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3.3. Confined and Unconfined Users

Each Linux user is mapped to an SELinux user using SELinux policy. This allows Linux users to inherit the restrictions on SELinux users. This Linux user mapping is seen by running the semanage login -l command as root:
~]# semanage login -l

Login Name           SELinux User         MLS/MCS Range        Service

__default__          unconfined_u         s0-s0:c0.c1023       *
root                 unconfined_u         s0-s0:c0.c1023       *
system_u             system_u             s0-s0:c0.c1023       *
In Fedora, Linux users are mapped to the SELinux __default__ login by default, which is mapped to the SELinux unconfined_u user. The following line defines the default mapping:
__default__               unconfined_u              s0-s0:c0.c1023
The following procedure demonstrates how to add a new Linux user to the system and how to map that user to the SELinux unconfined_u user. It assumes that the root user is running unconfined, as it does by default in Fedora:
Procedure 3.4. Mapping a New Linux User to the SELinux unconfined_u User
  1. As root, run the following command to create a new Linux user named newuser:
    ~]# useradd newuser
  2. To assign a password to the Linux newuser user. Run the following command as root:
    ~]# passwd newuser
    Changing password for user newuser.
    New UNIX password: Enter a password 
    Retype new UNIX password: Enter the same password again 
    passwd: all authentication tokens updated successfully.
    
  3. Log out of your current session, and log in as the Linux newuser user. When you log in, the pam_selinux PAM module automatically maps the Linux user to an SELinux user (in this case, unconfined_u), and sets up the resulting SELinux context. The Linux user's shell is then launched with this context. Run the following command to view the context of a Linux user:
    [newuser@localhost ~]$ id -Z 
    unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
    

    Note

    If you no longer need the newuser user on your system, log out of the Linux newuser's session, log in with your account, and run the userdel -r newuser command as root. It will remove newuser along with their home directory.
Confined and unconfined Linux users are subject to executable and writeable memory checks, and are also restricted by MCS or MLS.
If an unconfined Linux user executes an application that SELinux policy defines as one that can transition from the unconfined_t domain to its own confined domain, the unconfined Linux user is still subject to the restrictions of that confined domain. The security benefit of this is that, even though a Linux user is running unconfined, the application remains confined. Therefore, the exploitation of a flaw in the application can be limited by the policy.
Similarly, we can apply these checks to confined users. However, each confined Linux user is restricted by a confined user domain against the unconfined_t domain. The SELinux policy can also define a transition from a confined user domain to its own target confined domain. In such a case, confined Linux users are subject to the restrictions of that target confined domain. The main point is that special privileges are associated with the confined users according to their role. In the table below, you can see examples of basic confined domains for Linux users in Fedora:
Table 3.1. SELinux User Capabilities
User Role Domain X Window System su or sudo Execute in home directory and /tmp/ (default) Networking
sysadm_u sysadm_r sysadm_t yes su and sudo yes yes
staff_u staff_r staff_t yes only sudo yes yes
user_u user_r user_t yes no yes yes
guest_u guest_r guest_t no no no no
xguest_u xguest_r xguest_t yes no no Firefox only

Alongside with the already mentioned SELinux users, there are special roles, that can be mapped to those users. These roles determine what SELinux allows the user to do:
To list all available roles, run the following command:
~]$ seinfo -r
Note that the seinfo command is provided by the setools-console package, which is not installed by default.

3.3.1. The sudo Transition and SELinux Roles

In certain cases, confined users need to perform an administrative task that require root privileges. To do so, such a confined user has to gain a confined administrator SELinux role using the sudo command. The sudo command is used to give trusted users administrative access. When users precede an administrative command with sudo, they are prompted for their own password. Then, when they have been authenticated and assuming that the command is permitted, the administrative command is executed as if they were the root user.
As shown in Table 3.1, “SELinux User Capabilities”, only the staff_u and sysadm_u SELinux confined users are permitted to use sudo by default. When such users execute a command with sudo, their role is changed based on the rules specified in the /etc/sudoers configuration file or in a respective file in the /etc/sudoers.d/ directory if such a file exists.
For more information about sudo, see Fedora System Administrator's Guide available at http://docs.fedoraproject.org/.
Procedure 3.5. Configuring the sudo Transition
This procedure shows how to set up sudo to transition a newly-created SELinux_user_u confined user to a administrator_r confined administrator. To configure a confined administrator role for an already existing SELinux user, skip the first two steps. Also note that the following commands must be run as the root user.
  1. Create a new SELinux user and specify the default SELinux role and a supplementary confined administrator role for this user:
    ~]# semanage user -a -r s0-s0:c0.c1023 -R "default_role_r administrator_r" SELinux_user_u
    In the example below, the default role of the newly-created confined_u SELinux user is staff_r and the confined administrator role is webadm_r:
    ~]# semanage user -a -r s0-s0:c0.c1023 -R "staff_r webadm_r" confined_u
  2. Set up the default SElinux policy context file. For example, to have the same SELinux rules as the staff_u SELinux user, copy the staff_u context file:
    ~]# cp /etc/selinux/targeted/contexts/users/staff_u /etc/selinux/targeted/contexts/users/SELinux_user
  3. Map the newly-created SELinux user to an existing Linux user:
    ~]# semanage login -a -s SELinux_user_u -rs0:c0.c1023 linux_user
  4. Create a new configuration file with the same name as your Linux user in the /etc/sudoers.d/ directory and add the following string to it:
    ~]# echo "linux_user ALL=(ALL) TYPE=administaror_t ROLE=administrator_r /bin/sh " > /etc/sudoers.d/linux_user
    For example:
    ~]# echo "linux_user ALL=(ALL) TYPE=webadm_t ROLE=webadm_r /bin/sh " > /etc/sudoers.d/linux_user
  5. Use the restorecon utility to relabel the linux_user home directory:
    ~]# restorecon -R -v /home/linux_user
  6. Reboot the system:
    ~]# systemctl reboot
  7. When you log in to the system as the newly-created Linux user, the user is labeled with the default SELinux role:
    ~]$ id -Z
    SELinux_user_u:default_role_r:default_role_t:s0:c0.c1023
    After running sudo, the user's SELinux context changes to the supplementary SELinux role as specified in /etc/sudoers.d/linux_user. The -i option used with sudo caused that an interactive shell is executed:
    ~]$ sudo -i
    ~]# id -Z
    SELinux_user_u:administrator_r:administrator_t:s0-s0:c0.c1023
    For the confined_u SELinux user from the example specified in the first step the output looks like below:
    ~]$ id -Z
    confined_u:staff_r:staff_t:s0:c0.c1023
    ~]$ sudo -i	
    ~]# id -Z		
    confined_u:webadm_r:webadm_t:s0:c0.c1023