4.3. Confined and Unconfined Users

Each Linux user is mapped to an SELinux user via SELinux policy. This allows Linux users to inherit the restrictions on SELinux users. This Linux user mapping is seen by running the /usr/sbin/semanage login -l command as the Linux root user:

# /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

In Fedora 10, Linux users are mapped to the SELinux __default__ login by default (which is mapped to the SELinux unconfined_u user). The following defines the default-mapping:

__default__               unconfined_u              s0-s0:c0.c1023

The following example demonstates adding a new Linux user, and that Linux user being mapped to the SELinux unconfined_u user. It assumes that the Linux root user is running unconfined, as it does by default in Fedora 10:

As the Linux root user, run the /usr/sbin/useradd newuser command to create a new Linux user named newuser.

As the Linux root user, run the passwd newuser command to assign a password to the Linux newuser user:

# 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.

Log out of your current session, and log in as the Linux newuser user. When you log in, pam_selinux 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 SELinux context. To view the SELinux context for a Linux user, run the id -Z command:
```
[newuser@localhost ~]$ id -Z
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
```
Log out of the Linux newuser's session, and log back in with your account. If you do not want the Linux newuser user, as the Linux root user, run the /usr/sbin/userdel -r newuser command to remove it, along with the Linux newuser's home directory.

Confined and unconfined Linux users are subject to executable and writeable memory checks, and are also restricted by MCS (and MLS, if the MLS policy is used). 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.

The following confined SELinux users are available in Fedora 10:

User	Domain	X Window System	su and sudo	Execute in home directory and /tmp/	Networking
guest_u	guest_t	no	no	no	no
xguest_u	xguest_t	yes	no	no	only Firefox
user_u	user_t	yes	no	no	yes
staff_u	staff_t	yes	only `sudo`	yes	yes

Table 4.1. SELinux User Capabilities

Linux users in the guest_t, xguest_t, and user_t domains can only run set user ID (setuid) applications if SELinux policy permits it (such as passwd). They can not run the su and /usr/bin/sudo setuid applications, and therefore, can not use these applications to become the Linux root user.
Linux users in the guest_t domain have no network access, and can only log in via a terminal (including ssh; they can log in via ssh, but can not use ssh to connect to another system).
The only network access Linux users in the xguest_t domain have is Firefox connecting to web pages.
By default, Linux users in the guest_t, xguest_t, and user_t domains can not execute applications in their home directories or /tmp/, preventing them from executing applications (which inherit users' permissions) in directories that they have write access to. This prevents flawed or malicious applications from modifying files users' own.
Linux users in the xguest_t, user_t and staff_t domains can log in via the X Window System and a terminal.
By default, Linux users in the staff_t domain do not have permissions to execute applications with /usr/bin/sudo. These permissions must be configured by an administrator.