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.

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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 information 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 reset 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 information 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.

PAM_BUF_ERR

    Memory buffer error.

PAM_ABORT

    Error resuming an old conversation.

PAM_CONV_AGAIN

    The conversation method supplied by the application is waiting for an
    event.

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 discrepancy between the way
Linux-PAM handles the const struct pam_message **msg conversation function
argument and the way that Solaris' PAM (and derivatives, 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 for freeing 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 that the PAM_FAIL_DELAY item is set to NULL by default. This indicates
that PAM should perform a random delay as described above when authentication
fails and a delay has been suggested. If an application does not want the PAM
library to perform any delay on authentication failure, then the application
must define a custom delay function that executes no statements and set the
PAM_FAIL_DELAY item to point to this function.

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

3.2.1. Service function for user authentication

#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

#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

3.3.1. Service function for account management

#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

3.4.1. Service function to start session management

#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

#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

3.5.1. Service function to alter authentication token

#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
modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright
   notice, and the entire permission notice in its entirety,
   including the disclaimer of warranties.

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
   written permission.


Alternatively, this product may be distributed under the terms of the GNU
General Public License (GPL), in which case the provisions of the GNU GPL are
required instead of the above restrictions. (This clause is necessary due to a
potential bad interaction between the GNU GPL and the restrictions contained in
a BSD-style copyright.)

THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH


