The Linux-PAM Module Writers' Guide
Andrew G. Morgan, morgan@kernel.org
DRAFT v0.75 2001/02/21
This manual documents what a programmer needs to know in order to
write a module that conforms to the [1mLinux-PAM [22mstandard. It also dis-
cusses some security issues from the point of view of the module pro-
grammer.
______________________________________________________________________
Table of Contents
1. Introduction
1.1 Synopsis
1.2 Description
2. What can be expected by the module
2.1 Getting and setting PAM_ITEMs and data
2.1.1 Setting data
2.1.2 Getting data
2.1.3 Setting items
2.1.4 Getting items
2.1.5 The conversation mechanism
2.1.6 Getting the name of a user
2.1.7 Setting a Linux-PAM environment variable
2.1.8 Getting a Linux-PAM environment variable
2.1.9 Listing the Linux-PAM environment
2.2 Other functions provided by libpam
2.2.1 Understanding errors
2.2.2 Planning for delays
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
3.3 Account management
3.4 Session management
3.5 Password management
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
5.4 Added requirements for statically loaded modules.
6. An example module file
7. Files
8. See also
9. Notes
10. Author/acknowledgments
11. Bugs/omissions
12. Copyright information for this document
______________________________________________________________________
[1m1. Introduction[0m
[1m1.1. Synopsis[0m
#include <security/pam_modules.h>
gcc -fPIC -c pam_module-name.c
ld -x --shared -o pam_module-name.so pam_module-name.o
[1m1.2. Description[0m
[1mLinux-PAM [22m(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 [1mLinux-PAM[0m
library see the [1mLinux-PAM [22mSystem Administrators' Guide.
A [1mLinux-PAM [22mmodule is a single executable binary file that can be
loaded by the [1mLinux-PAM [22minterface 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
/usr/lib/security and take the form of dynamically loadable object
files (see dlopen(3)). Alternatively, the modules can be statically
linked into the [1mLinux-PAM [22mlibrary; this is mostly to allow [1mLinux-PAM[0m
to be used on platforms without dynamic linking available, but the two
forms can be used together. It is the [1mLinux-PAM [22minterface that is
called by an application and it is the responsibility of the library
to locate, load and call the appropriate functions in a [1mLinux-PAM[22m-
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.
[1m2. What can be expected by the module[0m
Here we list the interface that the conventions that all [1mLinux-PAM[0m
modules must adhere to.
[1m2.1. Getting and setting PAM_ITEM s and data[0m
First, we cover what the module should expect from the [1mLinux-PAM[0m
library and a [1mLinux-PAM [4m[22maware[24m application. Essesntially this is the
libpam.* library.
[1m2.1.1. Setting data[0m
Synopsis:
extern int pam_set_data(pam_handle_t *pamh,
const char *module_data_name,
void *data,
void (*cleanup)(pam_handle_t *pamh,
void *data, int error_status) );
The modules may be dynamically loadable objects. In general such files
should not contain static variables. This and the subsequent function
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 [4mnot[24m by an
application.
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() (see the Linux-PAM Application Developers' Guide).
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() is called by
the module, the error_status carries the return value of the
pam_authenticate() or other libpam function as appropriate. Based on
this value the Kerberos module may choose to delete the ticket file
([4mauthentication[24m [4mfailure[24m) 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().
PAM_DATA_SILENT
Which indicates that the process would prefer to perform the
cleanup() quietly. That is, discourages logging/messages to the
user.
[1m2.1.2. Getting data[0m
Synopsis:
extern int pam_get_data(const pam_handle_t *pamh,
const char *module_data_name,
const void **data);
This function together with the previous one provides a method of
associating module-specific data with the handle pamh. A successful
call to pam_get_data will result in *data pointing to the data
associated with the module_data_name. Note, this data is [4mnot[24m a copy
and should be treated as [4mconstant[24m by the module.
Note, if there is an entry but it has the value NULL, then this call
returns PAM_NO_MODULE_DATA.
[1m2.1.3. Setting items[0m
Synopsis:
extern int pam_set_item(pam_handle_t *pamh,
int item_type,
const void *item);
This function is used to (re)set the value of one of the item_types.
The reader is urged to read the entry for this function in the [1mLinux-[0m
[1mPAM [22mapplication developers' manual.
In addition to the items listed there, the module can set the
following two item_types:
PAM_AUTHTOK
The authentication token (often a password). This token should
be ignored by all module functions besides pam_sm_authenticate()
and pam_sm_chauthtok(). 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().
Both of these items are reset before returning to the application.
When resetting these items, the [1mLinux-PAM [22mlibrary first writes 0's to
the current tokens and then free()'s the associated memory.
The return values for this function are listed in the [1mLinux-PAM[0m
Application Developers' Guide.
[1m2.1.4. Getting items[0m
Synopsis:
extern int pam_get_item(const pam_handle_t *pamh,
int item_type,
const void **item);
This function is used to obtain the value of the specified item_type.
It is better documented in the [1mLinux-PAM [22mApplication Developers'
Guide. However, there are three things worth stressing here:
+o Generally, if the module wishes to obtain the name of the user, it
should not use this function, but instead perform a call to
pam_get_user() (see section ``below'').
+o The module is additionally privileged to read the authentication
tokens, PAM_AUTHTOK and PAM_OLDAUTHTOK (see the section above on
pam_set_data()).
+o The module should [4mnot[24m free() or alter the data pointed to by *item
after a successful return from pam_get_item(). This pointer points
directly at the data contained within the *pamh structure. Should
a module require that a change is made to the this ITEM it should
make the appropriate call to pam_set_item().
[1m2.1.5. The conversation mechanism[0m
Following the call pam_get_item(pamh,PAM_CONV,&item), the pointer item
points to a [4mconversation[24m-function that provides limited but direct
access to the application. The purpose of this function is to allow
the module to prompt the user for their password and pass other
information in a manner consistent with the application. For example,
an X-windows based program might pop up a dialog box to report a login
failure. Just as the application should not be concerned with the
method of authentication, so the module should not dictate the manner
in which input (output) is obtained from (presented to) to the user.
The reader is strongly urged to read the more complete description of
the pam_conv structure, written from the perspective of the
application developer, in the [1mLinux-PAM [22mApplication Developers' Guide.
The pam_response structure returned after a call to the pam_conv
function must be free()'d by the module. Since the call to the
conversation function originates from the module, it is clear that
either this pam_response structure could be either statically or
dynamically (using malloc() etc.) allocated within the application.
Repeated calls to the conversation function would likely overwrite
static memory, so it is required that for a successful return from the
conversation function the memory for the response structure is
dynamically allocated by the application with one of the malloc()
family of commands and [4mmust[24m be free()'d by the module.
If the pam_conv mechanism is used to enter authentication tokens, the
module should either pass the result to the pam_set_item() library
function, or copy it itself. In such a case, once the token has been
stored (by one of these methods or another one), the memory returned
by the application should be overwritten with 0's, and then free()'d.
The return values for this function are listed in the [1mLinux-PAM[0m
Application Developers' Guide.
[1m2.1.6. Getting the name of a user[0m
Synopsis:
extern int pam_get_user(pam_handle_t *pamh,
const char **user,
const char *prompt);
This is a [1mLinux-PAM [22mlibrary function that returns the (prospective)
name of the user. To determine the username it does the following
things, in this order:
+o checks what pam_get_item(pamh, PAM_USER, ... ); would have
returned. If this is not NULL this is what it returns. Otherwise,
+o obtains a username from the application via the pam_conv mechanism,
it prompts the user with the first non-NULL string in the following
list:
+o The prompt argument passed to the function
+o What is returned by pam_get_item(pamh,PAM_USER_PROMPT, ... );
+o The default prompt: ``Please enter username: ''
By whatever means the username is obtained, a pointer to it is
returned as the contents of *user. Note, this memory should [4mnot[24m be
free()'d by the module. Instead, it will be liberated on the next call
to pam_get_user(), or by pam_end() when the application ends its
interaction with [1mLinux-PAM[22m.
Also, in addition, it should be noted that this function sets the
PAM_USER item that is associated with the pam_[gs]et_item() function.
The return value of this function is one of the following:
+o PAM_SUCCESS - username obtained.
+o PAM_CONV_AGAIN - converstation did not complete and the caller is
required to return control to the application, until such time as
the application has completed the conversation process. A module
calling pam_get_user() that obtains this return code, should return
PAM_INCOMPLETE and be prepared (when invoked the next time) to
recall pam_get_user() to fill in the user's name, and then pick up
where it left off as if nothing had happened. This procedure is
needed to support an event-driven application programming model.
+o PAM_CONV_ERR - the conversation method supplied by the application
failed to obtain the username.
[1m2.1.7. Setting a Linux-PAM environment variable[0m
Synopsis:
extern int pam_putenv(pam_handle_t *pamh, const char *name_value);
[1mLinux-PAM [22mcomes equipped with a series of functions for maintaining a
set of [4menvironment[24m variables. The environment is initialized by the
call to pam_start() and is [1merased [22mwith a call to pam_end(). This
[4menvironment[24m is associated with the pam_handle_t pointer returned by
the former call.
The default environment is all but empty. It contains a single NULL
pointer, which is always required to terminate the variable-list. The
pam_putenv() function can be used to add a new environment variable,
replace an existing one, or delete an old one.
+o Adding/replacing a variable
To add or overwrite a [1mLinux-PAM [22menvironment variable the value of
the argument name_value, should be of the following form:
name_value="VARIABLE=VALUE OF VARIABLE"
Here, VARIABLE is the environment variable's name and what follows the
`=' is its (new) value. (Note, that "VARIABLE=" is a valid value for
name_value, indicating that the variable is set to "".)
+o Deleting a variable
To delete a [1mLinux-PAM [22menvironment variable the value of the
argument name_value, should be of the following form:
name_value="VARIABLE"
Here, VARIABLE is the environment variable's name and the absence of
an `=' indicates that the variable should be removed.
In all cases PAM_SUCCESS indicates success.
[1m2.1.8. Getting a Linux-PAM environment variable[0m
Synopsis:
extern const char *pam_getenv(pam_handle_t *pamh, const char *name);
This function can be used to return the value of the given variable.
If the returned value is NULL, the variable is not known.
[1m2.1.9. Listing the Linux-PAM environment[0m
Synopsis:
extern char * const *pam_getenvlist(pam_handle_t *pamh);
This function returns a pointer to the entire [1mLinux-PAM [22menvironment
array. At first sight the [4mtype[24m of the returned data may appear a
little confusing. It is basically a [4mread-only[24m array of character
pointers, that lists the NULL terminated list of environment variables
set so far.
Although, this is not a concern for the module programmer, we mention
here that an application should be careful to copy this entire array
before executing pam_end() otherwise all the variable information will
be lost. (There are functions in libpam_misc for this purpose:
pam_misc_copy_env() and pam_misc_drop_env().)
[1m2.2. Other functions provided by libpam[0m
[1m2.2.1. Understanding errors[0m
+o extern const char *pam_strerror(pam_handle_t *pamh, int errnum);
This function returns some text describing the [1mLinux-PAM [22merror
associated with the argument errnum. If the error is not
recognized ``Unknown Linux-PAM error'' is returned.
[1m2.2.2. Planning for delays[0m
+o extern int pam_fail_delay(pam_handle_t *pamh, unsigned int
micro_sec)
This function is offered by [1mLinux-PAM [22mto facilitate time delays
following a failed call to pam_authenticate() and before control is
returned to the application. When using this function the module
programmer should check if it is available with,
#ifdef PAM_FAIL_DELAY
....
#endif /* PAM_FAIL_DELAY */
Generally, an application requests that a user is authenticated by
[1mLinux-PAM [22mthrough a call to pam_authenticate() or pam_chauthtok().
These functions call each of the [4mstacked[24m authentication modules listed
in the [1mLinux-PAM [22mconfiguration file. As directed by this file, one of
more of the modules may fail causing the pam_...() call to return an
error. It is desirable for there to also be a pause before the
application continues. The principal reason for such a delay is
security: a delay acts to discourage [4mbrute[24m [4mforce[24m dictionary attacks
primarily, but also helps hinder [4mtimed[24m (cf. covert channel) attacks.
The pam_fail_delay() function provides the mechanism by which an
application or module can suggest a minimum delay (of micro_sec [4mmicro-[0m
[4mseconds[24m). [1mLinux-PAM [22mkeeps a record of the longest time requested with
this function. Should pam_authenticate() 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 [1mLinux-PAM [22mreturns control to the application.
[1m3. What is expected of a module[0m
The module must supply a sub-set of the six functions listed below.
Together they define the function of a [1mLinux-PAM module[22m. Module
developers are strongly urged to read the comments on security that
follow this list.
[1m3.1. Overview[0m
The six module functions are grouped into four independent management
groups. These groups are as follows: [4mauthentication[24m, [4maccount[24m, [4msession[0m
and [4mpassword[24m. 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 [4mall[24m four groups.
[1m3.1.1. Functional independence[0m
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 [1mLinux-PAM [22mauthenticate 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
[4mKERBEROS[24m 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.
[1m3.1.2. Minimizing administration problems[0m
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.
[1m3.1.3. Arguments supplied to the module[0m
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 [4mtext[24m (errors or warnings) to the application.
The argc and argv arguments are taken from the line appropriate to
this module---that is, with the [4mservice_name[24m matching that of the
application---in the configuration file (see the [1mLinux-PAM [22mSystem
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 [1mnot [22mthe name of the module.
[1m3.2. Authentication management[0m
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.
+o 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.
+o 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 [4mafter[24m 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 [1mLinux-PAM-0.75[22m, 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
[4mauthentication[24m 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 [1mLinux-PAM-0.75 [22mand 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 [4mfailing[24m. The first such error will dominate in the
return value of pam_setcred().
[1m3.3. Account management[0m
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.
+o 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.
[1m3.4. Session management[0m
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).
+o 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.
+o 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.
[1m3.5. Password management[0m
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.
+o 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 [4mmust[24m be combined with one of
the following two flags. Note, however, the following two
options are [4mmutually[24m [4mexclusive[24m.
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 [1mLinux-PAM [22mlibrary 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_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_USER_UNKNOWN
The user is not known to the authentication token changing
service.
[1m4. Generic optional arguments[0m
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 syslog(3) call to log debugging information to the
system log files.
no_warn
Instruct module to not give warning messages to the application.
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).
try_first_pass
The module should attempt authentication with the previously
typed password (by a call to pam_get_item() for the PAM_AUTHTOK
item). If that doesn't work, then the user is prompted for a
password. (This option is intended for auth modules only).
use_mapped_pass
[1mWARNING: [22mcoding this functionality may cause the module writer
to break [4mlocal[24m encryption laws. For example, in the U.S. there
are restrictions on the export computer code that is capable of
strong encryption. It has not been established whether this
option is affected by this law, but one might reasonably assume
that it does until told otherwise. For this reason, this option
is not supported by any of the modules distributed with [1mLinux-[0m
[1mPAM[22m.
The intended function of this argument, however, is that the
module should take the existing authentication token from a
previously invoked module and use it as a key to retrieve the
authentication token for this module. For example, the module
might create a strong hash of the PAM_AUTHTOK item (established
by a previously executed module). Then, with logical-exclusive-
or, use the result as a [4mkey[24m to safely store/retrieve the
authentication token for this module in/from a local file [4metc[24m. .
expose_account
In general the leakage of some information about user accounts
is not a secure policy for modules to adopt. Sometimes
information such as users names or home directories, or
preferred shell, can be used to attack a user's account. In some
circumstances, however, this sort of information is not deemed a
threat: displaying a user's full name when asking them for a
password in a secured environment could also be called being
'friendly'. The expose_account argument is a standard module
argument to encourage a module to be less discrete about account
information as it is deemed appropriate by the local
administrator.
[1m5. Programming notes[0m
Here we collect some pointers for the module writer to bear in mind
when writing/developing a [1mLinux-PAM [22mcompatible module.
[1m5.1. Security issues for module creation[0m
[1m5.1.1. Sufficient resources[0m
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.
[1m5.1.2. Who's who?[0m
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 [4msu[24m program: user joe executes [4msu[24m to become the
user [4mjane[24m. 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.
+o X, the identity of the user invoking the service request. This is
the user identifier; returned by the function getuid(2).
+o Y, the privileged identity of the application used to grant the
requested service. This is the [4meffective[24m user identifier; returned
by the function geteuid(2).
+o Z, the user under whose identity the service will be granted. This
is the username returned by pam_get_user(2) and also stored in the
[1mLinux-PAM [22mitem, PAM_USER.
+o [1mLinux-PAM [22mhas 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 [1mLinux-PAM [22mlibrary.
[1m5.1.3. Using the conversation function[0m
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.
[1m5.1.4. Authentication tokens[0m
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 [1mLinux-PAM [22mlibrary 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;
}
[1m5.2. Use of syslog(3)[0m
Only rarely should error information be directed to the user. Usually,
this is to be limited to ``[4msorry[24m [4myou[24m [4mcannot[24m [4mlogin[24m [4mnow[24m'' 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 [4mfacility-type[0m
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:
+o 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 [1mrequired [22mto be logged at the LOG_ERR level.
+o Debugging information, as activated by the debug argument to the
module in /etc/pam.conf, should be logged at the LOG_DEBUG level.
+o 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.
+o Shortages of system resources, such as a failure to manipulate a
file or malloc() failures should be logged at level LOG_CRIT.
+o Authentication failures, associated with an incorrectly typed
password should be logged at level, LOG_NOTICE.
[1m5.3. Modules that require system libraries[0m
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 -l[4mXXX[24m arguments for the desired
libraries when you create the shared module object. To make sure a
module is linked to the lib[4mwhatever[24m.so library when it is dlopen()ed,
try:
% gcc -shared -Xlinker -x -o pam_module.so pam_module.o -lwhatever
[1m5.4. Added requirements for statically loaded modules.[0m
Modules may be statically linked into libpam. This should be true of
all the modules distributed with the basic [1mLinux-PAM [22mdistribution. To
be statically linked, a module needs to export information about the
functions it contains in a manner that does not clash with other
modules.
The extra code necessary to build a static module should be delimited
with #ifdef PAM_STATIC and #endif. The static code should do the
following:
+o Define a single structure, struct pam_module, called
_pam_[4mmodname[24m_modstruct, where [4mmodname[24m is the name of the module [1mas[0m
[1mused in the filesystem [22mbut without the leading directory name
(generally /usr/lib/security/ or the suffix (generally .so).
As a simple example, consider the following module code which defines
a module that can be compiled to be [4mstatic[24m or [4mdynamic[24m:
#include <stdio.h> /* for NULL define */
#define PAM_SM_PASSWORD /* the only pam_sm_... function declared */
#include <security/pam_modules.h>
PAM_EXTERN int pam_sm_chauthtok(pam_handle_t *pamh, int flags,
int argc, const char **argv)
{
return PAM_SUCCESS;
}
#ifdef PAM_STATIC /* for the case that this module is static */
struct pam_module _pam_modname_modstruct = { /* static module data */
"pam_modname",
NULL,
NULL,
NULL,
NULL,
NULL,
pam_sm_chauthtok,
};
#endif /* end PAM_STATIC */
To be linked with [4mlibpam[24m, staticly-linked modules must be built from
within the Linux-PAM-X.YY/modules/ subdirectory of the [1mLinux-PAM[0m
source directory as part of a normal build of the [1mLinux-PAM [22msystem.
The [4mMakefile[24m, for the module in question, must execute the
register_static shell script that is located in the Linux-PAM-
X.YY/modules/ subdirectory. This is to ensure that the module is
properly registered with [4mlibpam[24m.
The [1mtwo [22mmanditory arguments to register_static are the title, and the
pathname of the object file containing the module's code. The pathname
is specified relative to the Linux-PAM-X.YY/modules directory. The
pathname may be an empty string---this is for the case that a single
object file needs to register more than one struct pam_module. In such
a case, exactly one call to register_static must indicate the object
file.
Here is an example; a line in the [4mMakefile[24m might look like this:
register:
ifdef STATIC
(cd ..; ./register_static pam_modname pam_modname/pam_modname.o)
endif
For some further examples, see the modules subdirectory of the current
[1mLinux-PAM [22mdistribution.
[1m6. An example module file[0m
At some point, we may include a fully commented example of a module in
this document. For now, we point the reader to these two locations in
the public CVS repository:
+o A module that always succeeds: http://cvs.sourceforge.net/cgi-
bin/cvsweb.cgi/Linux-PAM/modules/pam_permit/?cvsroot=pam
+o A module that always fails: http://cvs.sourceforge.net/cgi-
bin/cvsweb.cgi/Linux-PAM/modules/pam_deny/?cvsroot=pam
[1m7. Files[0m
/usr/lib/libpam.so.*
the shared library providing applications with access to [1mLinux-[0m
[1mPAM[22m.
/etc/pam.conf
the [1mLinux-PAM [22mconfiguration file.
/usr/lib/security/pam_*.so
the primary location for [1mLinux-PAM [22mdynamically loadable object
files; the modules.
[1m8. See also[0m
+o The [1mLinux-PAM [22mSystem Administrators' Guide.
+o The [1mLinux-PAM [22mApplication Writers' Guide.
+o V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH PLUGGABLE
AUTHENTICATION MODULES'', Open Software Foundation Request For
Comments 86.0, October 1995.
[1m9. Notes[0m
I intend to put development comments here... like ``at the moment this
isn't actually supported''. At release time what ever is in this
section will be placed in the Bugs section below! :)
+o Perhaps we should keep a registry of data-names as used by
pam_[gs]et_data() so there are no unintentional problems due to
conflicts?
+o pam_strerror() should be internationalized....
+o There has been some debate about whether initgroups() should be in
an application or in a module. It was settled by Sun who stated
that initgroups is an action of the [4mapplication[24m. The modules are
permitted to add additional groups, however.
+o Refinements/futher suggestions to syslog(3) usage by modules are
needed.
[1m10. Author/acknowledgments[0m
This document was written by Andrew G. Morgan (morgan@transmeta.com)
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, 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
[1mLinux-PAM[22m, Sun graciously made the documentation for their
implementation of PAM available. This act greatly accelerated the
development of [1mLinux-PAM[22m.
[1m11. Bugs/omissions[0m
Few PAM modules currently exist. Few PAM-aware applications exist.
This document is hopelessly unfinished. Only a partial list of people
is credited for all the good work they have done.
[1m12. Copyright information for this document[0m
Copyright (c) Andrew G. Morgan 1996, 1997. All rights reserved.
Email: <morgan@transmeta.com>
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
+o 1. Redistributions of source code must retain the above copyright
notice, and the entire permission notice in its entirety, including
the disclaimer of warranties.
+o 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.
+o 3. The name of the author may not be used to endorse or promote
products derived from this software without specific prior written
permission.
[1mAlternatively[22m, 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 [1minstead of [22mthe 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
DAMAGE.
$Id: pam_modules.sgml,v 1.6 2001/02/22 04:58:51 agmorgan Exp $
