

        Note

        You can update the SELINUX directive with any of the available SELinux states or modes.
      • Reboot your Linode for the changes to take effect:

        sudo reboot
        
      • Connect to your Linode via SSH (replace 192.0.2.0 with your own Linode’s IP address) and verify your SELinux installation’s status:

        ssh [email protected]
        sudo sestatus
        

        Its output should display disabled

          
        SELinux status:                 disabled
            
        

      SELinux Modes

      When SELinux is enabled, it can run in either enforcing or permissive modes.

      Note

      If SELinux is currently disabled, update your SELinux configuration file with the SELINUX directive set to enabled, then reboot your system, and SSH back into your Linode. These steps are outlined in the SELinux States section of the guide.
      • In enforcing mode, SELinux enforces its policies on your system and denies access based on those policies. Use the following command to view SELinux policy modules currently loaded into memory:

        sudo semodule -l
        
      • Permissive mode does not enforce any of your SELinux policies, instead, it logs any actions that would have been denied to your /var/log/audit/audit.log file.

      • You can check which mode your system is running by issuing the following command:

        sudo getenforce
        
      • To place SELinux in permissive mode, use the following command:

        sudo setenforce 0
        

        Permissive mode is useful when configuring your system, because you and your system’s components can interact with your files, scripts, and programs without restriction. However, you can use audit logs and system messages to understand what would be restricted in enforcing mode. This will help you better construct the necessary policies for your system’s user’s and programs.

      • Use the sealert utility to generate a report from your audit log. The log will include information about what SELinux is preventing and how to allow the action, if desired.

        sudo sealert -a /var/log/audit/audit.log
        

        The output resembles the example, however, it varies depending on the programs and configurations on your system. The example was generated using a Linode running the Apache webserver with a virtual hosts configuration.

          
        SELinux is preventing /usr/sbin/httpd from write access on the directory logs.
        
        *****  Plugin httpd_write_content (92.2 confidence) suggests   ***************
        
        If you want to allow httpd to have write access on the logs directory
        Then you need to change the label on 'logs'
        Do
        # semanage fcontext -a -t httpd_sys_rw_content_t 'logs'
        # restorecon -v 'logs'
            
        
      • To allow /usr/sbin/httpd write access to the directory logs, as shown in the output, you can execute the suggested commands, semanage fcontext -a -t httpd_sys_rw_content_t 'logs' and restorecon -v 'logs'.

      SELinux Context

      SELinux marks every single object on a machine with a context. Every file, user, and process has a context. The context is broken into three parts: user, role, and type. An SELinux policy controls which users can get which roles. Each specific role places a constraint on what type of files that user can access. When a user logs in to a system, a role is assigned to the user as seen in the ls -Z example, the output unconfined_u is a user role.

      1. Create a directory in your home folder:

        mkdir ~/example_dir
        
      2. Print the SELinux security context of your home folder’s directories and files:

        ls -Z ~/
        

        The output is similar to:

          
        drwxrwxr-x. example_user example_user unconfined_u:object_r:user_home_t:s0 example_dir
            
        

        The SELinux specific information is contained in the unconfined_u:object_r:user_home_t:s0 portion, which follows the following syntax: user:role:type:level. To learn more about users, roles, and related access control, see the CentOS SELinux documentation.

      SELinux Boolean

      An SELinux Boolean is a variable that can be toggled on and off without needing to reload or recompile an SELinux policy.

      1. You can view the list of boolean variables using the getsebool -a command. Pipe the command through grep to narrow down your results.

        sudo getsebool -a | grep "httpd_can"
        

        You will see a similar output:

          
        httpd_can_check_spam --> off
        httpd_can_connect_ftp --> off
        httpd_can_connect_ldap --> off
        httpd_can_connect_mythtv --> off
        httpd_can_connect_zabbix --> off
        httpd_can_network_connect --> off
        httpd_can_network_connect_cobbler --> off
        httpd_can_network_connect_db --> off
        httpd_can_network_memcache --> off
        httpd_can_network_relay --> off
        httpd_can_sendmail --> off
            
        

        You can change the value of any variable using the setsebool command. If you set the -P flag, the setting will persist through reboots. If, for example, you want to allow HTTPD scripts and modules to connect to the network, update the corresponding boolean variable.

        sudo setsebool -P httpd_can_network_connect ON
        

        When viewing a list of your boolean variables, you should now see that it is set to ON.

        sudo getsebool -a | grep "httpd_can"
        
          
        httpd_can_check_spam --> off
        httpd_can_connect_ftp --> off
        httpd_can_connect_ldap --> off
        httpd_can_connect_mythtv --> off
        httpd_can_connect_zabbix --> off
        httpd_can_network_connect --> on
        httpd_can_network_connect_cobbler --> off
        httpd_can_network_connect_db --> off
        httpd_can_network_memcache --> off
        httpd_can_network_relay --> off
        httpd_can_sendmail --> off
              
        

      Next Steps

      This guide provides a brief and basic introduction to SELinux administration. You can now take a deeper dive into SELinux by consulting some of the resources include in the More Information section of this guide.

      More Information

      You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

      This guide is published under a CC BY-ND 4.0 license.



      Source link

      How to Install SELinux on Debian 10


      Updated by Linode

      Contributed by
      Linode

      Ubuntu has a Mandatory Access Control (MAC) system similar to SELinux, named AppArmor. Both SELinux and AppArmor provide a set of tools to isolate applications from each other to protect the host system from being compromised. AppArmor offers Ubuntu users mandatory access control options, without the perceived difficulty or learning curve that SELinux may have. However, if you are switching to Debian 10, are already familiar with SELinux, and would like to use it to enforce security on your system, you can install it by following the steps in this guide.

      Before You Begin

      1. Ensure that you have followed the Getting Started and Securing Your Server guides.

        Note

        This guide is written for a non-root user. Commands that require elevated privileges are prefixed with sudo. If you’re not familiar with the sudo command, you can check our Users and Groups guide.
      2. Update your system:

        sudo apt update
        

        Note

        The Linode kernel does not support SELinux by default. If your system is running a Linode kernel, you will need to change to an upstream kernel in order to use SELinux. See the How to Change Your Linode’s Kernel for more steps. Once you’re kernel is set to the upstream kernel, continue on with the steps in this guide.

      Remove AppArmor

      1. Stop AppArmor using systemctl:

        sudo systemctl stop apparmor
        
      2. Purge AppArmor from the system.

        Caution

        Do not purge AppArmor if you believe you may reuse it in the future. If you would like to preserve your AppArmor configuration files, use the remove command, instead:

        sudo apt remove apparmor
        
        sudo apt purge apparmor
        

        Note

        If after issuing the `purge command you receive warnings about remaining AppArmor files or directories. You can remove them manually, if desired. This step is not necessary to get a working SELinux installation.

      3. Update your system:

        sudo apt update && sudo apt upgrade -yuf
        
      4. Reboot your Linode

        sudo reboot
        

      Install SELinux

      1. Install the SELinux package along with supporting packages to help you manage your installation.

        sudo apt-get install selinux-basics selinux-policy-default auditd
        
      2. Activate your SELinux installation:

        sudo selinux-activate
        

        Your output should resemble the following:

          
        Activating SE Linux
        Generating grub configuration file ...
        Found linux image: /boot/vmlinuz-4.19.0-8-amd64
        Found initrd image: /boot/initrd.img-4.19.0-8-amd64
        done
        SE Linux is activated.  You may need to reboot now.
            
        
      3. Reboot your Linode for the installation to complete:

        sudo reboot
        

        Note

        After rebooting your system, SELinux should be enabled, but in permissive mode. Permissive mode means any actions that would have been disallowed are allowed, but logged in your system’s audit log located in the /var/log/audit/audit.log file.

      4. Log back into your Linode via SSH. Replace 192.0.2.0 with your own Linode’s IP address.

        ssh [email protected]
        
      5. Verify the status of your SELinux installation:

        sudo sestatus
        

        You should see a similar output:

          
        SELinux status:                 enabled
        SELinuxfs mount:                /sys/fs/selinux
        SELinux root directory:         /etc/selinux
        Loaded policy name:             default
        Current mode:                   permissive
        Mode from config file:          permissive
        Policy MLS status:              enabled
        Policy deny_unknown status:     allowed
        Memory protection checking:     requested (insecure)
        Max kernel policy version:      31
            
        
      6. To put SELinux into enforcing mode, use the setenforce command. When in enforcing mode, any actions not permitted by your system’s are blocked and the corresponding event is logged in the audit log.

        sudo setenforce 1
        
      7. To maintain enforcing mode after reboot, modify the SELinux configuration file in /etc/selinux/config from the default SELINUX=permissive to SELINUX=enforcing:

        /etc/selinx/config
        1
        2
        3
        4
        5
        6
        7
        
        # This file controls the state of SELinux on the system.
        # SELINUX= can take one of these three values:
        # enforcing - SELinux security policy is enforced.
        # permissive - SELinux prints warnings instead of enforcing.
        # disabled - No SELinux policy is loaded.
        SELINUX=enforcing
            

        Note

        If you have set SELinux to enforcing mode, ensure that your SSH port has access before logging out of your session.

        sudo semanage port -l | grep 'ssh'
        

        You should see a similar output if TCP is allowed on port 22.

          
        ssh_port_t                     tcp      22
        
        

        If you do not see the this entry, open the port with the following command:

        sudo semanage port -a -t ssh_port_t -p tcp 22
        

      Next Steps

      After installing SELinux on your system, use our Getting Started with SELinux Guide to learn the basics of SELinux security.

      This guide is published under a CC BY-ND 4.0 license.



      Source link