Ubuntu Manpage: pam_afs_session - AFS PAG and token PAM module

Provided by: libpam-afs-session_2.5-3_amd64

NAME

       pam_afs_session - AFS PAG and token PAM module

SYNOPSIS

         auth          optional        pam_afs_session.so
         session       required        pam_afs_session.so

DESCRIPTION

The AFS session service module for PAM, typically installed at
/lib/security/pam_afs_session.so, establishes new AFS sessions and obtains AFS tokens when
a new session is opened for a user. It is a shared object that is dynamically loaded by
the PAM subsystem as necessary, based on the system PAM configuration. PAM is a system
for plugging in external authentication and session management modules so that each
application doesn't have to know the best way to check user authentication or create a
user session on that system. For details on how to configure PAM on your system, see the
PAM man page, often pam(7).

This module provides pam_setcred, pam_open_session, and pam_close_session implementations
for AFS. Because pam_setcred is part of the auth PAM group, it also implements a dummy
pam_authenticate that always succeeds (otherwise, it can't provide a pam_setcred).

Make sure that this module is NEVER listed as "sufficient" or as the only "required"
module in the auth group. Doing so will potentially allow users to log on without any
password. There unfortunately isn't a way to work around this and still provide
pam_setcred without running afoul of a bug in (at least) Linux PAM 0.99.7.1 and probably
earlier that causes authentication to fail when the final module in the auth group returns
PAM_IGNORE and "[default=done]" was given as the action.

Here are the actions of this module:

pam_open_session
When a new session is opened, this module will first check to see if AFS is running on
the system. If not, it will log a message and exit successfully. If AFS is running,
it will place the user's session in a new PAG (Process Authentication Group, often
implemented as supplemental groups, which limits user tokens to only processes in that
PAG). It will then attempt to obtain tokens, either directly if built with the
Heimdal libkafs library and Kerberos support or by running an external aklog program.
If PAG creation fails, the module will fail; if obtaining tokens fails, the module
will log a warning but still return success.

The module will only attempt to obtain tokens if the environment variable KRB5CCNAME
is set in the environment, unless otherwise configured (see the always_aklog option).
It will always create a new PAG, however.

pam_close_session
If and only if pam_open_session successfully obtained AFS tokens and AFS is still
running on the system, pam_close_session will delete the tokens in the current PAG
(equivalent to running unlog). To leave the tokens after session close, set the
retain_after_close option.

pam_setcred
When pam_setcred is called with the PAM_ESTABLISH_CRED flag, it will do the same as if
pam_open_session was called. When pam_setcred is called with the PAM_DELETE_CRED
flag, it will do the same as if pam_close_session was called. When called with the
PAM_REINITIALIZE_CRED flag or the PAM_REFRESH_CRED flag, it won't create a new PAG but
instead will only attempt to get new tokens (still skipping this unless KRB5CCNAME is
set in the environment or always_aklog is set).

This module is primarily intended for use with a Kerberos v5 authentication module. It
does not itself do any user authentication; it cannot, for instance, be used to
authenticate users to a kaserver. While it is intended for use with an aklog that uses
Kerberos v5 ticket caches to obtain tokens, it can be used with any aklog implementation
(always_aklog may have to be set if no Kerberos v5 ticket cache will be present).

This module performs no authorization checks and does not hook into password changes; it
only implements the session functions and pam_setcred.

CONFIGURATION

The AFS session PAM module supports the following configuration options, which may be set
in the PAM configuration as arguments listed after "pam_afs_session.so" or in the system
krb5.conf.

Some of them take arguments, in which case the argument will be given after "=". The rest
are boolean options. To set a boolean option in the PAM configuration, just give the name
of the option in the arguments. To set an option that takes an argument, follow the
option name with an equal sign ("=") and the value, with no separating whitespace.
Whitespace in option arguments is not supported in the PAM configuration files of most PAM
implementations.

To set an option for the PAM module in the system krb5.conf file, put that option in the
[appdefaults] section. The AFS session PAM module will look for options either at the top
level of the [appdefaults] section or in a subsection named "pam-afs-session" (currently,
realm-specific configuration is not checked). For example, the following fragment of a
krb5.conf file would set aklog_homedir to true and minimum_uid to 100.

[appdefaults]
aklog_homedir = true
pam-afs-session = {
minimum_uid = 100
}

There is no difference to the PAM module whether options are specified at the top level or
in a "pam-afs-session" section; the "pam-afs-session" section is supported in case there
are options that should be set for the PAM module but not for other applications. For
more information on the syntax of krb5.conf, see krb5.conf(5).

If the same option is set in krb5.conf and in the PAM configuration, the latter takes
precedent. Note, however, that due to the configuration syntax, there's no way to turn
off a boolean option in the PAM configuration that was turned on in krb5.conf.

afs_cells=cell[,cell...]
Obtain tokens for the listed cells instead of the default local cell. If more than
one cell is listed, try to obtain tokens for each cell. If specified in krb5.conf,
the cells can be separated by any combination of spaces and commas; if specified in
the PAM configuration, they must be separated by commas.

If the AFS session PAM module is running an external program, this option is
implemented by passing a -c flag with the cell as its argument for each listed cell to
that program. If aklog_homedir is also set, the -c flags and the -p flag will all be
passed to the external program.

aklog_homedir
Try to obtain the necessary tokens to access the user's home directory. If the
libkafs token-obtaining API is used, setting this will cause the AFS session PAM
module to pass the user's home directory into that API and request that the
appropriate tokens be obtained. If running an external aklog program, aklog will be
called with -p home-directory where home-directory is the home directory of the local
user for which the session is being opened or refreshed. This generally will tell
aklog to check that path, find all AFS cells involved in access to that path, and
attempt to obtain tokens for each one. Note that this means that if the user's home
directory is not in AFS, no tokens will be obtained.

In either case, the user's home directory is obtained via getpwnam() based on the
username PAM says we are authenticating.

always_aklog
Normally, the AFS session PAM module only tries to obtain tokens if KRB5CCNAME is set
in the PAM environment. If this option is set, it will always attempt to obtain
tokens. This is only useful if it is configured to run an external aklog program.

This can be used if your environment doesn't correctly set KRB5CCNAME in the
environment for some reason, or if your aklog doesn't rely on a Kerberos ticket cache
to obtain tokens (or can find the cache on its own via some other means).

debug
If this option is set, additional trace information will be logged to syslog with
priority LOG_DEBUG.

ignore_root
If this option is set, the AFS session PAM module won't take any action (and will exit
successfully) if the account for which the session is being established is named
"root".

kdestroy
If this option is set and the AFS session PAM module was built with Kerberos support,
the user's ticket cache will be destroyed after tokens are obtained successfully. If
tokens are not obtained successfully, the ticket cache will be left intact. Please
note that this is not properly a security feature, since the ticket cache will still
be written to disk between the time the Kerberos PAM module authenticates the user and
the time the AFS session PAM module is run. It can, however, be used to reduce the
window during which Kerberos ticket caches are lying about if the only use one has for
ticket caches is to obtain AFS tokens.

minimum_uid=uid
If this option is set, the AFS session PAM module won't take any action (and will exit
successfully) if the account for which the session is being established has a UID
lower than uid.

nopag
If this option is set, no PAG will be created. Be careful when using this option,
since it means that the user will inherit a PAG from the process managing the login.
If sshd, for instance, is started in a PAG, every user who logs in via ssh will be put
in the same PAG and will share tokens if this option is used.

This is the default on Mac OS X, where PAGs are not supported.

notokens
If this option is set, the AFS session PAM module will only create a PAG and not
attempt to obtain tokens. Setting this option overrides all other settings related to
acquiring tokens, including always_aklog. If both nopag and notokens are set, the
module essentially does nothing.

Setting notokens also implies retain_after_close, meaning that the AFS session PAM
module will also not attempt to delete tokens when the user's session ends.

program=path
The path to the aklog program to run. Setting this option tells the AFS session PAM
module to always run an external program to obtain tokens and never use the libkafs
interface, even if the latter is available.

You may pass options to this program by separating them with commas (or spaces or tabs
in krb5.conf or if the configuration syntax of your PAM configuration allows this).
For example, the setting:

program=/usr/bin/aklog,-noprdb,-524

will run "/usr/bin/aklog -noprdb -524" as the program to obtain tokens. The arguments
are passed directly, not parsed by the shell.

If this option is not set, the default behavior is to call the libkafs function to
obtain tokens, if available, and otherwise to use a default path to aklog determined
at compile time (the first aklog found on the compiler's path by default). If no
aklog could be found at compile time and libkafs isn't used, this option must be set.

retain_after_close
If this option is set, pam_close_session will do nothing (successfully) rather than
deleting tokens. This will allow programs started in the user's PAG that are still
running when the log out to continue to use the user's tokens until they expire.
Normally, the AFS kernel module will automatically clean up tokens once every process
in that PAG has terminated.

ENVIRONMENT

       KRB5CCNAME
           This module looks for KRB5CCNAME in the PAM environment and by default does not run
           aklog if it is not set.

       The entire PAM environment is passed to aklog as its environment (rather than the
       environment of the process running the PAM functions).

WARNINGS

       As mentioned above, this module implements a dummy pam_authenticate function so that it
       can provide a pam_setcred function.  Never list this module as "sufficient" or as the only
       "required" module or you may allow users to log on without a password.

       While spawning an external aklog program, the AFS session PAM module resets the SIGCHLD
       signal handler to the default handler while the program runs and then restores it
       afterward.  This is done to avoid having aklog interfere with process handling in the
       calling application, but it means that there's a race condition that can cause children to
       be incorrectly handled if they exit while aklog is running.  There is unfortunately no
       good solution to this other than building against Heimdal and using the libkafs interface
       to obtain tokens instead of an external program.

       To detect whether AFS is running on the system, the AFS session PAM module temporarily
       sets a SIGSYS handler before attempting an AFS system call.  That handler may also modify
       a static variable.  Neither of these should ideally be done in a PAM module, but there is
       no other good way of checking for the non-existence of a system call that doesn't crash
       the application on some operating systems.  The PAM module will attempt to restore the
       previous SIGSYS handler, if any, after the test is done, and the static variable is used
       in such a way that running it from multiple threads shouldn't be an issue, but be aware
       that the PAM module is doing this behind the back of the application and may interfere
       with unusual SIGSYS handlers or similar application actions.

NOTES

       When using the libkafs interface to obtain tokens, be sure that it is configured properly
       for the type of AFS tokens expected at your site.  As of Heimdal 0.7, the default behavior
       is to contact the krb524 service to translate Kerberos v5 tickets into Kerberos v4 tickets
       to use as tokens.  AFS cells running current server software no longer need this, and if
       your site doesn't run the krb524 service, this may break token acquisition.

       Sites running AFS servers that understand Kerberos-v5-derived tokens should add
       configuration like:

           libkafs = {
               EXAMPLE.ORG = {
                   afs-use-524 = no
               }
           }

       to the [appdefaults] section of their krb5.conf files to disable use of the krb524
       service.  See the Heimdal kafs(3) man page for more information.