The Linux-PAM Application Developers' Guide
Andrew G. Morgan
<morgan@kernel.org>
Thorsten Kukuk
<kukuk@thkukuk.de>
Version 0.99.6.0, 5. August 2006
Abstract
This manual documents what an application developer needs to know about the
Linux-PAM library. It describes how an application might use the Linux-PAM
library to authenticate users. In addition it contains a description of the
funtions to be found in libpam_misc library, that can be used in general
applications. Finally, it contains some comments on PAM related security issues
for the application developer.
-------------------------------------------------------------------------------
Table of Contents
1. Introduction
1.1. Description
1.2. Synopsis
2. Overview
3. The public interface to Linux-PAM
3.1. What can be expected by the application
3.1.1. Initialization of PAM transaction
3.1.2. Termination of PAM transaction
3.1.3. Setting PAM items
3.1.4. Getting PAM items
3.1.5. Strings describing PAM error codes
3.1.6. Request a delay on failure
3.1.7. Authenticating the user
3.1.8. Setting user credentials
3.1.9. Account validation management
3.1.10. Updating authentication tokens
3.1.11. Start PAM session management
3.1.12. terminating PAM session management
3.1.13. Set or change PAM environment variable
3.1.14. Get a PAM environment variable
3.1.15. Getting the PAM environment
3.2. What is expected of an application
3.2.1. The conversation function
3.3. Programming notes
4. Security issues of Linux-PAM
4.1. Care about standard library calls
4.2. Choice of a service name
4.3. The conversation function
4.4. The identity of the user
4.5. Sufficient resources
5. A library of miscellaneous helper functions
5.1. Functions supplied
5.1.1. Text based conversation function
5.1.2. Transcribing an environment to that of PAM
5.1.3. Liberating a locally saved environment
5.1.4. BSD like PAM environment variable setting
6. Porting legacy applications
7. Glossary of PAM related terms
8. An example application
9. Files
10. See also
11. Author/acknowledgments
12. 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.
It is the purpose of the Linux-PAM project to liberate the development of
privilege granting software from the development of secure and appropriate
authentication schemes. This is accomplished by providing a documented library
of functions that an application may use for all forms of user authentication
management. This library dynamically loads locally configured authentication
modules that actually perform the authentication tasks.
From the perspective of an application developer the information contained in
the local configuration of the PAM library should not be important. Indeed it
is intended that an application treat the functions documented here as a 'black
box' that will deal with all aspects of user authentication. 'All aspects'
includes user verification, account management, session initialization/
termination and also the resetting of passwords (authentication tokens).
1.2. Synopsis
For general applications that wish to use the services provided by Linux-PAM
the following is a summary of the relevant linking information:
#include <security/pam_appl.h>
cc -o application .... -lpam
In addition to libpam, there is a library of miscellaneous functions that make
the job of writing PAM-aware applications easier (this library is not covered
in the DCE-RFC for PAM and is specific to the Linux-PAM distribution):
#include <security/pam_appl.h>
#include <security/pam_misc.h>
cc -o application .... -lpam -lpam_misc
Chapter 2. Overview
Most service-giving applications are restricted. In other words, their service
is not available to all and every prospective client. Instead, the applying
client must jump through a number of hoops to convince the serving application
that they are authorized to obtain service.
The process of authenticating a client is what PAM is designed to manage. In
addition to authentication, PAM provides account management, credential
management, session management and authentication-token (password changing)
management services. It is important to realize when writing a PAM based
application that these services are provided in a manner that is transparent to
the application. That is to say, when the application is written, no
assumptions can be made about how the client will be authenticated.
The process of authentication is performed by the PAM library via a call to
pam_authenticate(). The return value of this function will indicate whether a
named client (the user) has been authenticated. If the PAM library needs to
prompt the user for any information, such as their name or a password then it
will do so. If the PAM library is configured to authenticate the user using
some silent protocol, it will do this too. (This latter case might be via some
hardware interface for example.)
It is important to note that the application must leave all decisions about
when to prompt the user at the discretion of the PAM library.
The PAM library, however, must work equally well for different styles of
application. Some applications, like the familiar login and passwd are terminal
based applications, exchanges of information with the client in these cases is
as plain text messages. Graphically based applications, however, have a more
sophisticated interface. They generally interact with the user via specially
constructed dialogue boxes. Additionally, network based services require that
text messages exchanged with the client are specially formatted for automated
processing: one such example is ftpd which prefixes each exchanged message with
a numeric identifier.
The presentation of simple requests to a client is thus something very
dependent on the protocol that the serving application will use. In spite of
the fact that PAM demands that it drives the whole authentication process, it
is not possible to leave such protocol subtleties up to the PAM library. To
overcome this potential problem, the application provides the PAM library with
a conversation function. This function is called from within the PAM library
and enables the PAM to directly interact with the client. The sorts of things
that this conversation function must be able to do are prompt the user with
text and/or obtain textual input from the user for processing by the PAM
library. The details of this function are provided in a later section.
For example, the conversation function may be called by the PAM library with a
request to prompt the user for a password. Its job is to reformat the prompt
request into a form that the client will understand. In the case of ftpd, this
might involve prefixing the string with the number 331 and sending the request
over the network to a connected client. The conversation function will then
obtain any reply and, after extracting the typed password, will return this
string of text to the PAM library. Similar concerns need to be addressed in the
case of an X-based graphical server.
There are a number of issues that need to be addressed when one is porting an
existing application to become PAM compliant. A section below has been devoted
to this: Porting legacy applications.
Besides authentication, PAM provides other forms of management. Session
management is provided with calls to pam_open_session() and pam_close_session
(). What these functions actually do is up to the local administrator. But
typically, they could be used to log entry and exit from the system or for
mounting and unmounting the user's home directory. If an application provides
continuous service for a period of time, it should probably call these
functions, first open after the user is authenticated and then close when the
service is terminated.
Account management is another area that an application developer should include
with a call to pam_acct_mgmt(). This call will perform checks on the good
health of the user's account (has it expired etc.). One of the things this
function may check is whether the user's authentication token has expired - in
such a case the application may choose to attempt to update it with a call to
pam_chauthtok(), although some applications are not suited to this task (ftp
for example) and in this case the application should deny access to the user.
PAM is also capable of setting and deleting the users credentials with the call
pam_setcred(). This function should always be called after the user is
authenticated and before service is offered to the user. By convention, this
should be the last call to the PAM library before the PAM session is opened.
What exactly a credential is, is not well defined. However, some examples are
given in the glossary below.
Chapter 3. The public interface to Linux-PAM
Firstly, the relevant include file for the Linux-PAM library is <security/
pam_appl.h>. It contains the definitions for a number of functions. After
listing these functions, we collect some guiding remarks for programmers.
3.1. What can be expected by the application
3.1.1. Initialization of PAM transaction
#include <security/pam_appl.h>
int pam_start( service_name,
user,
pam_conversation,
pamh);
const char * service_name;
const char * user;
const struct pam_conv * pam_conversation;
pam_handle_t ** pamh;
3.1.1.1. DESCRIPTION
The pam_start function creates the PAM context and initiates the PAM
transaction. It is the first of the PAM functions that needs to be called by an
application. The transaction state is contained entirely within the structure
identified by this handle, so it is possible to have multiple transactions in
parallel. But it is not possible to use the same handle for different
transactions, a new one is needed for every new context.
The service_name argument specifies the name of the service to apply and will
be stored as PAM_SERVICE item in the new context. The policy for the service
will be read from the file /etc/pam.d/service_name or, if that file does not
exist, from /etc/pam.conf.
The user argument can specify the name of the target user and will be stored as
PAM_USER item. If the argument is NULL, the module has to ask for this item if
necessary.
The pam_conversation argument points to a struct pam_conv describing the
conversation function to use. An application must provide this for direct
communication between a loaded module and the application.
Following a successful return (PAM_SUCCESS) the contents of pamh is a handle
that contains the PAM context for successive calls to the PAM functions. In an
error case is the content of pamh undefined.
The pam_handle_t is a blind structure and the application should not attempt to
probe it directly for information. Instead the PAM library provides the
functions pam_set_item(3) and pam_get_item(3). The PAM handle cannot be used
for mulitiple authentications at the same time as long as pam_end was not
called on it before.
3.1.1.2. RETURN VALUES
PAM_ABORT
General failure.
PAM_BUF_ERR
Memory buffer error.
PAM_SUCCESS
Transaction was successful created.
PAM_SYSTEM_ERR
System error, for example a NULL pointer was submitted instead of a pointer
to data.
3.1.2. Termination of PAM transaction
#include <security/pam_appl.h>
int pam_end( pamh,
pam_status);
pam_handle_t * pamh;
int pam_status;
3.1.2.1. DESCRIPTION
The pam_end function terminates the PAM transaction and is the last function an
application should call in the PAM contenxt. Upon return the handle pamh is no
longer valid and all memory associated with it will be invalid.
The pam_status argument should be set to the value returned to the application
by the last PAM library call.
The value taken by pam_status is used as an argument to the module specific
callback function, cleanup() (See pam_set_data(3) and pam_get_data(3)). In this
way the module can be given notification of the pass/fail nature of the
tear-down process, and perform any last minute tasks that are appropriate to
the module before it is unlinked. This argument can be logically OR'd with
PAM_DATA_SILENT to indicate to indicate that the module should not treat the
call too seriously. It is generally used to indicate that the current closing
of the library is in a fork(2)ed process, and that the parent will take care of
cleaning up things that exist outside of the current process space (files
etc.).
This function free's all memory for items associated with the pam_set_item(3)
and pam_get_item(3) functions. Pointers associated with such objects are not
valid anymore after pam_end was called.
3.1.2.2. RETURN VALUES
PAM_SUCCESS
Transaction was successful terminated.
PAM_SYSTEM_ERR
System error, for example a NULL pointer was submitted as PAM handle or the
function was called by a module.
3.1.3. 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;
3.1.3.1. 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).
PAM_FAIL_DELAY
A function pointer to redirect centrally managed failure delays. See
pam_fail_delay(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.
3.1.3.2. 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.
3.1.4. 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;
3.1.4.1. 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).
PAM_FAIL_DELAY
A function pointer to redirect centrally managed failure delays. See
pam_fail_delay(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.
3.1.4.2. 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.
3.1.5. Strings describing PAM error codes
#include <security/pam_appl.h>
const char *pam_strerror( pamh,
errnum);
pam_handle_t * pamh;
int errnum;
3.1.5.1. 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.
3.1.5.2. RETURN VALUES
This function returns always a pointer to a string.
3.1.6. Request a delay on failure
#include <security/pam_appl.h>
int pam_fail_delay( pamh,
usec);
pam_handle_t * pamh;
unsigned int usec;
3.1.6.1. 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 25%) 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);
3.1.6.2. RETURN VALUES
PAM_SUCCESS
Delay was successful adjusted.
PAM_SYSTEM_ERR
A NULL pointer was submitted as PAM handle.
3.1.7. Authenticating the user
#include <security/pam_appl.h>
int pam_authenticate( pamh,
flags);
pam_handle_t * pamh;
int flags;
3.1.7.1. DESCRIPTION
The pam_authenticate function is used to authenticate the user. The user is
required to provide an authentication token depending upon the authentication
service, usually this is a password, but could also be a finger print.
The PAM service module may request that the user enter their username vio the
the conversation mechanism (see pam_start(3) and pam_conv(3)). The name of the
authenticated user will be present in the PAM item PAM_USER. This item may be
recovered with a call to pam_get_item(3).
The pamh argument is an authentication handle obtained by a prior call to
pam_start(). The flags argument is the binary or of zero or more of the
following values:
PAM_SILENT
Do not emit any messages.
PAM_DISALLOW_NULL_AUTHTOK
The PAM module service should return PAM_AUTH_ERR if the user does not have
a registered authentication token.
3.1.7.2. RETURN VALUES
PAM_ABORT
The application should exit immediately after calling pam_end(3) first.
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_UNVAIL
The modules were not able to access the authentication information. This
might be due to a network or hardware failure etc.
PAM_MAXTRIES
One or more of the authentication modules has reached its limit of tries
authenticating the user. Do not try again.
PAM_SUCCESS
The user was successfully authenticated.
PAM_USER_UNKNOWN
User unknown to authentication service.
3.1.8. Setting user credentials
#include <security/pam_appl.h>
int pam_setcred( pamh,
flags);
pam_handle_t * pamh;
int flags;
3.1.8.1. DESCRIPTION
The pam_setcred function is used to establish, maintain and delete the
credentials of a user. It should be called after a user has been authenticated
and before a session is opened for the user (with pam_open_session(3)).
A credential is something that the user possesses. It is some property, such as
a Kerberos ticket, or a supplementary group membership that make up the
uniqueness of a given user. On a Linux system the user's UID and GID's are
credentials too. However, it has been decided that these properties (along with
the default supplementary groups of which the user is a member) are credentials
that should be set directly by the application and not by PAM. Such credentials
should be established, by the application, prior to a call to this function.
For example, initgroups(2) (or equivalent) should have been performed.
Valid flags, any one of which, may be logically OR'd with PAM_SILENT, are:
PAM_ESTABLISH_CRED
Initialize the credentials for the user.
PAM_DELETE_CRED
Delete the user's credentials.
PAM_REINITIALIZE_CRED
Fully reinitialize the user's credentials.
PAM_REFRESH_CRED
Extend the lifetime of the existing credentials.
3.1.8.2. RETURN VALUES
PAM_BUF_ERR
Memory buffer error.
PAM_CRED_ERR
Failed to set user credentials.
PAM_CRED_EXPIRED
User credentials are expired.
PAM_CRED_UNAVAIL
Failed to retrieve user credentials.
PAM_SUCCESS
Data was successful stored.
PAM_SYSTEM_ERR
A NULL pointer was submitted as PAM handle, the function was called by a
module or another system error occured.
PAM_USER_UNKNOWN
User is not known to an authentication module.
3.1.9. Account validation management
#include <security/pam_appl.h>
int pam_acct_mgmt( pamh,
flags);
pam_handle_t * pamh;
int flags;
3.1.9.1. DESCRIPTION
The pam_acct_mgmt function is used to determine if the users account is valid.
It checks for authentication token and account expiration and verifies access
restrictions. It is typically called after the user has been authenticated.
The pamh argument is an authentication handle obtained by a prior call to
pam_start(). The flags argument is the binary or of zero or more of the
following values:
PAM_SILENT
Do not emit any messages.
PAM_DISALLOW_NULL_AUTHTOK
The PAM module service should return PAM_NEW_AUTHTOK_REQD if the user has a
null authentication token.
3.1.9.2. RETURN VALUES
PAM_ACCT_EXPIRED
User account has expired.
PAM_AUTH_ERR
Authentication failure.
PAM_NEW_AUTHTOK_REQD
The user account is valid but their authentication token is expired. The
correct response to this return-value is to require that the user satisfies
the pam_chauthtok() function before obtaining service. It may not be
possible for some applications to do this. In such cases, the user should
be denied access until such time as they can update their password.
PAM_PERM_DENIED
Permission denied.
PAM_SUCCESS
The authentication token was successfully updated.
PAM_USER_UNKNOWN
User unknown to password service.
3.1.10. Updating authentication tokens
#include <security/pam_appl.h>
int pam_chauthtok( pamh,
flags);
pam_handle_t * pamh;
int flags;
3.1.10.1. DESCRIPTION
The pam_chauthtok function is used to change the authentication token for a
given user (as indicated by the state associated with the handle pamh).
The pamh argument is an authentication handle obtained by a prior call to
pam_start(). The flags argument is the binary or of zero or more of the
following values:
PAM_SILENT
Do not emit any messages.
PAM_CHANGE_EXPIRED_AUTHTOK
This argument indicates to the modules that the users authentication token
(password) should only be changed if it has expired. If this argument is
not passed, the application requires that all authentication tokens are to
be changed.
3.1.10.2. RETURN VALUES
PAM_AUTHTOK_ERR
A module was unable to obtain the new authentication token.
PAM_AUTHTOK_RECOVERY_ERR
A module was unable to obtain the old authentication token.
PAM_AUTHTOK_LOCK_BUSY
One or more of the modules was unable to change the authentication token
since it is currently locked.
PAM_AUTHTOK_DISABLE_AGING
Authentication token aging has been disabled for at least one of the
modules.
PAM_PERM_DENIED
Permission denied.
PAM_SUCCESS
The authentication token was successfully updated.
PAM_TRY_AGAIN
Not all of the modules were in a position to update the authentication
token(s). In such a case none of the user's authentication tokens are
updated.
PAM_USER_UNKNOWN
User unknown to password service.
3.1.11. Start PAM session management
#include <security/pam_appl.h>
int pam_open_session( pamh,
flags);
pam_handle_t * pamh;
int flags;
3.1.11.1. DESCRIPTION
The pam_open_session function sets up a user session for a previously
successful authenticated user. The session should later be terminated with a
call to pam_close_session(3).
It should be noted that the effective uid, geteuid(2). of the application
should be of sufficient privilege to perform such tasks as creating or mounting
the user's home directory for example.
The flags argument is the binary or of zero or more of the following values:
PAM_SILENT
Do not emit any messages.
3.1.11.2. RETURN VALUES
PAM_ABORT
General failure.
PAM_BUF_ERR
Memory buffer error.
PAM_SESSION_ERR
Session failure.
PAM_SUCCESS
Session was successful created.
3.1.12. terminating PAM session management
#include <security/pam_appl.h>
int pam_close_session( pamh,
flags);
pam_handle_t * pamh;
int flags;
3.1.12.1. DESCRIPTION
The pam_close_session function is used to indicate that an authenticated
session has ended. The session should have been created with a call to
pam_open_session(3).
It should be noted that the effective uid, geteuid(2). of the application
should be of sufficient privilege to perform such tasks as unmounting the
user's home directory for example.
The flags argument is the binary or of zero or more of the following values:
PAM_SILENT
Do not emit any messages.
3.1.12.2. RETURN VALUES
PAM_ABORT
General failure.
PAM_BUF_ERR
Memory buffer error.
PAM_SESSION_ERR
Session failure.
PAM_SUCCESS
Session was successful terminated.
3.1.13. 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;
3.1.13.1. 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.
3.1.13.2. 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.
3.1.14. Get a PAM environment variable
#include <security/pam_appl.h>
const char *pam_getenv( pamh,
name);
pam_handle_t * pamh;
const char * name;
3.1.14.1. DESCRIPTION
The pam_getenv function searches the PAM environment list as associated with
the handle pamh for a string that matches the string pointed to by name. The
return values are of the form: "name=value".
3.1.14.2. RETURN VALUES
The pam_getenv function returns NULL on failure.
3.1.15. Getting the PAM environment
#include <security/pam_appl.h>
char **pam_getenvlist(pamh);
pam_handle_t *pamh;
3.1.15.1. 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.
3.1.15.2. RETURN VALUES
The pam_getenvlist function returns NULL on failure.
3.2. What is expected of an application
3.2.1. 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;
};
3.2.1.1. 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_meg read
only 'struct pam_message' pointers. Solaris' PAM implementation interprets this
argument as a pointer to a pointer to an array of num_meg 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])
3.2.1.2. RETURN VALUES
PAM_BUF_ERR
Memory buffer error.
PAM_CONV_ERR
Conversation failure. The application should not set *resp.
PAM_SUCCESS
Success.
3.3. Programming notes
Note, all of the authentication service function calls accept the token
PAM_SILENT, which instructs the modules to not send messages to the
application. This token can be logically OR'd with any one of the permitted
tokens specific to the individual function calls. PAM_SILENT does not override
the prompting of the user for passwords etc., it only stops informative
messages from being generated.
Chapter 4. Security issues of Linux-PAM
PAM, from the perspective of an application, is a convenient API for
authenticating users. PAM modules generally have no increased privilege over
that possessed by the application that is making use of it. For this reason,
the application must take ultimate responsibility for protecting the
environment in which PAM operates.
A poorly (or maliciously) written application can defeat any Linux-PAM module's
authentication mechanisms by simply ignoring it's return values. It is the
applications task and responsibility to grant privileges and access to
services. The Linux-PAM library simply assumes the responsibility of
authenticating the user; ascertaining that the user is who they say they are.
Care should be taken to anticipate all of the documented behavior of the
Linux-PAM library functions. A failure to do this will most certainly lead to a
future security breach.
4.1. Care about standard library calls
In general, writers of authorization-granting applications should assume that
each module is likely to call any or all 'libc' functions. For 'libc' functions
that return pointers to static/dynamically allocated structures (ie. the
library allocates the memory and the user is not expected to 'free()' it) any
module call to this function is likely to corrupt a pointer previously obtained
by the application. The application programmer should either re-call such a
'libc' function after a call to the Linux-PAM library, or copy the structure
contents to some safe area of memory before passing control to the Linux-PAM
library.
Two important function classes that fall into this category are getpwnam(3) and
syslog(3).
4.2. Choice of a service name
When picking the service-name that corresponds to the first entry in the
Linux-PAM configuration file, the application programmer should avoid the
temptation of choosing something related to argv[0]. It is a trivial matter for
any user to invoke any application on a system under a different name and this
should not be permitted to cause a security breach.
In general, this is always the right advice if the program is setuid, or
otherwise more privileged than the user that invokes it. In some cases,
avoiding this advice is convenient, but as an author of such an application,
you should consider well the ways in which your program will be installed and
used. (Its often the case that programs are not intended to be setuid, but end
up being installed that way for convenience. If your program falls into this
category, don't fall into the trap of making this mistake.)
To invoke some target application by another name, the user may symbolically
link the target application with the desired name. To be precise all the user
need do is, ln -s /target/application ./preferred_name and then run ./
preferred_name.
By studying the Linux-PAM configuration file(s), an attacker can choose the
preferred_name to be that of a service enjoying minimal protection; for example
a game which uses Linux-PAM to restrict access to certain hours of the day. If
the service-name were to be linked to the filename under which the service was
invoked, it is clear that the user is effectively in the position of dictating
which authentication scheme the service uses. Needless to say, this is not a
secure situation.
The conclusion is that the application developer should carefully define the
service-name of an application. The safest thing is to make it a single
hard-wired name.
4.3. The conversation function
Care should be taken to ensure that the conv() function is robust. Such a
function is provided in the library libpam_misc (see below).
4.4. The identity of the user
The Linux-PAM modules will need to determine the identity of the user who
requests a service, and the identity of the user who grants the service. These
two users will seldom be the same. Indeed there is generally a third user
identity to be considered, the new (assumed) identity of the user once the
service is granted.
The need for keeping tabs on these identities is clearly an issue of security.
One convention that is actively used by some modules is that the identity of
the user requesting a service should be the current UID (userid) of the running
process; the identity of the privilege granting user is the EUID (effective
userid) of the running process; the identity of the user, under whose name the
service will be executed, is given by the contents of the PAM_USER pam_get_item
(3). Note, modules can change the values of PAM_USER and PAM_RUSER during any
of the pam_*() library calls. For this reason, the application should take care
to use the pam_get_item() every time it wishes to establish who the
authenticated user is (or will currently be).
For network-serving databases and other applications that provide their own
security model (independent of the OS kernel) the above scheme is insufficient
to identify the requesting user.
A more portable solution to storing the identity of the requesting user is to
use the PAM_RUSER pam_get_item(3). The application should supply this value
before attempting to authenticate the user with pam_authenticate(). How well
this name can be trusted will ultimately be at the discretion of the local
administrator (who configures PAM for your application) and a selected module
may attempt to override the value where it can obtain more reliable data. If an
application is unable to determine the identity of the requesting entity/user,
it should not call pam_set_item(3) to set PAM_RUSER.
In addition to the PAM_RUSER item, the application should supply the PAM_RHOST
(requesting host) item. As a general rule, the following convention for its
value can be assumed: NULL = unknown; localhost = invoked directly from the
local system; other.place.xyz = some component of the user's connection
originates from this remote/requesting host. At present, PAM has no established
convention for indicating whether the application supports a trusted path to
communication from this host.
4.5. Sufficient resources
Care should be taken to ensure that the proper execution of an application is
not compromised by a lack of system resources. If an application is unable to
open sufficient files to perform its service, it should fail gracefully, or
request additional resources. Specifically, the quantities manipulated by the
setrlimit(2) family of commands should be taken into consideration.
This is also true of conversation prompts. The application should not accept
prompts of arbitrary length with out checking for resource allocation failure
and dealing with such extreme conditions gracefully and in a mannor that
preserves the PAM API. Such tolerance may be especially important when
attempting to track a malicious adversary.
Chapter 5. A library of miscellaneous helper functions
To aid the work of the application developer a library of miscellaneous
functions is provided. It is called libpam_miscy, and contains a text based
conversation function, and routines for enhancing the standard PAM-environment
variable support.
The functions, structures and macros, made available by this library can be
defined by including <security/pam_misc.h>. It should be noted that this
library is specific to Linux-PAM and is not referred to in the defining DCE-RFC
(see See also) below.
5.1. Functions supplied
5.1.1. Text based conversation function
#include <security/pam_misc.h>
void misc_conv( num_msg,
msgm,
response,
appdata_ptr);
int num_msg;
const struct pam_message ** msgm;
struct pam_response ** response;
void * appdata_ptr;
5.1.1.1. DESCRIPTION
The misc_conv function is part of libpam_misc and not of the standard libpam
library. This function will prompt the user with the appropriate comments and
obtain the appropriate inputs as directed by authentication modules.
In addition to simply slotting into the appropriate pam_conv(3), this function
provides some time-out facilities. The function exports five variables that can
be used by an application programmer to limit the amount of time this
conversation function will spend waiting for the user to type something. The
five variabls are as follows:
time_t pam_misc_conv_warn_time;
This variable contains the time (as returned by time(2)) that the user
should be first warned that the clock is ticking. By default it has the
value 0, which indicates that no such warning will be given. The
application may set its value to sometime in the future, but this should be
done prior to passing control to the Linux-PAM library.
const char *pam_misc_conv_warn_line;
Used in conjuction with pam_misc_conv_warn_time, this variable is a pointer
to the string that will be displayed when it becomes time to warn the user
that the timeout is approaching. Its default value is a translated version
of ?...Time is running out...?, but this can be changed by the application
prior to passing control to Linux-PAM.
time_t pam_misc_conv_die_time;
This variable contains the time (as returned by time(2)) that the will time
out. By default it has the value 0, which indicates that the conversation
function will not timeout. The application may set its value to sometime in
the future, but this should be done prior to passing control to the
Linux-PAM library.
const char *pam_misc_conv_die_line;
Used in conjuction with pam_misc_conv_die_time, this variable is a pointer
to the string that will be displayed when the conversation times out. Its
default value is a translated version of ?...Sorry, your time is up!?, but
this can be changed by the application prior to passing control to
Linux-PAM.
int pam_misc_conv_died;
Following a return from the Linux-PAM libraray, the value of this variable
indicates whether the conversation has timed out. A value of 1 indicates
the time-out occurred.
The following two function pointers are available for supporting binary prompts
in the conversation function. They are optimized for the current incarnation of
the libpamc library and are subject to change.
int (*pam_binary_handler_fn)(void *appdata, pamc_bp_t *prompt_p);
This function pointer is initialized to NULL but can be filled with a
function that provides machine-machine (hidden) message exchange. It is
intended for use with hidden authentication protocols such as RSA or
Diffie-Hellman key exchanges. (This is still under development.)
int (*pam_binary_handler_free)(void *appdata, pamc_bp_t *delete_me);
This function pointer is initialized to PAM_BP_RENEW(delete_me, 0, 0), but
can be redefined as desired by the application.
5.1.2. Transcribing an environment to that of PAM
#include <security/pam_misc.h>
int pam_misc_paste_env( pamh,
user);
pam_handle_t * pamh;
const char * const * user;
5.1.2.1. DESCRIPTION
This function takes the supplied list of environment pointers and uploads its
contents to the PAM environment. Success is indicated by PAM_SUCCESS.
5.1.3. Liberating a locally saved environment
#include <security/pam_misc.h>
int pam_misc_drop_env(env);
char **env;
5.1.3.1. DESCRIPTION
This function is defined to complement the pam_getenvlist(3) function. It
liberates the memory associated with env, overwriting with 0 all memory before
free()ing it.
5.1.4. BSD like PAM environment variable setting
#include <security/pam_misc.h>
int pam_misc_setenv( pamh,
name,
value,
readonly);
pam_handle_t * pamh;
const char * name;
const char * value;
int readonly;
5.1.4.1. DESCRIPTION
This function performs a task equivalent to pam_putenv(3), its syntax is,
however, more like the BSD style function; setenv(). The name and value are
concatenated with an '=' to form a name=value and passed to pam_putenv(). If,
however, the PAM variable is already set, the replacement will only be applied
if the last argument, readonly, is zero.
Chapter 6. Porting legacy applications
The point of PAM is that the application is not supposed to have any idea how
the attached authentication modules will choose to authenticate the user. So
all they can do is provide a conversation function that will talk directly to
the user(client) on the modules' behalf.
Consider the case that you plug a retinal scanner into the login program. In
this situation the user would be prompted: "please look into the scanner". No
username or password would be needed - all this information could be deduced
from the scan and a database lookup. The point is that the retinal scanner is
an ideal task for a "module".
While it is true that a pop-daemon program is designed with the POP protocol in
mind and no-one ever considered attaching a retinal scanner to it, it is also
the case that the "clean" PAM'ification of such a daemon would allow for the
possibility of a scanner module being be attached to it. The point being that
the "standard" pop-authentication protocol(s) [which will be needed to satisfy
inflexible/legacy clients] would be supported by inserting an appropriate
pam_qpopper module(s). However, having rewritten popd once in this way any new
protocols can be implemented in-situ.
One simple test of a ported application would be to insert the pam_permit
module and see if the application demands you type a password... In such a
case, xlock would fail to lock the terminal - or would at best be a
screen-saver, ftp would give password free access to all etc.. Neither of these
is a very secure thing to do, but they do illustrate how much flexibility PAM
puts in the hands of the local admin.
The key issue, in doing things correctly, is identifying what is part of the
authentication procedure (how many passwords etc..) the exchange protocol
(prefixes to prompts etc., numbers like 331 in the case of ftpd) and what is
part of the service that the application delivers. PAM really needs to have
total control in the authentication "procedure", the conversation function
should only deal with reformatting user prompts and extracting responses from
raw input.
Chapter 7. Glossary of PAM related terms
The following are a list of terms used within this document.
Authentication token
Generally, this is a password. However, a user can authenticate him/herself
in a variety of ways. Updating the user's authentication token thus
corresponds to refreshing the object they use to authenticate themself with
the system. The word password is avoided to keep open the possibility that
the authentication involves a retinal scan or other non-textual mode of
challenge/response.
Credentials
Having successfully authenticated the user, PAM is able to establish
certain characteristics/attributes of the user. These are termed
credentials. Examples of which are group memberships to perform privileged
tasks with, and tickets in the form of environment variables etc. . Some
user-credentials, such as the user's UID and GID (plus default group
memberships) are not deemed to be PAM-credentials. It is the responsibility
of the application to grant these directly.
Chapter 8. An example application
To get a flavor of the way a Linux-PAM application is written we include the
following example. It prompts the user for their password and indicates whether
their account is valid on the standard output, its return code also indicates
the success (0 for success; 1 for failure).
/*
This program was contributed by Shane Watts
[modifications by AGM and kukuk]
You need to add the following (or equivalent) to the
/etc/pam.d/check_user file:
# check authorization
auth required pam_unix.so
account required pam_unix.so
*/
#include <security/pam_appl.h>
#include <security/pam_misc.h>
#include <stdio.h>
static struct pam_conv conv = {
misc_conv,
NULL
};
int main(int argc, char *argv[])
{
pam_handle_t *pamh=NULL;
int retval;
const char *user="nobody";
if(argc == 2) {
user = argv[1];
}
if(argc > 2) {
fprintf(stderr, "Usage: check_user [username]\n");
exit(1);
}
retval = pam_start("check_user", user, &conv, &pamh);
if (retval == PAM_SUCCESS)
retval = pam_authenticate(pamh, 0); /* is user really user? */
if (retval == PAM_SUCCESS)
retval = pam_acct_mgmt(pamh, 0); /* permitted access? */
/* This is where we have been authorized or not. */
if (retval == PAM_SUCCESS) {
fprintf(stdout, "Authenticated\n");
} else {
fprintf(stdout, "Not Authenticated\n");
}
if (pam_end(pamh,retval) != PAM_SUCCESS) { /* close Linux-PAM */
pamh = NULL;
fprintf(stderr, "check_user: failed to release authenticator\n");
exit(1);
}
return ( retval == PAM_SUCCESS ? 0:1 ); /* indicate success */
}
Chapter 9. Files
/usr/include/security/pam_appl.h
Header file with interfaces for Linux-PAM applications.
/usr/include/security/pam_misc.h
Header file for useful library functions for making applications easier to
write.
Chapter 10. See also
* The Linux-PAM System Administrators' Guide.
* The Linux-PAM Module Writers' 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 11. 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 12. 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
