6.22. pam_namespace - setup a private namespace

pam_namespace.so [ debug ] [ unmnt_remnt ] [ unmnt_only ] [ require_selinux ] [ gen_hash ] [ ignore_config_error ] [ ignore_instance_parent_mode ] [ no_unmount_on_close ] [ use_current_context ] [ use_default_context ]

6.22.1. DESCRIPTION

The pam_namespace PAM module sets up a private namespace for a session with polyinstantiated directories. A polyinstantiated directory provides a different instance of itself based on user name, or when using SELinux, user name, security context or both. If an executable script /etc/security/namespace.init exists, it is used to initialize the namespace every time a new instance directory is setup. The script receives the polyinstantiated directory path, the instance directory path, flag whether the instance directory was newly created (0 for no, 1 for yes), and the user name as its arguments.

The pam_namespace module disassociates the session namespace from the parent namespace. Any mounts/unmounts performed in the parent namespace, such as mounting of devices, are not reflected in the session namespace. To propagate selected mount/unmount events from the parent namespace into the disassociated session namespace, an administrator may use the special shared-subtree feature. For additional information on shared-subtree feature, please refer to the mount(8) man page and the shared-subtree description at http://lwn.net/Articles/159077 and http://lwn.net/Articles/159092.

6.22.2. DESCRIPTION

The pam_namespace.so module allows setup of private namespaces with polyinstantiated directories. Directories can be polyinstantiated based on user name or, in the case of SELinux, user name, sensitivity level or complete security context. If an executable script /etc/security/namespace.init exists, it is used to initialize the namespace every time a new instance directory is setup. The script receives the polyinstantiated directory path and the instance directory path as its arguments.

The /etc/security/namespace.conf file specifies which directories are polyinstantiated, how they are polyinstantiated, how instance directories would be named, and any users for whom polyinstantiation would not be performed.

When someone logs in, the file namespace.conf is scanned. Comments are marked by # characters. Each non comment line represents one polyinstantiated directory. The fields are separated by spaces but can be quoted by " characters also escape sequences \b, \n, and \t are recognized. The fields are as follows:

polydir instance_prefix method list_of_uids

The first field, polydir, is the absolute pathname of the directory to polyinstantiate. The special string $HOME is replaced with the user's home directory, and $USER with the username. This field cannot be blank.

The second field, instance_prefix is the string prefix used to build the pathname for the instantiation of <polydir>. Depending on the polyinstantiation method it is then appended with "instance differentiation string" to generate the final instance directory path. This directory is created if it did not exist already, and is then bind mounted on the <polydir> to provide an instance of <polydir> based on the <method> column. The special string $HOME is replaced with the user's home directory, and $USER with the username. This field cannot be blank.

The third field, method, is the method used for polyinstantiation. It can take these values; "user" for polyinstantiation based on user name, "level" for polyinstantiation based on process MLS level and user name, "context" for polyinstantiation based on process security context and user name, "tmpfs" for mounting tmpfs filesystem as an instance dir, and "tmpdir" for creating temporary directory as an instance dir which is removed when the user's session is closed. Methods "context" and "level" are only available with SELinux. This field cannot be blank.

The fourth field, list_of_uids, is a comma separated list of user names for whom the polyinstantiation is not performed. If left blank, polyinstantiation will be performed for all users. If the list is preceded with a single "~" character, polyinstantiation is performed only for users in the list.

The method field can contain also following optional flags separated by : characters.

create=mode,owner,group - create the polyinstantiated directory. The mode, owner and group parameters are optional. The default for mode is determined by umask, the default owner is the user whose session is opened, the default group is the primary group of the user.

iscript=path - path to the instance directory init script. The base directory for relative paths is /etc/security/namespace.d.

noinit - instance directory init script will not be executed.

shared - the instance directories for "context" and "level" methods will not contain the user name and will be shared among all users.

The directory where polyinstantiated instances are to be created, must exist and must have, by default, the mode of 0000. The requirement that the instance parent be of mode 0000 can be overridden with the command line option ignore_instance_parent_mode

In case of context or level polyinstantiation the SELinux context which is used for polyinstantiation is the context used for executing a new process as obtained by getexeccon. This context must be set by the calling application or pam_selinux.so module. If this context is not set the polyinstatiation will be based just on user name.

The "instance differentiation string" is <user name> for "user" method and <user name>_<raw directory context> for "context" and "level" methods. If the whole string is too long the end of it is replaced with md5sum of itself. Also when command line option gen_hash is used the whole string is replaced with md5sum of itself.

6.22.3. OPTIONS

debug

A lot of debug information is logged using syslog

unmnt_remnt

For programs such as su and newrole, the login session has already setup a polyinstantiated namespace. For these programs, polyinstantiation is performed based on new user id or security context, however the command first needs to undo the polyinstantiation performed by login. This argument instructs the command to first undo previous polyinstantiation before proceeding with new polyinstantiation based on new id/context

unmnt_only

For trusted programs that want to undo any existing bind mounts and process instance directories on their own, this argument allows them to unmount currently mounted instance directories

require_selinux

If selinux is not enabled, return failure

gen_hash

Instead of using the security context string for the instance name, generate and use its md5 hash.

ignore_config_error

If a line in the configuration file corresponding to a polyinstantiated directory contains format error, skip that line process the next line. Without this option, pam will return an error to the calling program resulting in termination of the session.

ignore_instance_parent_mode

Instance parent directories by default are expected to have the restrictive mode of 000. Using this option, an administrator can choose to ignore the mode of the instance parent. This option should be used with caution as it will reduce security and isolation goals of the polyinstantiation mechanism.

no_unmount_on_close

For certain trusted programs such as newrole, open session is called from a child process while the parent perfoms close session and pam end functions. For these commands use this option to instruct pam_close_session to not unmount the bind mounted polyinstantiated directory in the parent.

use_current_context

Useful for services which do not change the SELinux context with setexeccon call. The module will use the current SELinux context of the calling process for the level and context polyinstantiation.

use_default_context

Useful for services which do not use pam_selinux for changing the SELinux context with setexeccon call. The module will use the default SELinux context of the user for the level and context polyinstantiation.

6.22.4. MODULE SERVICES PROVIDED

The session service is supported. The module must not be called from multithreaded processes.

6.22.5. RETURN VALUES

PAM_SUCCESS

Namespace setup was successful.

PAM_SERVICE_ERR

Unexpected system error occurred while setting up namespace.

PAM_SESSION_ERR

Unexpected namespace configuration error occurred.

6.22.6. FILES

/etc/security/namespace.conf

Main configuration file

/etc/security/namespace.d

Directory for additional configuration files

/etc/security/namespace.init

Init script for instance directories

6.22.7. EXAMPLES

These are some example lines which might be specified in /etc/security/namespace.conf.


      # The following three lines will polyinstantiate /tmp,
      # /var/tmp and user's home directories. /tmp and /var/tmp
      # will be polyinstantiated based on the security level
      # as well as user name, whereas home directory will be
      # polyinstantiated based on the full security context and user name.
      # Polyinstantiation will not be performed for user root
      # and adm for directories /tmp and /var/tmp, whereas home
      # directories will be polyinstantiated for all users.
      #
      # Note that instance directories do not have to reside inside
      # the polyinstantiated directory. In the examples below,
      # instances of /tmp will be created in /tmp-inst directory,
      # where as instances of /var/tmp and users home directories
      # will reside within the directories that are being
      # polyinstantiated.
      #
      /tmp     /tmp-inst/               level      root,adm
      /var/tmp /var/tmp/tmp-inst/    level      root,adm
      $HOME    $HOME/$USER.inst/inst- context
    

For the <service>s you need polyinstantiation (login for example) put the following line in /etc/pam.d/<service> as the last line for session group:

session required pam_namespace.so [arguments]

This module also depends on pam_selinux.so setting the context.

6.22.8. AUTHORS

The namespace setup scheme was designed by Stephen Smalley, Janak Desai and Chad Sellers. The pam_namespace PAM module was developed by Janak Desai <janak@us.ibm.com>, Chad Sellers <csellers@tresys.com> and Steve Grubb <sgrubb@redhat.com>. Additional improvements by Xavier Toth <txtoth@gmail.com> and Tomas Mraz <tmraz@redhat.com>.