6.2. Confining New Linux Users: useradd
Linux users mapped to the SELinux unconfined_u user run in the unconfined_t domain. This is seen by running the id -Z command while logged-in as a Linux user mapped to unconfined_u:
$ id -Z
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
When Linux users run in the unconfined_t domain, SELinux policy rules are applied, but policy rules exist that allow Linux users running in the unconfined_t domain almost all access. If unconfined Linux users execute an application that SELinux policy defines can transition from the unconfined_t domain to its own confined domain, unconfined Linux users are 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, and therefore, the exploitation of a flaw in the application can be limited by policy. Note: this does not protect the system from the user. Instead, the user and the system are being protected from possible damage caused by a flaw in the application.
When creating Linux users with useradd, use the -Z option to specify which SELinux user they are mapped to. The following example creates a new Linux user, useruuser, and maps that user to the SELinux user_u user. Linux users mapped to the SELinux user_u user run in the user_t domain. In this domain, Linux users are unable to run setuid applications unless SELinux policy permits it (such as passwd), and can not run su or sudo, preventing them from becoming the Linux root user with these commands.
As the Linux root user, run the /usr/sbin/useradd -Z user_u useruuser command to create a new Linux user (useruuser) that is mapped to the SELinux user_u user.
As the Linux root user, run the semanage login -l command to view the mapping between the Linux useruuser user and user_u:
# /usr/sbin/semanage login -l
Login Name SELinux User MLS/MCS Range
__default__ unconfined_u s0-s0:c0.c1023
root unconfined_u s0-s0:c0.c1023
system_u system_u s0-s0:c0.c1023
useruuser user_u s0
As the Linux root user, run the passwd useruuser command to assign a password to the Linux useruuser user:
# passwd useruuser
Changing password for user useruuser.
New UNIX password: Enter a password
Retype new UNIX password: Enter the same password again
passwd: all authentication tokens updated successfully.
Log out of your current session, and log in as the Linux useruuser user. When you log in, pam_selinux maps the Linux user to an SELinux user (in this case, user_u), and sets up the resulting SELinux context. The Linux user's shell is then launched with this context. Run the id -Z command to view the context of a Linux user:
[useruuser@localhost ~]$ id -Z
user_u:user_r:user_t:s0
Log out of the Linux useruuser's session, and log back in with your account. If you do not want the Linux useruuser user, run the /usr/sbin/userdel -r useruuser command as the Linux root user to remove it, along with its home directory.
