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.

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.

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.

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) to the 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.

• PAM_EXTERN int pam_sm_authenticate(pam_handle_t *pamh, int flags, int argc, const char **argv);

  This function performs the task of authenticating the user.

  The flags argument can be a logically OR'd with PAM_SILENT and optionally take the following value:

  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.

  Besides PAM_SUCCESS return values that can be sent by this function are one of the following:

  PAM_AUTH_ERR

  The user was not authenticated

  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_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.

• PAM_EXTERN int pam_sm_setcred(pam_handle_t *pamh, int flags, int argc, const char **argv);

  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.

  Permitted flags, one of which, may be logically OR'd with PAM_SILENT are,

  PAM_ESTABLISH_CRED

  Set the credentials for the authentication service,

  PAM_DELETE_CRED

  Delete the credentials associated with the authentication service,

  PAM_REINITIALIZE_CRED

  Reinitialize the user credentials, and

  PAM_REFRESH_CRED

  Extend the lifetime of the user credentials.

  Prior to Linux-PAM-0.75, and due to a deficiency with the way the auth stack was handled in the case of the setcred stack being processed, the module was required to attempt to return the same error code as pam_sm_authenticate did. This was necessary to preserve the logic followed by libpam as it executes the stack of authentication modules, when the application called either pam_authenticate() or pam_setcred(). Failing to do this, led to confusion on the part of the System Administrator.

  For Linux-PAM-0.75 and later, libpam handles the credential stack much more sanely. 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.

  Besides PAM_SUCCESS, the module may return one of the following errors:

  PAM_CRED_UNAVAIL

  This module cannot retrieve the user's credentials.

  PAM_CRED_EXPIRED

  The user's credentials have expired.

  PAM_USER_UNKNOWN

  The user is not known to this authentication module.

  PAM_CRED_ERR

  This module was unable to set the credentials of the user.

  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 prototype for a static module is properly declared.

• PAM_EXTERN int pam_sm_acct_mgmt(pam_handle_t *pamh, int flags, int argc, const char **argv);

  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 the same as those applicable to the flags argument of pam_sm_authenticate.

  This function may return one of the following errors,

  PAM_ACCT_EXPIRED

  The user is no longer permitted access to the system.

  PAM_AUTH_ERR

  There was an authentication error.

  PAM_AUTHTOKEN_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_USER_UNKNOWN

  The user is not known to the module's account management component.

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.

The following two functions are defined to handle the initialization/termination of a session. For example, at the beginning of a session the module may wish to log a message with the system regarding the user. Similarly, at the end of the session the module would inform the system that the user's session has ended.

It should be possible for sessions to be opened by one application and closed by another. This either requires that the module uses only information obtained from pam_get_item(), or that information regarding the session is stored in some way by the operating system (in a file for example).

• PAM_EXTERN int pam_sm_open_session(pam_handle_t *pamh, int flags, int argc, const char **argv);

  This function is called to commence a session. The only valid, but optional, flag is PAM_SILENT.

  As a return value, PAM_SUCCESS signals success and PAM_SESSION_ERR failure.

• PAM_EXTERN int pam_sm_close_session(pam_handle_t *pamh, int flags, int argc, const char **argv);

  This function is called to terminate a session. The only valid, but optional, flag is PAM_SILENT.

  As a return value, PAM_SUCCESS signals success and PAM_SESSION_ERR failure.

3.5 Password management

To be correctly initialized, PAM_SM_PASSWORD must be #define'd prior to including <security/pam_modules.h>. This will ensure that the prototype for a static module is properly declared.

• PAM_EXTERN int pam_sm_chauthtok(pam_handle_t *pamh, int flags, int argc, const char **argv);

  This function is used to (re-)set the authentication token of the user. A valid flag, which may be logically OR'd with PAM_SILENT, can be built from the following list,

  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.

  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.

  Note, the Linux-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.

  PAM_SUCCESS is the only successful return value, valid error-returns are:

  PAM_AUTHTOK_ERR

  The module was unable to obtain the new authentication token.

  PAM_AUTHTOK_RECOVER_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_USER_UNKNOWN

  The user is not known to the authentication token changing service.