To improve security, it's often wise to use more than one method of protection (also called "defense in depth"). That way, if one method is breached, another method remains operational and prevents further intrusion. This article describes a way to add another layer of depth to your security strategy: using PAM to polyinstantiate world-writeable shared directories. This means that a new instance of a directory, such as /tmp, is created for each user.
Polyinstantiation of world-writeable directories prevents the following types of attacks, as Russell Coker illustrates in "Polyinstantiation of directories in an SELinux system" (see Resources):
- Race-condition attacks with symbolic links
- Exposing a file name considered secret information or useful to an attacker
- Attacks by one user on another user
- Attacks by a user on a daemon
- Attacks by a non-root daemon on a user
However, polyinstantiation does NOT prevent these types of attacks:
- Attacks by a root daemon on a user
- Attacks by root (account or escalated privilege) on any user
How PAM and polyinstantiation work
PAM creates a polyinstantiated private /tmp directory at login time within a system instance directory; this redirection is transparent to the user logging in. The user sees a standard /tmp directory and can read and write to it normally. That user cannot see any other user's (including root's) /tmp space or the actual /tmp file system.
Polyinstantiated user directories are neither hidden nor protected from the root user. If you are interested in that level of protection, SELinux can help. The configuration examples provided here should work whether or not you have enabled SELinux. See Resources for links to more information about using SELinux.
This section shows you how to enable polyinstantiation of /tmp and /var/tmp directories for users on your system. It also describes the optional configuration steps necessary to accommodate X Windows or a graphical display manager. I used Red Hat Enterprise Linux 5.1 (RHEL 5.1) to write this article, but you can try the procedures described here on any Linux distribution that includes the pam_namespace module.
First we'll edit namespace.conf.
The first file you'll edit is /etc/security/namespace.conf, which controls the
pam_namespace module. In this file, list the directories that you want PAM to
polyinstantiate on login. Some example directories are listed in the file included
with PAM and are commented out. Type
to view a comprehensive manual page. The syntax for each line in this file is
polydir instance_prefix method list_of_uids.
Briefly, here is what these variables represent:
polydiris the absolute pathname of the directory to polyinstantiate.
instance_prefixis the base directory of the new polyinstantiated user directory.
methodcan be user, level, or context.
list_of_uidsis a list of user names for which PAM will NOT polyinstantiate their directories.
In this example, you are not using SELinux, so you must specify the user
for the method. You can use the variables
$HOME within the configuration file if needed.
Listing 1 creates a private /tmp and /var/tmp namespace instance for each user on the system except root and adm.
Listing 1. /etc/security/namespace.conf
#$HOME $HOME/$USER.inst/ user root,adm /tmp /tmp/tmp-inst/ user root,adm /var/tmp /var/tmp/tmp-inst/ user root,adm
The /tmp and /var/tmp directories do not have to be located on separate filesystems; they can be directories on a single file system. The directories /tmp/tmp-inst and /var/tmp/tmp-inst must be created once, manually, with file mode 000 before polyinstantiation will work. If the directories are not created correctly, logins will fail.
Type the following commands while logged in as the root user to create these directories:
# mkdir /tmp/tmp-inst # mkdir /var/tmp/tmp-inst # chown root:root /tmp/tmp-inst /var/tmp/tmp-inst # chmod 000 /tmp/tmp-inst /var/tmp/tmp-inst
Next, modify the PAM configuration files to add the pam_namspace.so module to the list of required modules to run on login from the console and from the secure shell. Edit the /etc/pam.d/login and /etc/pam.d/sshd files to place the pam_namespace.so module on the last line in each file. Listing 2 and Listing 3 show where to add the module in /etc/pam.d/login and /etc/pam.d/sshd, respectively:
Listing 2. Adding the PAM module to /etc/pam.d/login
#%PAM-1.0 auth [user_unknown=ignore success=ok ignore=ignore default=bad] pam_securetty.so auth include system-auth account required pam_nologin.so account include system-auth password include system-auth # pam_selinux.so close should be the first session rule session required pam_selinux.so close session include system-auth session required pam_loginuid.so # session optional pam_console.so # pam_selinux.so open should only be followed by sessions to be executed in the user context session required pam_selinux.so open session optional pam_keyinit.so force revoke # Polyinstantiation: session required pam_namespace.so
Listing 3. Adding the PAM module to /etc/pam.d/sshd
#%PAM-1.0 auth include system-auth account required pam_nologin.so account include system-auth password include system-auth session optional pam_keyinit.so force revoke session include system-auth session required pam_loginuid.so # Polyinstantiation: session required pam_namespace.so
Enable X Windows
Because of the way the X Window system uses temporary directories, graphical sessions might fail for users with a polyinstantiated /tmp directory. PAM executes the contents of the /etc/security/namespace.init file during login if pam_namespace is specified in any files in the /etc/pam.d directory. Use this file to make the necessary changes to allow X Windows to start correctly. A default namespace.init file is included with RHEL 5.1, but I have modified it slightly in Listing 4.
Listing 4. Enables X Windows to start correctly
if [ $1 = /tmp ]; then if [ ! -f /.tmp/.X11-unix ]; then mkdir -p /.tmp/.X11-unix fi mount --bind /tmp/.X11-unix /.tmp/.X11-unix [ -f /tmp/.X0-lock ] && cp -fp -- /tmp/.X0-lock "$2/.X0-lock" mkdir -p -- "$2/.X11-unix" ln -fs -- /.tmp/.X11-unix/X0 "$2/.X11-unix/X0" fi exit 0
Configure the Gnome Display Manager
Configuring the Gnome Display Manager (GDM) is easy. Add the pam_namespace.so module to the list of required modules in the /etc/pam.d/gdm file. Listing 5 shows an example.
Listing 5. Configuring the Gnome Display Manager
#%PAM-1.0 auth required pam_env.so auth include system-auth account required pam_nologin.so account include system-auth password include system-auth session optional pam_keyinit.so force revoke session include system-auth session required pam_loginuid.so session optional pam_console.so # Polyinstantiation: session required pam_namespace.so
If you are using the X Display Manager (XDM) instead of GDM, configure the /etc/pam.d/xdm file in the same way. Now both the graphical logins and the command-line logins result in polyinstantiated /tmp and /var/tmp directories.
Finishing up: Allowing for errors
If PAM encounters an error when running the pam_namespace.so module, the login
session for the user trying to login will fail. Until you are sure that things are
operating as you intend, allow logins to continue in case of an error. To enable
ignore_config_error option, add it to the end of
the line in each file in the /etc/pam.d directory where you added the
For example, in the /etc/pam.d/login file, edit the line containing the pam_namspace.so module as follows:
session required pam_namespace.so ignore_config_error
For a complete list of options, see the pam_namespace manual page. After a user
logs in, check the file /var/log/secure for errors. When you are satisfied that
your PAM configuration is correct, remove the
Finishing up: Results
After you have modified and saved the configuration files, choose a non-root user account to test and log out all instances of that user from the system. Log in again and a new polyinstantiated /tmp and /var/tmp directory will be created for that user. Listing 6 and Listing 7 show what this looks like on the system and from the user's perspective. In this example, the username is robb.
Listing 6. Console session from the user's perspective
[robb@testbox tmp]$ cd /tmp [robb@testbox tmp]$ touch foo [robb@testbox tmp]$ ls foo
Listing 7. Console session on the system as root
[root@testbox ~]# ls /tmp tmp-inst [root@testbox ~]# ls /tmp/tmp-inst/ robb [root@testbox ~]# ls /tmp/tmp-inst/robb/ foo
Because of polyinstantiation, robb's /tmp directory is isolated in a separate directory under /tmp/tmp-inst/, and robb cannot see the system /tmp directory or any files within it.
While polyinstantiation will not prevent every type of attack, it is a useful
addition to your security toolkit that is straightforward to configure. Feel free
to experiment by polyinstantiating other directories such as /home. With the
ignore_config_error option, mistakes are not fatal, but
don't forget to remove that option after you have finished testing your
- "Polyinstantiation of directories in an SE Linux system," by Russell Coker, describes the problems related to shared directories such as /tmp and /var/tmp, as well as problems related to having multiple SELinux security contexts used for accessing a single home directory.
- "Applying mount namespaces" (developerWorks, September 2007) shows you how to build your own filesystem setup without being constrained by the sysadmin-dictated structure; you'll discover some practical applications for advanced Linux mounts features.
- "Secure programmer: Minimizing privileges" (developerWorks, May 2004) discusses how to minimize privileges by minimizing the privileged modules, the privileges granted, and the time the privileges are active.
- "What's new in SELinux for Red Hat Enterprise Linux 5" (Red Hat Magazine, May 2007) is an exhaustive overview of SELinux for RHEL5.
- This posting on shared subtrees (LWN.net, November 2005) answers the question "A process wants to clone its own namespace, but still wants to access the CD that got mounted recently" with a detailed list of features, semantics, mount state descriptions, and implementation issues.
- "Role-based access control in SELinux" (developerWorks, February 2008) shows you how to work with RBAC in SELinux, and how the SELinux policy, kernel, and userspace work together to enforce the RBAC and tie users to a type enforcement policy.
- The following resources can help you get a handle on SELinux: SELinux Frequently Asked Questions (FAQ), SELinux unofficial technical FAQ, and Configuring the SELinux Policy.
- Learn more about Security-Enhanced Linux at the NSA Web site.
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