Here we collect some pointers for the module writer to bear in mind when writing/developing a Linux-PAM compatible module.
Care should be taken to ensure that the proper execution of a module
is not compromised by a lack of system resources. If a module is
unable to open sufficient files to perform its task, it should fail
gracefully, or request additional resources. Specifically, the
quantities manipulated by the setrlimit(2) family of commands
should be taken into consideration.
Generally, the module may wish to establish the identity of the user
requesting a service. This may not be the same as the username
returned by pam_get_user(). Indeed, that is only going to be the
name of the user under whose identity the service will be given. This
is not necessarily the user that requests the service.
In other words, user X runs a program that is setuid-Y, it grants the
user to have the permissions of Z. A specific example of this sort of
service request is the su program: user joe executes
su to become the user jane. In this situation X=joe,
Y=root and Z=jane. Clearly, it is important that the module
does not confuse these different users and grant an inappropriate
level of privilege.
The following is the convention to be adhered to when juggling user-identities.
getuid(2).
geteuid(2).
pam_get_user(2) and also stored
in the Linux-PAM item, PAM_USER.
PAM_RUSER item.
Generally, network sensitive modules/applications may wish to set/read
this item to establish the identity of the user requesting a service
from a remote location.
Note, if a module wishes to modify the identity of either the uid
or euid of the running process, it should take care to restore
the original values prior to returning control to the Linux-PAM
library.
Prior to calling the conversation function, the module should reset the contents of the pointer that will return the applications response. This is a good idea since the application may fail to fill the pointer and the module should be in a position to notice!
The module should be prepared for a failure from the conversation. The
generic error would be PAM_CONV_ERR, but anything other than
PAM_SUCCESS should be treated as indicating failure.
To ensure that the authentication tokens are not left lying around the
items, PAM_AUTHTOK and PAM_OLDAUTHTOK, are not available to
the application: they are defined in
<security/pam_modules.h>. This is ostensibly for
security reasons, but a maliciously programmed application will always
have access to all memory of the process, so it is only superficially
enforced. As a general rule the module should overwrite
authentication tokens as soon as they are no longer needed.
Especially before free()'ing them. The Linux-PAM library is
required to do this when either of these authentication token items
are (re)set.
Not to dwell too little on this concern; should the module store the
authentication tokens either as (automatic) function variables or
using pam_[gs]et_data() the associated memory should be
over-written explicitly before it is released. In the case of the
latter storage mechanism, the associated cleanup() function
should explicitly overwrite the *data before free()'ing it:
for example,
/*
* An example cleanup() function for releasing memory that was used to
* store a password.
*/
int cleanup(pam_handle_t *pamh, void *data, int error_status)
{
char *xx;
if ((xx = data)) {
while (*xx)
*xx++ = '\0';
free(data);
}
return PAM_SUCCESS;
}
syslog(3)
Only rarely should error information be directed to the user. Usually,
this is to be limited to ``sorry you cannot login now'' type
messages. Information concerning errors in the configuration file,
/etc/pam.conf, or due to some system failure encountered by
the module, should be written to syslog(3) with
facility-type LOG_AUTHPRIV.
With a few exceptions, the level of logging is, at the discretion of the module developer. Here is the recommended usage of different logging levels:
LOG_ERR level. However, information regarding an unrecognized
argument, passed to a module from an entry in the
/etc/pam.conf file, is required to be logged at the
LOG_ERR level.
debug argument to the
module in /etc/pam.conf, should be logged at the
LOG_DEBUG level.
LOG_ALERT.
malloc() failures should be logged at level LOG_CRIT.
LOG_NOTICE.
Writing a module is much like writing an application. You have to
provide the "conventional hooks" for it to work correctly, like
pam_sm_authenticate() etc., which would correspond to the
main() function in a normal function.
Typically, the author may want to link against some standard system
libraries. As when one compiles a normal program, this can be done for
modules too: you simply append the -lXXX arguments
for the desired libraries when you create the shared module object. To
make sure a module is linked to the libwhatever.so
library when it is dlopen()ed, try:
% gcc -shared -Xlinker -x -o pam_module.so pam_module.o -lwhatever
Modules may be statically linked into libpam. This should be true of all the modules distributed with the basic Linux-PAM distribution. To be statically linked, a module needs to export information about the functions it contains in a manner that does not clash with other modules.
The extra code necessary to build a static module should be delimited
with #ifdef PAM_STATIC and #endif. The static code should do
the following:
struct pam_module, called
_pam_modname_modstruct, where
modname is the name of the module as used in the
filesystem but without the leading directory name (generally
/usr/lib/security/ or the suffix (generally .so).
As a simple example, consider the following module code which defines a module that can be compiled to be static or dynamic:
#include <stdio.h> /* for NULL define */
#define PAM_SM_PASSWORD /* the only pam_sm_... function declared */
#include <security/pam_modules.h>
PAM_EXTERN int pam_sm_chauthtok(pam_handle_t *pamh, int flags,
int argc, const char **argv)
{
return PAM_SUCCESS;
}
#ifdef PAM_STATIC /* for the case that this module is static */
struct pam_module _pam_modname_modstruct = { /* static module data */
"pam_modname",
NULL,
NULL,
NULL,
NULL,
NULL,
pam_sm_chauthtok,
};
#endif /* end PAM_STATIC */
To be linked with libpam, staticly-linked modules must be built
from within the Linux-PAM-X.YY/modules/ subdirectory of the
Linux-PAM source directory as part of a normal build of the
Linux-PAM system.
The Makefile, for the module in question, must execute the
register_static shell script that is located in the
Linux-PAM-X.YY/modules/ subdirectory. This is to ensure that
the module is properly registered with libpam.
The two manditory arguments to register_static are the
title, and the pathname of the object file containing the module's
code. The pathname is specified relative to the
Linux-PAM-X.YY/modules directory. The pathname may be an
empty string---this is for the case that a single object file needs to
register more than one struct pam_module. In such a case, exactly
one call to register_static must indicate the object file.
Here is an example; a line in the Makefile might look like this:
register:
ifdef STATIC
(cd ..; ./register_static pam_modname pam_modname/pam_modname.o)
endif
For some further examples, see the modules subdirectory of
the current Linux-PAM distribution.