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The Linux-PAM Module Writers' Guide
Andrew G. Morgan
<morgan@kernel.org>
Thorsten Kukuk
<kukuk@thkukuk.de>
Version 1.1.2, 31. August 2010
Abstract
This manual documents what a programmer needs to know in order to write a
module that conforms to the Linux-PAM standard.It also discusses some
security issues from the point of view of the module programmer.
--------------------------------------------------------------------------
1. Introduction
1.1. Description
1.2. Synopsis
2. What can be expected by the module
2.1. Getting and setting PAM_ITEMs and data
2.1.1. Set module internal data
2.1.2. Get module internal data
2.1.3. Setting PAM items
2.1.4. Getting PAM items
2.1.5. Get user name
2.1.6. The conversation function
2.1.7. Set or change PAM environment variable
2.1.8. Get a PAM environment variable
2.1.9. Getting the PAM environment
2.2. Other functions provided by libpam
2.2.1. Strings describing PAM error codes
2.2.2. Request a delay on failure
3. What is expected of a module
3.1. Overview
3.1.1. Functional independence
3.1.2. Minimizing administration problems
3.1.3. Arguments supplied to the module
3.2. Authentication management
3.2.1. Service function for user authentication
3.2.2. Service function to alter credentials
3.3. Account management
3.3.1. Service function for account management
3.4. Session management
3.4.1. Service function to start session
management
3.4.2. Service function to terminate session
management
3.5. Authentication token management
3.5.1. Service function to alter authentication
token
4. Generic optional arguments
5. Programming notes
5.1. Security issues for module creation
5.1.1. Sufficient resources
5.1.2. Who's who?
5.1.3. Using the conversation function
5.1.4. Authentication tokens
5.2. Use of syslog(3)
5.3. Modules that require system libraries
6. An example module
7. See also
8. Author/acknowledgments
9. Copyright information for this document
Chapter 1. Introduction
1.1. Description
Linux-PAM (Pluggable Authentication Modules for Linux) is a library that
enables the local system administrator to choose how individual
applications authenticate users. For an overview of the Linux-PAM library
see the Linux-PAM System Administrators' Guide.
A Linux-PAM module is a single executable binary file that can be loaded
by the Linux-PAM interface library. This PAM library is configured locally
with a system file, /etc/pam.conf, to authenticate a user request via the
locally available authentication modules. The modules themselves will
usually be located in the directory /lib/security (or /lib64/security,
depending on the architecture) and take the form of dynamically loadable
object files (see dlopen(3). Alternatively, the modules can be statically
linked into the Linux-PAM library; this is mostly to allow Linux-PAM to be
used on platforms without dynamic linking available, but this is a
deprecated functionality. It is the Linux-PAM interface that is called by
an application and it is the responsibility of the library to locate, load
and call the appropriate functions in a Linux-PAM-module.
Except for the immediate purpose of interacting with the user (entering a
password etc..) the module should never call the application directly.
This exception requires a "conversation mechanism" which is documented
below.
1.2. Synopsis
#include <security/pam_modules.h>
gcc -fPIC -c pam_module.c
gcc -shared -o pam_module.so pam_module.o -lpam
Chapter 2. What can be expected by the module
Here we list the interface that the conventions that all Linux-PAM modules
must adhere to.
2.1. Getting and setting PAM_ITEMs and data
First, we cover what the module should expect from the Linux-PAM library
and a Linux-PAM aware application. Essentially this is the libpam.*
library.
2.1.1. Set module internal data
#include <security/pam_modules.h>
int pam_set_data( pamh,
module_data_name,
data,
(*cleanup)(pam_handle_t *pamh, void *data, int
error_status));
pam_handle_t *pamh;
const char *module_data_name;
void *data;
void (*cleanup)(pam_handle_t *pamh, void *data, int error_status);
2.1.1.1. DESCRIPTION
The pam_set_data function associates a pointer to an object with the
(hopefully) unique string module_data_name in the PAM context specified by
the pamh argument.
PAM modules may be dynamically loadable objects. In general such files
should not contain static variables. This function and its counterpart
pam_get_data(3), provide a mechanism for a module to associate some data
with the handle pamh. Typically a module will call the pam_set_data
function to register some data under a (hopefully) unique
module_data_name. The data is available for use by other modules too but
not by an application. Since this functions stores only a pointer to the
data, the module should not modify or free the content of it.
The function cleanup() is associated with the data and, if non-NULL, it is
called when this data is over-written or following a call to pam_end(3).
The error_status argument is used to indicate to the module the sort of
action it is to take in cleaning this data item. As an example, Kerberos
creates a ticket file during the authentication phase, this file might be
associated with a data item. When pam_end(3) is called by the module, the
error_status carries the return value of the pam_authenticate(3) or other
libpam function as appropriate. Based on this value the Kerberos module
may choose to delete the ticket file (authentication failure) or leave it
in place.
The error_status may have been logically OR'd with either of the following
two values:
PAM_DATA_REPLACE
When a data item is being replaced (through a second call to
pam_set_data) this mask is used. Otherwise, the call is assumed to
be from pam_end(3).
PAM_DATA_SILENT
Which indicates that the process would prefer to perform the
cleanup() quietly. That is, discourages logging/messages to the
user.
2.1.1.2. RETURN VALUES
PAM_BUF_ERR
Memory buffer error.
PAM_SUCCESS
Data was successful stored.
PAM_SYSTEM_ERR
A NULL pointer was submitted as PAM handle or the function was
called by an application.
2.1.2. Get module internal data
#include <security/pam_modules.h>
int pam_get_data( pamh,
module_data_name,
data);
const pam_handle_t *pamh;
const char *module_data_name;
const void **data;
2.1.2.1. DESCRIPTION
This function together with the pam_set_data(3) function is useful to
manage module-specific data meaningful only to the calling PAM module.
The pam_get_data function looks up the object associated with the
(hopefully) unique string module_data_name in the PAM context specified by
the pamh argument. A successful call to pam_get_data will result in data
pointing to the object. Note, this data is not a copy and should be
treated as constant by the module.
2.1.2.2. RETURN VALUES
PAM_SUCCESS
Data was successful retrieved.
PAM_SYSTEM_ERR
A NULL pointer was submitted as PAM handle or the function was
called by an application.
PAM_NO_MODULE_DATA
Module data not found or there is an entry, but it has the value
NULL.
2.1.3. Setting PAM items
#include <security/pam_modules.h>
int pam_set_item( pamh,
item_type,
item);
pam_handle_t *pamh;
int item_type;
const void *item;
2.1.3.1. DESCRIPTION
The pam_set_item function allows applications and PAM service modules to
access and to update PAM informations of item_type. For this a copy of the
object pointed to by the item argument is created. The following
item_types are supported:
PAM_SERVICE
The service name (which identifies that PAM stack that the PAM
functions will use to authenticate the program).
PAM_USER
The username of the entity under whose identity service will be
given. That is, following authentication, PAM_USER identifies the
local entity that gets to use the service. Note, this value can be
mapped from something (eg., "anonymous") to something else (eg.
"guest119") by any module in the PAM stack. As such an application
should consult the value of PAM_USER after each call to a PAM
function.
PAM_USER_PROMPT
The string used when prompting for a user's name. The default
value for this string is a localized version of "login: ".
PAM_TTY
The terminal name: prefixed by /dev/ if it is a device file; for
graphical, X-based, applications the value for this item should be
the $DISPLAY variable.
PAM_RUSER
The requesting user name: local name for a locally requesting user
or a remote user name for a remote requesting user.
Generally an application or module will attempt to supply the
value that is most strongly authenticated (a local account before
a remote one. The level of trust in this value is embodied in the
actual authentication stack associated with the application, so it
is ultimately at the discretion of the system administrator.
PAM_RUSER@PAM_RHOST should always identify the requesting user. In
some cases, PAM_RUSER may be NULL. In such situations, it is
unclear who the requesting entity is.
PAM_RHOST
The requesting hostname (the hostname of the machine from which
the PAM_RUSER entity is requesting service). That is
PAM_RUSER@PAM_RHOST does identify the requesting user. In some
applications, PAM_RHOST may be NULL. In such situations, it is
unclear where the authentication request is originating from.
PAM_AUTHTOK
The authentication token (often a password). This token should be
ignored by all module functions besides pam_sm_authenticate(3) and
pam_sm_chauthtok(3). In the former function it is used to pass the
most recent authentication token from one stacked module to
another. In the latter function the token is used for another
purpose. It contains the currently active authentication token.
PAM_OLDAUTHTOK
The old authentication token. This token should be ignored by all
module functions except pam_sm_chauthtok(3).
PAM_CONV
The pam_conv structure. See pam_conv(3).
The following additional items are specific to Linux-PAM and should not be
used in portable applications:
PAM_FAIL_DELAY
A function pointer to redirect centrally managed failure delays.
See pam_fail_delay(3).
PAM_XDISPLAY
The name of the X display. For graphical, X-based applications the
value for this item should be the $DISPLAY variable. This value
may be used independently of PAM_TTY for passing the name of the
display.
PAM_XAUTHDATA
A pointer to a structure containing the X authentication data
required to make a connection to the display specified by
PAM_XDISPLAY, if such information is necessary. See
pam_xauth_data(3).
PAM_AUTHTOK_TYPE
The default action is for the module to use the following prompts
when requesting passwords: "New UNIX password: " and "Retype UNIX
password: ". The example word UNIX can be replaced with this item,
by default it is empty. This item is used by pam_get_authtok(3).
For all item_types, other than PAM_CONV and PAM_FAIL_DELAY, item is a
pointer to a <NUL> terminated character string. In the case of PAM_CONV,
item points to an initialized pam_conv structure. In the case of
PAM_FAIL_DELAY, item is a function pointer: void (*delay_fn)(int retval,
unsigned usec_delay, void *appdata_ptr)
Both, PAM_AUTHTOK and PAM_OLDAUTHTOK, will be reseted before returning to
the application. Which means an application is not able to access the
authentication tokens.
2.1.3.2. RETURN VALUES
PAM_BAD_ITEM
The application attempted to set an undefined or inaccessible
item.
PAM_BUF_ERR
Memory buffer error.
PAM_SUCCESS
Data was successful updated.
PAM_SYSTEM_ERR
The pam_handle_t passed as first argument was invalid.
2.1.4. Getting PAM items
#include <security/pam_modules.h>
int pam_get_item( pamh,
item_type,
item);
const pam_handle_t *pamh;
int item_type;
const void **item;
2.1.4.1. DESCRIPTION
The pam_get_item function allows applications and PAM service modules to
access and retrieve PAM informations of item_type. Upon successful return,
item contains a pointer to the value of the corresponding item. Note, this
is a pointer to the actual data and should not be free()'ed or
over-written! The following values are supported for item_type:
PAM_SERVICE
The service name (which identifies that PAM stack that the PAM
functions will use to authenticate the program).
PAM_USER
The username of the entity under whose identity service will be
given. That is, following authentication, PAM_USER identifies the
local entity that gets to use the service. Note, this value can be
mapped from something (eg., "anonymous") to something else (eg.
"guest119") by any module in the PAM stack. As such an application
should consult the value of PAM_USER after each call to a PAM
function.
PAM_USER_PROMPT
The string used when prompting for a user's name. The default
value for this string is a localized version of "login: ".
PAM_TTY
The terminal name: prefixed by /dev/ if it is a device file; for
graphical, X-based, applications the value for this item should be
the $DISPLAY variable.
PAM_RUSER
The requesting user name: local name for a locally requesting user
or a remote user name for a remote requesting user.
Generally an application or module will attempt to supply the
value that is most strongly authenticated (a local account before
a remote one. The level of trust in this value is embodied in the
actual authentication stack associated with the application, so it
is ultimately at the discretion of the system administrator.
PAM_RUSER@PAM_RHOST should always identify the requesting user. In
some cases, PAM_RUSER may be NULL. In such situations, it is
unclear who the requesting entity is.
PAM_RHOST
The requesting hostname (the hostname of the machine from which
the PAM_RUSER entity is requesting service). That is
PAM_RUSER@PAM_RHOST does identify the requesting user. In some
applications, PAM_RHOST may be NULL. In such situations, it is
unclear where the authentication request is originating from.
PAM_AUTHTOK
The authentication token (often a password). This token should be
ignored by all module functions besides pam_sm_authenticate(3) and
pam_sm_chauthtok(3). In the former function it is used to pass the
most recent authentication token from one stacked module to
another. In the latter function the token is used for another
purpose. It contains the currently active authentication token.
PAM_OLDAUTHTOK
The old authentication token. This token should be ignored by all
module functions except pam_sm_chauthtok(3).
PAM_CONV
The pam_conv structure. See pam_conv(3).
The following additional items are specific to Linux-PAM and should not be
used in portable applications:
PAM_FAIL_DELAY
A function pointer to redirect centrally managed failure delays.
See pam_fail_delay(3).
PAM_XDISPLAY
The name of the X display. For graphical, X-based applications the
value for this item should be the $DISPLAY variable. This value
may be used independently of PAM_TTY for passing the name of the
display.
PAM_XAUTHDATA
A pointer to a structure containing the X authentication data
required to make a connection to the display specified by
PAM_XDISPLAY, if such information is necessary. See
pam_xauth_data(3).
PAM_AUTHTOK_TYPE
The default action is for the module to use the following prompts
when requesting passwords: "New UNIX password: " and "Retype UNIX
password: ". The example word UNIX can be replaced with this item,
by default it is empty. This item is used by pam_get_authtok(3).
If a service module wishes to obtain the name of the user, it should not
use this function, but instead perform a call to pam_get_user(3).
Only a service module is privileged to read the authentication tokens,
PAM_AUTHTOK and PAM_OLDAUTHTOK.
2.1.4.2. RETURN VALUES
PAM_BAD_ITEM
The application attempted to set an undefined or inaccessible
item.
PAM_BUF_ERR
Memory buffer error.
PAM_PERM_DENIED
The value of item was NULL.
PAM_SUCCESS
Data was successful updated.
PAM_SYSTEM_ERR
The pam_handle_t passed as first argument was invalid.
2.1.5. Get user name
#include <security/pam_modules.h>
int pam_get_user( pamh,
user,
prompt);
const pam_handle_t *pamh;
const char **user;
const char *prompt;
2.1.5.1. DESCRIPTION
The pam_get_user function returns the name of the user specified by
pam_start(3). If no user was specified it returns what pam_get_item (pamh,
PAM_USER, ... ); would have returned. If this is NULL it obtains the
username via the pam_conv(3) mechanism, it prompts the user with the first
non-NULL string in the following list:
* The prompt argument passed to the function.
* What is returned by pam_get_item (pamh, PAM_USER_PROMPT, ... );
* The default prompt: "login: "
By whatever means the username is obtained, a pointer to it is returned as
the contents of *user. Note, this memory should not be free()'d or
modified by the module.
This function sets the PAM_USER item associated with the pam_set_item(3)
and pam_get_item(3) functions.
2.1.5.2. RETURN VALUES
PAM_SUCCESS
User name was successful retrieved.
PAM_SYSTEM_ERR
A NULL pointer was submitted.
PAM_CONV_ERR
The conversation method supplied by the application failed to
obtain the username.
2.1.6. The conversation function
#include <security/pam_appl.h>
struct pam_message {
int msg_style;
const char *msg;
};
struct pam_response {
char *resp;
int resp_retcode;
};
struct pam_conv {
int (*conv)(int num_msg, const struct pam_message **msg,
struct pam_response **resp, void *appdata_ptr);
void *appdata_ptr;
};
2.1.6.1. DESCRIPTION
The PAM library uses an application-defined callback to allow a direct
communication between a loaded module and the application. This callback
is specified by the struct pam_conv passed to pam_start(3) at the start of
the transaction.
When a module calls the referenced conv() function, the argument
appdata_ptr is set to the second element of this structure.
The other arguments of a call to conv() concern the information exchanged
by module and application. That is to say, num_msg holds the length of the
array of pointers, msg. After a successful return, the pointer resp points
to an array of pam_response structures, holding the application supplied
text. The resp_retcode member of this struct is unused and should be set
to zero. It is the caller's responsibility to release both, this array and
the responses themselves, using free(3). Note, *resp is a struct
pam_response array and not an array of pointers.
The number of responses is always equal to the num_msg conversation
function argument. This does require that the response array is free(3)'d
after every call to the conversation function. The index of the responses
corresponds directly to the prompt index in the pam_message array.
On failure, the conversation function should release any resources it has
allocated, and return one of the predefined PAM error codes.
Each message can have one of four types, specified by the msg_style member
of struct pam_message:
PAM_PROMPT_ECHO_OFF
Obtain a string without echoing any text.
PAM_PROMPT_ECHO_ON
Obtain a string whilst echoing text.
PAM_ERROR_MSG
Display an error message.
PAM_TEXT_INFO
Display some text.
The point of having an array of messages is that it becomes possible to
pass a number of things to the application in a single call from the
module. It can also be convenient for the application that related things
come at once: a windows based application can then present a single form
with many messages/prompts on at once.
In passing, it is worth noting that there is a descrepency between the way
Linux-PAM handles the const struct pam_message **msg conversation function
argument from the way that Solaris' PAM (and derivitives, known to include
HP/UX, are there others?) does. Linux-PAM interprets the msg argument as
entirely equivalent to the following prototype const struct pam_message
*msg[] (which, in spirit, is consistent with the commonly used prototypes
for argv argument to the familiar main() function: char **argv; and char
*argv[]). Said another way Linux-PAM interprets the msg argument as a
pointer to an array of num_msg read only 'struct pam_message' pointers.
Solaris' PAM implementation interprets this argument as a pointer to a
pointer to an array of num_msg pam_message structures. Fortunately,
perhaps, for most module/application developers when num_msg has a value
of one these two definitions are entirely equivalent. Unfortunately,
casually raising this number to two has led to unanticipated compatibility
problems.
For what its worth the two known module writer work-arounds for trying to
maintain source level compatibility with both PAM implementations are:
* never call the conversation function with num_msg greater than one.
* set up msg as doubly referenced so both types of conversation function
can find the messages. That is, make
msg[n] = & (( *msg )[n])
2.1.6.2. RETURN VALUES
PAM_BUF_ERR
Memory buffer error.
PAM_CONV_ERR
Conversation failure. The application should not set *resp.
PAM_SUCCESS
Success.
2.1.7. Set or change PAM environment variable
#include <security/pam_appl.h>
int pam_putenv( pamh,
name_value);
pam_handle_t *pamh;
const char *name_value;
2.1.7.1. DESCRIPTION
The pam_putenv function is used to add or change the value of PAM
environment variables as associated with the pamh handle.
The pamh argument is an authentication handle obtained by a prior call to
pam_start(). The name_value argument is a single NUL terminated string of
one of the following forms:
NAME=value of variable
In this case the environment variable of the given NAME is set to
the indicated value: value of variable. If this variable is
already known, it is overwritten. Otherwise it is added to the PAM
environment.
NAME=
This function sets the variable to an empty value. It is listed
separately to indicate that this is the correct way to achieve
such a setting.
NAME
Without an '=' the pam_putenv() function will delete the
corresponding variable from the PAM environment.
pam_putenv() operates on a copy of name_value, which means in contrast to
putenv(3), the application is responsible to free the data.
2.1.7.2. RETURN VALUES
PAM_PERM_DENIED
Argument name_value given is a NULL pointer.
PAM_BAD_ITEM
Variable requested (for deletion) is not currently set.
PAM_ABORT
The pamh handle is corrupt.
PAM_BUF_ERR
Memory buffer error.
PAM_SUCCESS
The environment variable was successfully updated.
2.1.8. Get a PAM environment variable
#include <security/pam_appl.h>
const char *pam_getenv( pamh,
name);
pam_handle_t *pamh;
const char *name;
2.1.8.1. DESCRIPTION
The pam_getenv function searches the PAM environment list as associated
with the handle pamh for an item that matches the string pointed to by
name and returns a pointer to the value of the environment variable. The
application is not allowed to free the data.
2.1.8.2. RETURN VALUES
The pam_getenv function returns NULL on failure.
2.1.9. Getting the PAM environment
#include <security/pam_appl.h>
char **pam_getenvlist( pamh);
pam_handle_t *pamh;
2.1.9.1. DESCRIPTION
The pam_getenvlist function returns a complete copy of the PAM environment
as associated with the handle pamh. The PAM environment variables
represent the contents of the regular environment variables of the
authenticated user when service is granted.
The format of the memory is a malloc()'d array of char pointers, the last
element of which is set to NULL. Each of the non-NULL entries in this
array point to a NUL terminated and malloc()'d char string of the form:
"name=value".
It should be noted that this memory will never be free()'d by libpam. Once
obtained by a call to pam_getenvlist, it is the responsibility of the
calling application to free() this memory.
It is by design, and not a coincidence, that the format and contents of
the returned array matches that required for the third argument of the
execle(3) function call.
2.1.9.2. RETURN VALUES
The pam_getenvlist function returns NULL on failure.
2.2. Other functions provided by libpam
2.2.1. Strings describing PAM error codes
#include <security/pam_appl.h>
const char *pam_strerror( pamh,
errnum);
pam_handle_t *pamh;
int errnum;
2.2.1.1. DESCRIPTION
The pam_strerror function returns a pointer to a string describing the
error code passed in the argument errnum, possibly using the LC_MESSAGES
part of the current locale to select the appropriate language. This string
must not be modified by the application. No library function will modify
this string.
2.2.1.2. RETURN VALUES
This function returns always a pointer to a string.
2.2.2. Request a delay on failure
#include <security/pam_appl.h>
int pam_fail_delay( pamh,
usec);
pam_handle_t *pamh;
unsigned int usec;
2.2.2.1. DESCRIPTION
The pam_fail_delay function provides a mechanism by which an application
or module can suggest a minimum delay of usec micro-seconds. The function
keeps a record of the longest time requested with this function. Should
pam_authenticate(3) fail, the failing return to the application is delayed
by an amount of time randomly distributed (by up to 50%) about this
longest value.
Independent of success, the delay time is reset to its zero default value
when the PAM service module returns control to the application. The delay
occurs after all authentication modules have been called, but before
control is returned to the service application.
When using this function the programmer should check if it is available
with:
#ifdef HAVE_PAM_FAIL_DELAY
....
#endif /* HAVE_PAM_FAIL_DELAY */
For applications written with a single thread that are event driven in
nature, generating this delay may be undesirable. Instead, the application
may want to register the delay in some other way. For example, in a single
threaded server that serves multiple authentication requests from a single
event loop, the application might want to simply mark a given connection
as blocked until an application timer expires. For this reason the delay
function can be changed with the PAM_FAIL_DELAY item. It can be queried
and set with pam_get_item(3) and pam_set_item (3) respectively. The value
used to set it should be a function pointer of the following prototype:
void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr);
The arguments being the retval return code of the module stack, the
usec_delay micro-second delay that libpam is requesting and the
appdata_ptr that the application has associated with the current pamh.
This last value was set by the application when it called pam_start(3) or
explicitly with pam_set_item(3). Note, if PAM_FAIL_DELAY item is unset (or
set to NULL), then no delay will be performed.
2.2.2.2. RETURN VALUES
PAM_SUCCESS
Delay was successful adjusted.
PAM_SYSTEM_ERR
A NULL pointer was submitted as PAM handle.
Chapter 3. What is expected of a module
The module must supply a sub-set of the six functions listed below.
Together they define the function of a Linux-PAM module. Module developers
are strongly urged to read the comments on security that follow this list.
3.1. Overview
The six module functions are grouped into four independent management
groups. These groups are as follows: authentication, account, session and
password. To be properly defined, a module must define all functions
within at least one of these groups. A single module may contain the
necessary functions for all four groups.
3.1.1. Functional independence
The independence of the four groups of service a module can offer means
that the module should allow for the possibility that any one of these
four services may legitimately be called in any order. Thus, the module
writer should consider the appropriateness of performing a service without
the prior success of some other part of the module.
As an informative example, consider the possibility that an application
applies to change a user's authentication token, without having first
requested that Linux-PAM authenticate the user. In some cases this may be
deemed appropriate: when root wants to change the authentication token of
some lesser user. In other cases it may not be appropriate: when joe
maliciously wants to reset alice's password; or when anyone other than the
user themself wishes to reset their KERBEROS authentication token. A
policy for this action should be defined by any reasonable authentication
scheme, the module writer should consider this when implementing a given
module.
3.1.2. Minimizing administration problems
To avoid system administration problems and the poor construction of a
/etc/pam.conf file, the module developer may define all six of the
following functions. For those functions that would not be called, the
module should return PAM_SERVICE_ERR and write an appropriate message to
the system log. When this action is deemed inappropriate, the function
would simply return PAM_IGNORE.
3.1.3. Arguments supplied to the module
The flags argument of each of the following functions can be logically
OR'd with PAM_SILENT, which is used to inform the module to not pass any
text (errors or warnings) application.
The argc and argv arguments are taken from the line appropriate to this
module---that is, with the service_name matching that of the
application---in the configuration file (see the Linux-PAM System
Administrators' Guide). Together these two parameters provide the number
of arguments and an array of pointers to the individual argument tokens.
This will be familiar to C programmers as the ubiquitous method of passing
command arguments to the function main(). Note, however, that the first
argument (argv[0]) is a true argument and not the name of the module.
3.2. Authentication management
To be correctly initialized, PAM_SM_AUTH must be #define'd prior to
including <security/pam_modules.h>. This will ensure that the prototypes
for static modules are properly declared.
3.2.1. Service function for user authentication
#define PAM_SM_AUTH
#include <security/pam_modules.h>
int pam_sm_authenticate( pamh,
flags,
argc,
argv);
pam_handle_t *pamh;
int flags;
int argc;
const char **argv;
3.2.1.1. DESCRIPTION
The pam_sm_authenticate function is the service module's implementation of
the pam_authenticate(3) interface.
This function performs the task of authenticating the user.
Valid flags, which may be logically OR'd with PAM_SILENT, are:
PAM_SILENT
Do not emit any messages.
PAM_DISALLOW_NULL_AUTHTOK
Return PAM_AUTH_ERR if the database of authentication tokens for
this authentication mechanism has a NULL entry for the user.
Without this flag, such a NULL token will lead to a success
without the user being prompted.
3.2.1.2. RETURN VALUES
PAM_AUTH_ERR
Authentication failure.
PAM_CRED_INSUFFICIENT
For some reason the application does not have sufficient
credentials to authenticate the user.
PAM_AUTHINFO_UNAVAIL
The modules were not able to access the authentication
information. This might be due to a network or hardware failure
etc.
PAM_SUCCESS
The authentication token was successfully updated.
PAM_USER_UNKNOWN
The supplied username is not known to the authentication service.
PAM_MAXTRIES
One or more of the authentication modules has reached its limit of
tries authenticating the user. Do not try again.
3.2.2. Service function to alter credentials
#define PAM_SM_AUTH
#include <security/pam_modules.h>
int pam_sm_setcred( pamh,
flags,
argc,
argv);
pam_handle_t *pamh;
int flags;
int argc;
const char **argv;
3.2.2.1. DESCRIPTION
The pam_sm_setcred function is the service module's implementation of the
pam_setcred(3) interface.
This function performs the task of altering the credentials of the user
with respect to the corresponding authorization scheme. Generally, an
authentication module may have access to more information about a user
than their authentication token. This function is used to make such
information available to the application. It should only be called after
the user has been authenticated but before a session has been established.
Valid flags, which may be logically OR'd with PAM_SILENT, are:
PAM_SILENT
Do not emit any messages.
PAM_ESTABLISH_CRED
Initialize the credentials for the user.
PAM_DELETE_CRED
Delete the credentials associated with the authentication service.
PAM_REINITIALIZE_CRED
Reinitialize the user credentials.
PAM_REFRESH_CRED
Extend the lifetime of the user credentials.
The way the auth stack is navigated in order to evaluate the pam_setcred()
function call, independent of the pam_sm_setcred() return codes, is
exactly the same way that it was navigated when evaluating the
pam_authenticate() library call. Typically, if a stack entry was ignored
in evaluating pam_authenticate(), it will be ignored when libpam evaluates
the pam_setcred() function call. Otherwise, the return codes from each
module specific pam_sm_setcred() call are treated as required.
3.2.2.2. RETURN VALUES
PAM_CRED_UNAVAIL
This module cannot retrieve the user's credentials.
PAM_CRED_EXPIRED
The user's credentials have expired.
PAM_CRED_ERR
This module was unable to set the credentials of the user.
PAM_SUCCESS
The user credential was successfully set.
PAM_USER_UNKNOWN
The user is not known to this authentication module.
These, non-PAM_SUCCESS, return values will typically lead to the
credential stack failing. The first such error will dominate in the return
value of pam_setcred().
3.3. Account management
To be correctly initialized, PAM_SM_ACCOUNT must be #define'd prior to
including <security/pam_modules.h>. This will ensure that the prototypes
for static modules are properly declared.
3.3.1. Service function for account management
#define PAM_SM_ACCOUNT
#include <security/pam_modules.h>
int pam_sm_acct_mgmt( pamh,
flags,
argc,
argv);
pam_handle_t *pamh;
int flags;
int argc;
const char **argv;
3.3.1.1. DESCRIPTION
The pam_sm_acct_mgmt function is the service module's implementation of
the pam_acct_mgmt(3) interface.
This function performs the task of establishing whether the user is
permitted to gain access at this time. It should be understood that the
user has previously been validated by an authentication module. This
function checks for other things. Such things might be: the time of day or
the date, the terminal line, remote hostname, etc. This function may also
determine things like the expiration on passwords, and respond that the
user change it before continuing.
Valid flags, which may be logically OR'd with PAM_SILENT, are:
PAM_SILENT
Do not emit any messages.
PAM_DISALLOW_NULL_AUTHTOK
Return PAM_AUTH_ERR if the database of authentication tokens for
this authentication mechanism has a NULL entry for the user.
3.3.1.2. RETURN VALUES
PAM_ACCT_EXPIRED
User account has expired.
PAM_AUTH_ERR
Authentication failure.
PAM_NEW_AUTHTOK_REQD
The user's authentication token has expired. Before calling this
function again the application will arrange for a new one to be
given. This will likely result in a call to pam_sm_chauthtok().
PAM_PERM_DENIED
Permission denied.
PAM_SUCCESS
The authentication token was successfully updated.
PAM_USER_UNKNOWN
User unknown to password service.
3.4. Session management
To be correctly initialized, PAM_SM_SESSION must be #define'd prior to
including <security/pam_modules.h>. This will ensure that the prototypes
for static modules are properly declared.
3.4.1. Service function to start session management
#define PAM_SM_SESSION
#include <security/pam_modules.h>
int pam_sm_open_session( pamh,
flags,
argc,
argv);
pam_handle_t *pamh;
int flags;
int argc;
const char **argv;
3.4.1.1. DESCRIPTION
The pam_sm_open_session function is the service module's implementation of
the pam_open_session(3) interface.
This function is called to commence a session. The only valid value for
flags is zero or:
PAM_SILENT
Do not emit any messages.
3.4.1.2. RETURN VALUES
PAM_SESSION_ERR
Cannot make/remove an entry for the specified session.
PAM_SUCCESS
The session was successfully started.
3.4.2. Service function to terminate session management
#define PAM_SM_SESSION
#include <security/pam_modules.h>
int pam_sm_close_session( pamh,
flags,
argc,
argv);
pam_handle_t *pamh;
int flags;
int argc;
const char **argv;
3.4.2.1. DESCRIPTION
The pam_sm_close_session function is the service module's implementation
of the pam_close_session(3) interface.
This function is called to terminate a session. The only valid value for
flags is zero or:
PAM_SILENT
Do not emit any messages.
3.4.2.2. RETURN VALUES
PAM_SESSION_ERR
Cannot make/remove an entry for the specified session.
PAM_SUCCESS
The session was successfully terminated.
3.5. Authentication token management
To be correctly initialized, PAM_SM_PASSWORD must be #define'd prior to
including <security/pam_modules.h>. This will ensure that the prototypes
for static modules are properly declared.
3.5.1. Service function to alter authentication token
#define PAM_SM_PASSWORD
#include <security/pam_modules.h>
int pam_sm_chauthtok( pamh,
flags,
argc,
argv);
pam_handle_t *pamh;
int flags;
int argc;
const char **argv;
3.5.1.1. DESCRIPTION
The pam_sm_chauthtok function is the service module's implementation of
the pam_chauthtok(3) interface.
This function is used to (re-)set the authentication token of the user.
Valid flags, which may be logically OR'd with PAM_SILENT, are:
PAM_SILENT
Do not emit any messages.
PAM_CHANGE_EXPIRED_AUTHTOK
This argument indicates to the module that the user's
authentication token (password) should only be changed if it has
expired. This flag is optional and must be combined with one of
the following two flags. Note, however, the following two options
are mutually exclusive.
PAM_PRELIM_CHECK
This indicates that the modules are being probed as to their ready
status for altering the user's authentication token. If the module
requires access to another system over some network it should
attempt to verify it can connect to this system on receiving this
flag. If a module cannot establish it is ready to update the
user's authentication token it should return PAM_TRY_AGAIN, this
information will be passed back to the application.
If the control value sufficient is used in the password stack, the
PAM_PRELIM_CHECK section of the modules following that control
value is not always executed.
PAM_UPDATE_AUTHTOK
This informs the module that this is the call it should change the
authorization tokens. If the flag is logically OR'd with
PAM_CHANGE_EXPIRED_AUTHTOK, the token is only changed if it has
actually expired.
The PAM library calls this function twice in succession. The first time
with PAM_PRELIM_CHECK and then, if the module does not return
PAM_TRY_AGAIN, subsequently with PAM_UPDATE_AUTHTOK. It is only on the
second call that the authorization token is (possibly) changed.
3.5.1.2. RETURN VALUES
PAM_AUTHTOK_ERR
The module was unable to obtain the new authentication token.
PAM_AUTHTOK_RECOVERY_ERR
The module was unable to obtain the old authentication token.
PAM_AUTHTOK_LOCK_BUSY
Cannot change the authentication token since it is currently
locked.
PAM_AUTHTOK_DISABLE_AGING
Authentication token aging has been disabled.
PAM_PERM_DENIED
Permission denied.
PAM_TRY_AGAIN
Preliminary check was unsuccessful. Signals an immediate return to
the application is desired.
PAM_SUCCESS
The authentication token was successfully updated.
PAM_USER_UNKNOWN
User unknown to password service.
Chapter 4. Generic optional arguments
Here we list the generic arguments that all modules can expect to be
passed. They are not mandatory, and their absence should be accepted
without comment by the module.
debug
Use the pam_syslog(3) call to log debugging information to the
system log files.
use_first_pass
The module should not prompt the user for a password. Instead, it
should obtain the previously typed password (by a call to
pam_get_item() for the PAM_AUTHTOK item), and use that. If that
doesn't work, then the user will not be authenticated. (This
option is intended for auth and passwd modules only).
Chapter 5. Programming notes
Here we collect some pointers for the module writer to bear in mind when
writing/developing a Linux-PAM compatible module.
5.1. Security issues for module creation
5.1.1. Sufficient resources
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.
5.1.2. Who's who?
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.
* X, the identity of the user invoking the service request. This is the
user identifier; returned by the function getuid(2).
* Y, the privileged identity of the application used to grant the
requested service. This is the effective user identifier; returned by
the function geteuid(2).
* Z, the user under whose identity the service will be granted. This is
the username returned by pam_get_user() and also stored in the
Linux-PAM item, PAM_USER.
* Linux-PAM has a place for an additional user identity that a module
may care to make use of. This is the 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.
5.1.3. Using the conversation function
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.
5.1.4. Authentication tokens
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;
}
5.2. Use of 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:
* As a general rule, errors encountered by a module should be logged at
the 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.
* Debugging information, as activated by the debug argument to the
module in /etc/pam.conf, should be logged at the LOG_DEBUG level.
* If a module discovers that its personal configuration file or some
system file it uses for information is corrupted or somehow unusable,
it should indicate this by logging messages at level, LOG_ALERT.
* Shortages of system resources, such as a failure to manipulate a file
or malloc() failures should be logged at level LOG_CRIT.
* Authentication failures, associated with an incorrectly typed password
should be logged at level, LOG_NOTICE.
5.3. Modules that require system libraries
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 -o pam_module.so pam_module.o -lwhatever
Chapter 6. An example module
At some point, we may include a fully commented example of a module in
this document. For now, please look at the modules directory of the
Linux-PAM sources.
Chapter 7. See also
* The Linux-PAM System Administrators' Guide.
* The Linux-PAM Application Developers' Guide.
* The V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH PLUGGABLE
AUTHENTICATION MODULES'', Open Software Foundation Request For
Comments 86.0, October 1995.
Chapter 8. Author/acknowledgments
This document was written by Andrew G. Morgan (morgan@kernel.org) with
many contributions from Chris Adams, Peter Allgeyer, Tim Baverstock, Tim
Berger, Craig S. Bell, Derrick J. Brashear, Ben Buxton, Seth Chaiklin,
Oliver Crow, Chris Dent, Marc Ewing, Cristian Gafton, Emmanuel Galanos,
Brad M. Garcia, Eric Hester, Roger Hu, Eric Jacksch, Michael K. Johnson,
David Kinchlea, Olaf Kirch, Marcin Korzonek, Thorsten Kukuk, Stephen
Langasek, Nicolai Langfeldt, Elliot Lee, Luke Kenneth Casson Leighton, Al
Longyear, Ingo Luetkebohle, Marek Michalkiewicz, Robert Milkowski, Aleph
One, Martin Pool, Sean Reifschneider, Jan Rekorajski, Erik Troan, Theodore
Ts'o, Jeff Uphoff, Myles Uyema, Savochkin Andrey Vladimirovich, Ronald
Wahl, David Wood, John Wilmes, Joseph S. D. Yao and Alex O. Yuriev.
Thanks are also due to Sun Microsystems, especially to Vipin Samar and
Charlie Lai for their advice. At an early stage in the development of
Linux-PAM, Sun graciously made the documentation for their implementation
of PAM available. This act greatly accelerated the development of
Linux-PAM.
Chapter 9. Copyright information for this document
Copyright (c) 2006 Thorsten Kukuk <kukuk@thkukuk.de>
Copyright (c) 1996-2002 Andrew G. Morgan <morgan@kernel.org>
Redistribution and use in source and binary forms, with or without
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2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. The name of the author may not be used to endorse or promote
products derived from this software without specific prior
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Alternatively, this product may be distributed under the terms of the GNU
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are required instead of the above restrictions. (This clause is necessary
due to a potential bad interaction between the GNU GPL and the
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