pam-afs-session 2.6

(PAM module for AFS PAGs and tokens)

Written by Russ Allbery <eagle@eyrie.org>

Copyright 2015 Russ Allbery <eagle@eyrie.org>. Copyright 2005, 2006, 2007, 2008, 2010, 2011 The Board of Trustees of the Leland Stanford Junior University. This software is distributed under a BSD-style license. Plese see the section LICENSE below for more information.

BLURB

pam-afs-session is a PAM module intended for use with a Kerberos PAM module to obtain an AFS PAG and AFS tokens on login. It puts every new session in a PAG regardless of whether it was authenticated with Kerberos and either uses Heimdal's libkafs or runs a configurable external program to obtain tokens. It supports using Heimdal's libkafs or OpenAFS's libkopenafs for the AFS interface and falls back to an internal implementation if libkafs isn't available.

DESCRIPTION

pam-afs-session is a PAM module that isolates each login in a separate AFS PAG (so that they will not trample on each other's AFS tokens) and supports either running an external program to obtain AFS tokens from a Kerberos ticket cache or using Heimdal's libkafs library. It does not obtain tickets itself and must be used in conjunction with a Kerberos PAM module to obtain tokens (setting up PAGs can be done without any Kerberos implementations). It provides only the setcred and session PAM functions.

There are two ways this module can obtain tokens:

For the AFS system call layer, pam-afs-session supports linking with the Heimdal libkafs library or OpenAFS's libkopenafs library. As a fallback, and to support a low-dependency build, it also comes with a simple AFS system call implementation for Linux, Mac OS X, Solaris, or platforms that use syscall to call AFS functions. It can also link with the older OpenAFS libraries when libkopenafs isn't available.

The module can optionally be linked with Kerberos libraries to obtain configuration information from krb5.conf, to support the kdestroy option, and to use libkafs's functions for obtaining tokens.

This module does not support getting tokens for the rmtsys AFS network redirection interface (generally used with the NFS translator).

pam-afs-session is primarily tested on Linux. I rely on reports from users to ensure it continues working on other platforms. Please let me know if any problems you run into.

REQUIREMENTS

The PAM implementations on Linux, Solaris, Mac OS X, HP-UX, and AIX are supported, although the module is primarily tested on Linux and only lightly tested (and not at all by me personally) on the other platforms. Use on platforms with other PAM implementations, such as IRIX or the *BSDs, will require more porting and will not currently work. Patches are welcome.

The module is written in C and should hopefully build on any system with an adequate PAM library that Libtool supports.

Either Heimdal's libkafs or OpenAFS's libkopenafs are the preferred ways of making AFS system calls. If neither are present during compile time, pam-afs-session will attempt to fall back on a built-in AFS system call layer. To use the built-in AFS system call interface on Linux, Mac OS X, and Solaris 11, the system must run a new enough version of OpenAFS or Arla to support AFS system calls through ioctl on a file in /proc or /dev. On other systems with a simple system call interface, configure must be able to find the AFS header afs/param.h in order to get the system call numbers for that platform. On AIX and IRIX, configure will attempt to locate the necessary OpenAFS libraries for lsetpag (either libafsauthent or libsys) but will not support deleting tokens at the end of a session.

The module can optionally use Heimdal's libkafs library to obtain tokens as well as create the PAG. If you are using Heimdal and obtaining tokens from Kerberos tickets, this is the recommended configuration, since it means that the PAM module doesn't have to fork an external process. For other users, an external aklog program that obtains tokens is necessary (and not provided by this module).

To obtain configuration information from krb5.conf and to support the kdestroy option, either MIT Kerberos or Heimdal are required.

Testing the module requires a system with AFS installed and working so that the PAG creation and manipulation can be tested. Running the complete test suite also requires that you have an existing ticket cache and working aklog program. Those portions of the test suite will be skipped if AFS or a Kerberos ticket cache do not appear to be available.

To run the POD test suite, you must have the Perl 5.006 or later and the modules Test::More and Test::Pod installed. Test::More comes with Perl 5.8 or later. Test::Pod is available from CPAN and currently must be installed separately, but the POD tests will be skipped without interfering with the rest of the tests if it's not installed.

To check spelling in the POD documentation, Pod::Spell (available from CPAN) and either aspell or ispell with the american dictionary are also required. The user's path is searched for aspell or ispell and aspell is preferred. Spelling tests are disabled by default since spelling dictionaries differ too much between systems. To enable those tests, set RRA_MAINTAINER_TESTS to a true value.

To bootstrap from a Git checkout, or if you change the Automake files and need to regenerate Makefile.in, you will need Automake 1.11 or later. For bootstrap or if you change configure.ac or any of the m4 files it includes and need to regenerate configure or config.h.in, you will need Autoconf 2.64 or later. Perl is also required to generate the manual pages from a fresh Git checkout.

COMPILING AND INSTALLING

If starting from a Git clone instead of a release tarball, first run:

    ./autogen

To build the module, just run:

    ./configure
    make

Kerberos support is enabled by default if configure can find Kerberos libraries. If your Kerberos libraries aren't installed in a location found by your compiler by default and krb5-config isn't on your PATH, use the --with-krb5=PATH option to configure. The Kerberos libraries will then be expected in PATH/lib and the headers in PATH/include.

To specify a particular krb5-config script to use, either set the KRB5_CONFIG environment variable or pass it to configure like:

    ./configure KRB5_CONFIG=/path/to/krb5-config

To not use krb5-config and force library probing even if there is a krb5-config script on your path, set KRB5_CONFIG to a nonexistent path:

    ./configure KRB5_CONFIG=/nonexistent

To disable Kerberos support, even if you have Kerberos libraries available, pass --without-krb5 to configure.

By default, the first aklog program found on the PATH will be compiled into the module as the default aklog program to run. This can be overridden by the program PAM option. To specify an explicit path to aklog, use:

    ./configure --with-aklog=/path/to/aklog

instead.

To install the module into /usr/local/lib/security and the man page into /usr/local/share/man/man5, run:

    make install

You can change the installation locations with the --prefix, --mandir, and --libdir options to configure. The module will normally be installed in a subdirectory of $libdir named security, but if --prefix is set to /usr (rather than /usr/local, the default), the module will be installed in /lib/security (or /lib32/security or /lib64/security if they exist) to match the default PAM configuration. Alternately, you can simply copy pam_afs_session.so to whatever directory you use for PAM modules. On Solaris, you will need to make the module executable.

By default, if libkafs or libkopenafs are available, pam-afs-session will use them in preference to its own system call implementation. If for some reason you want to build pam-afs-session without a dependency on those libraries even though you have them installed, pass the --without-libkafs flag to configure.

If you are building this module without libkafs on a platform other than Linux, configure needs to find the AFS header afs/param.h. If this is not in the normal include path, pass the --with-afs-headers option to configure to point the compiler at the correct path. For example, if your AFS headers are in /usr/afsws/include, run:

    ./configure --with-afs-headers=/usr/afsws/include

instead.

You can pass the --enable-reduced-depends flag to configure to try to minimize the shared library dependencies encoded in the binaries. This omits from the link line all the libraries included solely because libkafs or the Kerberos libraries depend on them and instead links the programs only against libraries whose APIs are called directly. This will only work with shared libraries and will only work on platforms where shared libraries properly encode their own dependencies (such as Linux). It is intended primarily for building packages for Linux distributions to avoid encoding unnecessary shared library dependencies that make shared library migrations more difficult. If none of the above made any sense to you, don't bother with this flag.

TESTING

pam-afs-session comes with a test suite, which you can run after building pam-afs-session with:

    make check

Don't run the tests themselves inside AFS, or use a world-writable AFS directory (not generally recommended). The test suite manipulates AFS tokens and creates PAGs, so it will temporarily not have access to your personal AFS tokens and won't be able to write its results to AFS build directories that aren't world-writable.

If a test case fails, please run the that individual test case with verbose output using:

    tests/runtests -o <name-of-test>

and send me the output when reporting the problem.

CONFIGURING

Just installing the module does not enable it or change anything about your system authentication configuration. You have to add the module to your PAM configuration, generally in the session group and possibly in the auth group as well. See the platform-specific instructions below.

On all platforms, options can be put after the module name in the PAM configuration file. This is useful if you don't have Kerberos libraries available or if you want different configurations for different services.

If Kerberos support was enabled, configuration options may also be put in the krb5.conf file used by your Kerberos libraries (usually /etc/krb5.conf or /usr/local/etc/krb5.conf) instead or in addition to the PAM configuration. See the man page for more details. This is recommended for general system configuration, since the krb5.conf configuration syntax is a little nicer and more flexible.

See the files in the examples directory for examples of PAM configuration for various operating systems. Contributions for additional operating systems are welcome.

Linux

To use it in conjunction with pam_krb5 on a Debian system, put something like:

    auth [success=ok default=1] pam_krb5.so
    auth [default=done]         pam_afs_session.so program=/usr/bin/aklog
    auth required               pam_unix.so try_first_pass nullok_secure

in /etc/pam.d/common-auth and something like:

    session optional pam_krb5.so
    session required pam_afs_session.so program=/usr/bin/aklog

in /etc/pam.d/common-session. The program= setting is optional if /usr/bin/aklog was in your path when the module was compiled or was specified via the --with-aklog option to configure.

You may want to stack your Kerberos PAM module and the Unix module differently, but note that this module should always run after the Kerberos PAM module. If there is no ticket cache available in the PAM environment, it will succeed silently.

On Red Hat systems, modify /etc/pam.d/system-auth instead; it contains all of the configuration for the different stacks.

Be aware that the pam_keyinit module can have a bad interaction with this module. pam_keyinit initializes a new kernel keyring and will remove the keyring created by pam_afs_session to represent your PAG and hold your tokens. pam_afs_session will attempt to detect this and add the PAG and tokens back, but to do so it must run after pam_keyinit in the session stack. To be safe, you may want to remove this PAM module unless you're using it.

Note that this is not an authentication module and will always return PAM_SUCCESS to any authentication attempt, so never make this module sufficient in your authentication stack. It's only listed as an auth module because it provides a pam_setcred implementation and some programs need to call pam_setcred rather than pam_open_session (screen savers, for instance, to refresh credentials).

Solaris

Solaris doesn't support the [] keywords that Linux PAM does, so if you want to mark the Kerberos PAM module as sufficient and fall back on the Unix module only if it fails, you won't be able to easily run pam_afs_session in the auth group. For most applications, this isn't a problem; running pam_afs_session only from the session group with something like:

    other session required /usr/local/lib/security/pam_afs_session.so 
        minimum_uid=100 retain_after_close

(all on one line) in /etc/pam.conf will be sufficient. Make sure this line is after the Kerberos PAM module's session call.

However, note that login and dtlogin, at least under Solaris 10, apparently call pam_open_session before pam_setcred, and the open_session function of the native Kerberos PAM module doesn't set up the ticket cache. This means that calling pam_afs_session only in the session group won't work, since it will be called before the setcred function of the Kerberos PAM module and hence won't have a ticket cache available yet. There are two possible solutions: replace the native Kerberos PAM module with another module, such as:

    http://www.eyrie.org/~eagle/software/pam-krb5/

that does the same thing in open_session as in setcred, or also call pam_afs_session from the auth group:

    login auth required /usr/local/lib/security/pam_afs_session.so
        minimum_uid=100

after the Kerberos PAM module (and likewise for dtlogin). This means that you cannot use sufficient in the auth group for the Kerberos PAM module, since that will then abort the group before pam_afs_session runs.

See the man page for pam.conf on Solaris for more configuration information.

HP-UX

HP-UX configuration is very similar to Solaris. Something like:

    dtaction session required /usr/lib/security/pam_afs_session.so
    dtlogin  session required /usr/lib/security/pam_afs_session.so
    login    session required /usr/lib/security/pam_afs_session.so
    OTHER    session required /usr/lib/security/pam_afs_session.so

will use pam-afs-session for most login sessions.

Mac OS X

For Mac OS X, PAM isn't used for system login and is therefore mostly useful for remote ssh. To use this module with sshd, add it to the session group of the sshd PAM configuration, and it will then obtain tokens with forwarded tickets via GSS-API or tickets obtained via KerberosAuthentication.

Note that PAGs are not supported on Mac OS X, so you will need to add the "nopag" parameter to the PAM configuration for this module. Note also that while the latest Mac OS X ssh daemon supports GSS-API authentication, it apparently doesn't make the forwarded tickets available to the PAM stack, so pam-afs-session cannot properly set up tokens during GSS-API login.

IMPLEMENTATION NOTES

pam-afs-session supports three basic usage patterns: creating a new session using either pam_open_session or pam_setcred(PAM_ESTABLISH_CRED) (or both), closing a session with pam_close_session or pam_setcred(PAM_DELETE_CRED), and refreshing credentials with pam_setcred(PAM_REINITIALIZE_CRED). In general, the same behavior occurs whether using the pam_*_session interface or the pam_setcred interface, since some PAM-using programs call one and some call the other. In all cases, pam-afs-session will log an error and then successfully exit if AFS doesn't appear to be running (checked with the k_hasafs interface).

pam-afs-session stores a PAM data key named "pam_afs_session" once it has successfully created a PAG and a token. If this key is present in the PAM data when pam_open_session or pam_setcred with the PAM_ESTABLISH_CRED flag is called, it will successfully do nothing to keep from doing duplicate work. Otherwise, it will always create a PAG and then will either call krb5_afslog or run an external program to obtain tokens provided that the environment variable KRB5CCNAME is set in the PAM environment.

If libkafs was found and used at compile time and if krb5_afslog is available, the default is to run krb5_afslog to obtain tokens. Otherwise, the default is to run whatever aklog program was found at compile time. If program is set explicitly in the configuration, pam-afs-session will always run that program rather than calling krb5_afslog.

When pam_close_session or pam_setcred(PAM_DELETE_CRED) is called, pam-afs-session will destroy the user's tokens only if the PAM data key "pam_afs_session" is present (meaning that pam-afs-session previously obtained tokens). This check is present to avoid deleting tokens when session initialization failed, since in that case no new PAG may have been created and we may be deleting tokens that are not ours to delete.

pam_setcred(PAM_REINITIALIZE_CRED) is treated similarly to the opening of a new session except that no PAG is created. Instead, only the program to obtain tokens is run, provided that KRB5CCNAME is set in the PAM environment. This interface is used by programs such as screen savers and lockers to refresh a user's credentials.

pam-afs-session requires that the user (obtained via pam_get_user) exists in the local passwd file or equivalent (using getpwnam). If running an external program, the program is run under that user's UID and primary GID. If calling krb5_afslog, that user's UID is used for the "AFS ID" field in the tokens display (which is entirely cosmetic).

HOMEPAGE AND SOURCE REPOSITORY

The pam-afs-session web page at:

    http://www.eyrie.org/~eagle/software/pam-afs-session/

will always have the current version of this package, the current documentation, and pointers to any additional resources.

pam-afs-session is maintained using Git. You can access the current source by cloning the repository at:

    git://git.eyrie.org/afs/pam-afs-session.git

or view the repository via the web at:

    http://git.eyrie.org/?p=afs/pam-afs-session.git

HISTORY AND ACKNOWLEDGEMENTS

Some of the ideas behind this PAM module (although not the code) are taken from the libpam-openafs-session Debian package written by Sam Hartman and the pam_afs2 module written by Douglas Engert. Thanks to both of them for their previous work. Any errors in this implementation are mine.

The Linux system call layer was based on inspection of the code in OpenAFS and on discussions with Jeffrey Hutzelman on how best to implement an AFS system call layer and on how the pieces work. I couldn't have written this code without his explanations.

Thanks to Douglas Engert for an initial code review, for Solaris porting, for suggesting the always_aklog and aklog_homedir options, and for catching various other problems and missing features.

Thanks to Sean O'Malley for additional Solaris porting information and for testing with the Sun C compiler.

Thanks to Joe Buehler for porting and testing on HP-UX.

Thanks to Markus Moeller for multiple patches and suggestions for improved portability for pam-krb5, from which this module has also benefitted.

LICENSE

The pam-afs-session package as a whole is covered by the following copyright statement and license:

  Copyright 2015 Russ Allbery <eagle@eyrie.org>
  Copyright 2005, 2006, 2007, 2008, 2009, 2010, 2011
      The Board of Trustees of the Leland Stanford Junior University
  Permission is hereby granted, free of charge, to any person obtaining
  a copy of this software and associated documentation files (the
  "Software"), to deal in the Software without restriction, including
  without limitation the rights to use, copy, modify, merge, publish,
  distribute, sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:
  The above copyright notice and this permission notice shall be
  included in all copies or substantial portions of the Software.
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
  CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
  TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
  SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

All individual files without an explicit exception below are released under this license. Some files may have additional copyright holders as noted in those files. There is detailed information about the licensing of each file in the LICENSE file in this distribution.

Some files in this distribution are individually released under different licenses, all of which are compatible with the above general package license but which may require preservation of additional notices. All required notices are preserved in the LICENSE file. Each file intended for copying into other software packages contains a copyright and license notice at the top or bottom of the file. Please take note of any attribution and notice requirements specified in that license.

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