The Linux-PAM Module Writers' Guide Andrew G. Morgan, morgan@kernel.org DRAFT v0.76 2002/05/09 This manual documents what a programmer needs to know in order to write a module that conforms to the Linux-PAM standard. 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 ______________________________________________________________________ 1. Introduction 1.1. Synopsis #include gcc -fPIC -c pam_module-name.c ld -x --shared -o pam_module-name.so pam_module-name.o 1.2. Description Linux-PAM (Pluggable Authentication Modules for Linux) is a library that enables the local system administrator to choose how individual applications authenticate users. For an overview of the Linux-PAM library see the Linux-PAM System Administrators' Guide. A Linux-PAM module is a single executable binary file that can be loaded by the Linux-PAM interface library. This PAM library is configured locally with a system file, /etc/pam.conf, to authenticate a user request via the locally available authentication modules. The modules themselves will usually be located in the directory /usr/lib/security and take the form of dynamically loadable object files (see dlopen(3)). Alternatively, the modules can be statically linked into the Linux-PAM library; this is mostly to allow Linux-PAM to be used on platforms without dynamic linking available, but the two forms can be used together. It is the Linux-PAM interface that is called by an application and it is the responsibility of the library to locate, load and call the appropriate functions in a Linux-PAM- module. Except for the immediate purpose of interacting with the user (entering a password etc..) the module should never call the application directly. This exception requires a "conversation mechanism" which is documented below. 2. What can be expected by the module Here we list the interface that the conventions that all Linux-PAM modules must adhere to. 2.1. Getting and setting PAM_ITEM s and data First, we cover what the module should expect from the Linux-PAM library and a Linux-PAM aware application. Essesntially this is the libpam.* library. 2.1.1. Setting data 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 not 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 (authentication failure) or leave it in place. The error_status may have been logically OR'd with either of the following two values: PAM_DATA_REPLACE When a data item is being replaced (through a second call to pam_set_data()) this mask is used. Otherwise, the call is assumed to be from pam_end(). PAM_DATA_SILENT Which indicates that the process would prefer to perform the cleanup() quietly. That is, discourages logging/messages to the user. 2.1.2. Getting data 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 not a copy and should be treated as constant by the module. Note, if there is an entry but it has the value NULL, then this call returns PAM_NO_MODULE_DATA. 2.1.3. Setting items 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 Linux- PAM application 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 Linux-PAM library 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 Linux-PAM Application Developers' Guide. 2.1.4. Getting items 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 Linux-PAM Application 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 not 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(). 2.1.5. The conversation mechanism Following the call pam_get_item(pamh,PAM_CONV,&item), the pointer item points to a structure containing an a pointer to a conversation- 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 Linux-PAM Application Developers' Guide. The return values for this function are listed in the Linux-PAM Application 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 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 must 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. There is a handy macro _pam_drop_reply() to be found in that can be used to conveniently cleanup a pam_response structure. (Note, this include file is specific to the Linux-PAM sources, and whilst it will work with Sun derived PAM implementations, it is not generally distributed by Sun.) 2.1.6. Getting the name of a user Synopsis: extern int pam_get_user(pam_handle_t *pamh, const char **user, const char *prompt); This is a Linux-PAM library 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 not 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 Linux-PAM. 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. 2.1.7. Setting a Linux-PAM environment variable Synopsis: extern int pam_putenv(pam_handle_t *pamh, const char *name_value); Linux-PAM comes equipped with a series of functions for maintaining a set of environment variables. The environment is initialized by the call to pam_start() and is erased with a call to pam_end(). This environment 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 Linux-PAM environment 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 Linux-PAM environment 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. 2.1.8. Getting a Linux-PAM environment variable 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. 2.1.9. Listing the Linux-PAM environment Synopsis: extern char * const *pam_getenvlist(pam_handle_t *pamh); This function returns a pointer to the entire Linux-PAM environment array. At first sight the type of the returned data may appear a little confusing. It is basically a read-only 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().) 2.2. Other functions provided by libpam 2.2.1. Understanding errors +o extern const char *pam_strerror(pam_handle_t *pamh, int errnum); This function returns some text describing the Linux-PAM error associated with the argument errnum. If the error is not recognized ``Unknown Linux-PAM error'' is returned. 2.2.2. Planning for delays +o extern int pam_fail_delay(pam_handle_t *pamh, unsigned int micro_sec) This function is offered by Linux-PAM to 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 Linux-PAM through a call to pam_authenticate() or pam_chauthtok(). These functions call each of the stacked authentication modules listed in the Linux-PAM configuration 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 brute force dictionary attacks primarily, but also helps hinder timed (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 micro- seconds). Linux-PAM keeps 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 Linux-PAM returns control to the application. 3. What is expected of a module The module must supply a sub-set of the six functions listed below. Together they define the function of a Linux-PAM module. Module developers are strongly urged to read the comments on security that follow this list. 3.1. Overview The six module functions are grouped into four independent management groups. These groups are as follows: authentication, account, session and password. To be properly defined, a module must define all functions within at least one of these groups. A single module may contain the necessary functions for all four groups. 3.1.1. Functional independence The independence of the four groups of service a module can offer means that the module should allow for the possibility that any one of these four services may legitimately be called in any order. Thus, the module writer should consider the appropriateness of performing a service without the prior success of some other part of the module. As an informative example, consider the possibility that an application applies to change a user's authentication token, without having first requested that Linux-PAM authenticate the user. In some cases this may be deemed appropriate: when root wants to change the authentication token of some lesser user. In other cases it may not be appropriate: when joe maliciously wants to reset alice's password; or when anyone other than the user themself wishes to reset their KERBEROS authentication token. A policy for this action should be defined by any reasonable authentication scheme, the module writer should consider this when implementing a given module. 3.1.2. Minimizing administration problems To avoid system administration problems and the poor construction of a /etc/pam.conf file, the module developer may define all six of the following functions. For those functions that would not be called, the module should return PAM_SERVICE_ERR and write an appropriate message to the system log. When this action is deemed inappropriate, the function would simply return PAM_IGNORE. 3.1.3. Arguments supplied to the module The flags argument of each of the following functions can be logically OR'd with PAM_SILENT, which is used to inform the module to not pass any text (errors or warnings) to the application. The argc and argv arguments are taken from the line appropriate to this module---that is, with the service_name matching that of the application---in the configuration file (see the Linux-PAM System Administrators' Guide). Together these two parameters provide the number of arguments and an array of pointers to the individual argument tokens. This will be familiar to C programmers as the ubiquitous method of passing command arguments to the function main(). Note, however, that the first argument (argv[0]) is a true argument and not the name of the module. 3.2. Authentication management To be correctly initialized, PAM_SM_AUTH must be #define'd prior to including . 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 after the user has been authenticated but before a session has been established. Permitted flags, one of which, may be logically OR'd with PAM_SILENT are, PAM_ESTABLISH_CRED Set the credentials for the authentication service, PAM_DELETE_CRED Delete the credentials associated with the authentication service, PAM_REINITIALIZE_CRED Reinitialize the user credentials, and PAM_REFRESH_CRED Extend the lifetime of the user credentials. Prior to Linux-PAM-0.75, and due to a deficiency with the way the auth stack was handled in the case of the setcred stack being processed, the module was required to attempt to return the same error code as pam_sm_authenticate did. This was necessary to preserve the logic followed by libpam as it executes the stack of authentication modules, when the application called either pam_authenticate() or pam_setcred(). Failing to do this, led to confusion on the part of the System Administrator. For Linux-PAM-0.75 and later, libpam handles the credential stack much more sanely. The way the auth stack is navigated in order to evaluate the pam_setcred() function call, independent of the pam_sm_setcred() return codes, is exactly the same way that it was navigated when evaluating the pam_authenticate() library call. Typically, if a stack entry was ignored in evaluating pam_authenticate(), it will be ignored when libpam evaluates the pam_setcred() function call. Otherwise, the return codes from each module specific pam_sm_setcred() call are treated as required. Besides PAM_SUCCESS, the module may return one of the following errors: PAM_CRED_UNAVAIL This module cannot retrieve the user's credentials. PAM_CRED_EXPIRED The user's credentials have expired. PAM_USER_UNKNOWN The user is not known to this authentication module. PAM_CRED_ERR This module was unable to set the credentials of the user. these, non-PAM_SUCCESS, return values will typically lead to the credential stack failing. The first such error will dominate in the return value of pam_setcred(). 3.3. Account management To be correctly initialized, PAM_SM_ACCOUNT must be #define'd prior to including . 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. 3.4. Session management To be correctly initialized, PAM_SM_SESSION must be #define'd prior to including . 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. 3.5. Password management To be correctly initialized, PAM_SM_PASSWORD must be #define'd prior to including . 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 must be combined with one of the following two flags. Note, however, the following two options are mutually exclusive. PAM_PRELIM_CHECK This indicates that the modules are being probed as to their ready status for altering the user's authentication token. If the module requires access to another system over some network it should attempt to verify it can connect to this system on receiving this flag. If a module cannot establish it is ready to update the user's authentication token it should return PAM_TRY_AGAIN, this information will be passed back to the application. PAM_UPDATE_AUTHTOK This informs the module that this is the call it should change the authorization tokens. If the flag is logically OR'd with PAM_CHANGE_EXPIRED_AUTHTOK, the token is only changed if it has actually expired. Note, the Linux-PAM library calls this function twice in succession. The first time with PAM_PRELIM_CHECK and then, if the module does not return PAM_TRY_AGAIN, subsequently with PAM_UPDATE_AUTHTOK. It is only on the second call that the authorization token is (possibly) changed. PAM_SUCCESS is the only successful return value, valid error- returns are: PAM_AUTHTOK_ERR The module was unable to obtain the new authentication token. PAM_AUTHTOK_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. 4. Generic optional arguments Here we list the generic arguments that all modules can expect to be passed. They are not mandatory, and their absence should be accepted without comment by the module. debug Use the 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 WARNING: coding this functionality may cause the module writer to break local 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 Linux- PAM. 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 key to safely store/retrieve the authentication token for this module in/from a local file etc. . 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. 5. Programming notes Here we collect some pointers for the module writer to bear in mind when writing/developing a Linux-PAM compatible module. 5.1. Security issues for module creation 5.1.1. Sufficient resources Care should be taken to ensure that the proper execution of a module is not compromised by a lack of system resources. If a module is unable to open sufficient files to perform its task, it should fail gracefully, or request additional resources. Specifically, the quantities manipulated by the setrlimit(2) family of commands should be taken into consideration. 5.1.2. Who's who? Generally, the module may wish to establish the identity of the user requesting a service. This may not be the same as the username returned by pam_get_user(). Indeed, that is only going to be the name of the user under whose identity the service will be given. This is not necessarily the user that requests the service. In other words, user X runs a program that is setuid-Y, it grants the user to have the permissions of Z. A specific example of this sort of service request is the su program: user joe executes su to become the user jane. In this situation X=joe, Y=root and Z=jane. Clearly, it is important that the module does not confuse these different users and grant an inappropriate level of privilege. The following is the convention to be adhered to when juggling user- identities. +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 effective 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 Linux-PAM item, PAM_USER. +o Linux-PAM has a place for an additional user identity that a module may care to make use of. This is the PAM_RUSER item. Generally, network sensitive modules/applications may wish to set/read this item to establish the identity of the user requesting a service from a remote location. Note, if a module wishes to modify the identity of either the uid or euid of the running process, it should take care to restore the original values prior to returning control to the Linux-PAM library. 5.1.3. Using the conversation function Prior to calling the conversation function, the module should reset the contents of the pointer that will return the applications response. This is a good idea since the application may fail to fill the pointer and the module should be in a position to notice! The module should be prepared for a failure from the conversation. The generic error would be PAM_CONV_ERR, but anything other than PAM_SUCCESS should be treated as indicating failure. 5.1.4. Authentication tokens To ensure that the authentication tokens are not left lying around the items, PAM_AUTHTOK and PAM_OLDAUTHTOK, are not available to the application: they are defined in . This is ostensibly for security reasons, but a maliciously programmed application will always have access to all memory of the process, so it is only superficially enforced. As a general rule the module should overwrite authentication tokens as soon as they are no longer needed. Especially before free()'ing them. The Linux-PAM library is required to do this when either of these authentication token items are (re)set. Not to dwell too little on this concern; should the module store the authentication tokens either as (automatic) function variables or using pam_[gs]et_data() the associated memory should be over-written explicitly before it is released. In the case of the latter storage mechanism, the associated cleanup() function should explicitly overwrite the *data before free()'ing it: for example, /* * An example cleanup() function for releasing memory that was used to * store a password. */ int cleanup(pam_handle_t *pamh, void *data, int error_status) { char *xx; if ((xx = data)) { while (*xx) *xx++ = '\0'; free(data); } return PAM_SUCCESS; } 5.2. Use of syslog(3) Only rarely should error information be directed to the user. Usually, this is to be limited to ``sorry you cannot login now'' type messages. Information concerning errors in the configuration file, /etc/pam.conf, or due to some system failure encountered by the module, should be written to syslog(3) with facility-type LOG_AUTHPRIV. With a few exceptions, the level of logging is, at the discretion of the module developer. Here is the recommended usage of different logging levels: +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 required to 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. 5.3. Modules that require system libraries Writing a module is much like writing an application. You have to provide the "conventional hooks" for it to work correctly, like pam_sm_authenticate() etc., which would correspond to the main() function in a normal function. Typically, the author may want to link against some standard system libraries. As when one compiles a normal program, this can be done for modules too: you simply append the -lXXX arguments for the desired libraries when you create the shared module object. To make sure a module is linked to the libwhatever.so library when it is dlopen()ed, try: % gcc -shared -Xlinker -x -o pam_module.so pam_module.o -lwhatever 5.4. Added requirements for statically loaded modules. Modules may be statically linked into libpam. This should be true of all the modules distributed with the basic Linux-PAM distribution. 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_modname_modstruct, where modname is the name of the module as used in the filesystem but 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 static or dynamic: #include /* for NULL define */ #define PAM_SM_PASSWORD /* the only pam_sm_... function declared */ #include 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 libpam, staticly-linked modules must be built from within the Linux-PAM-X.YY/modules/ subdirectory of the Linux-PAM source directory as part of a normal build of the Linux-PAM system. The Makefile, 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 libpam. The two manditory 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 Makefile 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 Linux-PAM distribution. 6. An example module file 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/viewcvs.cgi/pam/Linux-PAM/modules/pam_permit/ +o A module that always fails: http://cvs.sourceforge.net/cgi- bin/viewcvs.cgi/pam/Linux-PAM/modules/pam_deny/ 7. Files /usr/lib/libpam.so.* the shared library providing applications with access to Linux- PAM. /etc/pam.conf the Linux-PAM configuration file. /usr/lib/security/pam_*.so the primary location for Linux-PAM dynamically loadable object files; the modules. 8. See also +o The Linux-PAM System Administrators' Guide. +o The Linux-PAM Application 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. 9. Notes 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 application. The modules are permitted to add additional groups, however. +o Refinements/futher suggestions to syslog(3) usage by modules are needed. 10. 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, 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. 11. Bugs/omissions 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. 12. Copyright information for this document Copyright (c) Andrew G. Morgan 1996-2002. All rights reserved. Email: 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. 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 DAMAGE. $Id: pam_modules.sgml,v 1.9 2002/05/10 06:00:12 agmorgan Exp $