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 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>

   PAM_EXTERN 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>

   PAM_EXTERN 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>

   PAM_EXTERN 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>

   PAM_EXTERN 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>

   PAM_EXTERN 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>

   PAM_EXTERN 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 users
           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:

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    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
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