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 LLiinnuuxx--PPAAMM 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 2.1.1 Setting data 2.1.2 Getting data 2.1.3 Setting items 2.1.4 Getting items 2.1.5 The 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 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 5.3 Modules that require system libraries 5.4 Added requirements for 6. An example module file 7. Files 8. See also 9. Notes 10. Author/acknowledgments 11. Bugs/omissions 12. Copyright information for this document ______________________________________________________________________ 11.. IInnttrroodduuccttiioonn 11..11.. SSyynnooppssiiss #include <security/pam_modules.h> gcc -fPIC -c pam_module-name.c ld -x --shared -o pam_module-name.so pam_module-name.o 11..22.. DDeessccrriippttiioonn LLiinnuuxx--PPAAMM (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 LLiinnuuxx--PPAAMM library see the LLiinnuuxx--PPAAMM System Administrators' Guide. A LLiinnuuxx--PPAAMM module is a single executable binary file that can be loaded by the LLiinnuuxx--PPAAMM 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 LLiinnuuxx--PPAAMM library; this is mostly to allow LLiinnuuxx--PPAAMM to be used on platforms without dynamic linking available, but the two forms can be used together. It is the LLiinnuuxx--PPAAMM 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 LLiinnuuxx--PPAAMM- 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. 22.. WWhhaatt ccaann bbee eexxppeecctteedd bbyy tthhee mmoodduullee Here we list the interface that the conventions that all LLiinnuuxx--PPAAMM modules must adhere to. 22..11.. GGeettttiinngg aanndd sseettttiinngg PPAAMM__IITTEEMM ss aanndd ddaattaa First, we cover what the module should expect from the LLiinnuuxx--PPAAMM library and a LLiinnuuxx--PPAAMM _a_w_a_r_e application. Essesntially this is the libpam.* library. 22..11..11.. SSeettttiinngg ddaattaa 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 _n_o_t 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 (_a_u_t_h_e_n_t_i_c_a_t_i_o_n _f_a_i_l_u_r_e) 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. 22..11..22.. GGeettttiinngg ddaattaa 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 _n_o_t a copy and should be treated as _c_o_n_s_t_a_n_t by the module. Note, if there is an entry but it has the value NULL, then this call returns PAM_NO_MODULE_DATA. 22..11..33.. SSeettttiinngg iitteemmss 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 LLiinnuuxx-- PPAAMM 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 LLiinnuuxx--PPAAMM 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 LLiinnuuxx--PPAAMM Application Developers' Guide. 22..11..44.. GGeettttiinngg iitteemmss 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 LLiinnuuxx--PPAAMM 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 _n_o_t 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(). 22..11..55.. TThhee ccoonnvveerrssaattiioonn mmeecchhaanniissmm Following the call pam_get_item(pamh,PAM_CONV,&item), the pointer item points to a _c_o_n_v_e_r_s_a_t_i_o_n-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 LLiinnuuxx--PPAAMM 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 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 _m_u_s_t 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 LLiinnuuxx--PPAAMM Application Developers' Guide. 22..11..66.. GGeettttiinngg tthhee nnaammee ooff aa uusseerr Synopsis: extern int pam_get_user(pam_handle_t *pamh, const char **user, const char *prompt); This is a LLiinnuuxx--PPAAMM 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 _n_o_t 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 LLiinnuuxx--PPAAMM. 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. 22..11..77.. SSeettttiinngg aa LLiinnuuxx--PPAAMM eennvviirroonnmmeenntt vvaarriiaabbllee Synopsis: extern int pam_putenv(pam_handle_t *pamh, const char *name_value); LLiinnuuxx--PPAAMM comes equipped with a series of functions for maintaining a set of _e_n_v_i_r_o_n_m_e_n_t variables. The environment is initialized by the call to pam_start() and is eerraasseedd with a call to pam_end(). This _e_n_v_i_r_o_n_m_e_n_t 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 LLiinnuuxx--PPAAMM 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 LLiinnuuxx--PPAAMM 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. 22..11..88.. GGeettttiinngg aa LLiinnuuxx--PPAAMM eennvviirroonnmmeenntt vvaarriiaabbllee 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. 22..11..99.. LLiissttiinngg tthhee LLiinnuuxx--PPAAMM eennvviirroonnmmeenntt Synopsis: extern char * const *pam_getenvlist(pam_handle_t *pamh); This function returns a pointer to the entire LLiinnuuxx--PPAAMM environment array. At first sight the _t_y_p_e of the returned data may appear a little confusing. It is basically a _r_e_a_d_-_o_n_l_y 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().) 22..22.. OOtthheerr ffuunnccttiioonnss pprroovviiddeedd bbyy lliibbppaamm 22..22..11.. UUnnddeerrssttaannddiinngg eerrrroorrss +o extern const char *pam_strerror(pam_handle_t *pamh, int errnum); This function returns some text describing the LLiinnuuxx--PPAAMM error associated with the argument errnum. If the error is not recognized ``Unknown Linux-PAM error'' is returned. 22..22..22.. PPllaannnniinngg ffoorr ddeellaayyss +o extern int pam_fail_delay(pam_handle_t *pamh, unsigned int micro_sec) This function is offered by LLiinnuuxx--PPAAMM 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 LLiinnuuxx--PPAAMM through a call to pam_authenticate() or pam_chauthtok(). These functions call each of the _s_t_a_c_k_e_d authentication modules listed in the LLiinnuuxx--PPAAMM 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 _b_r_u_t_e _f_o_r_c_e dictionary attacks primarily, but also helps hinder _t_i_m_e_d (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 _m_i_c_r_o_- _s_e_c_o_n_d_s). LLiinnuuxx--PPAAMM 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 LLiinnuuxx--PPAAMM returns control to the application. 33.. WWhhaatt iiss eexxppeecctteedd ooff aa mmoodduullee The module must supply a sub-set of the six functions listed below. Together they define the function of a LLiinnuuxx--PPAAMM mmoodduullee. Module developers are strongly urged to read the comments on security that follow this list. 33..11.. OOvveerrvviieeww The six module functions are grouped into four independent management groups. These groups are as follows: _a_u_t_h_e_n_t_i_c_a_t_i_o_n, _a_c_c_o_u_n_t, _s_e_s_s_i_o_n and _p_a_s_s_w_o_r_d. 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 _a_l_l four groups. 33..11..11.. FFuunnccttiioonnaall iinnddeeppeennddeennccee 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 LLiinnuuxx--PPAAMM 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 _K_E_R_B_E_R_O_S 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. 33..11..22.. MMiinniimmiizziinngg aaddmmiinniissttrraattiioonn pprroobblleemmss 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. 33..11..33.. AArrgguummeennttss ssuupppplliieedd ttoo tthhee mmoodduullee 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 _t_e_x_t (errors or warnings) to the application. The argc and argv arguments are taken from the line appropriate to this module---that is, with the _s_e_r_v_i_c_e___n_a_m_e matching that of the application---in the configuration file (see the LLiinnuuxx--PPAAMM 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 nnoott the name of the module. 33..22.. AAuutthheennttiiccaattiioonn mmaannaaggeemmeenntt 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 _a_f_t_e_r 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 LLiinnuuxx--PPAAMM--00..7755, 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 _a_u_t_h_e_n_t_i_c_a_t_i_o_n 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 LLiinnuuxx--PPAAMM--00..7755 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 _f_a_i_l_i_n_g. The first such error will dominate in the return value of pam_setcred(). 33..33.. AAccccoouunntt mmaannaaggeemmeenntt 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. 33..44.. SSeessssiioonn mmaannaaggeemmeenntt 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. 33..55.. PPaasssswwoorrdd mmaannaaggeemmeenntt 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 _m_u_s_t be combined with one of the following two flags. Note, however, the following two options are _m_u_t_u_a_l_l_y _e_x_c_l_u_s_i_v_e. 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 LLiinnuuxx--PPAAMM 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. 44.. GGeenneerriicc ooppttiioonnaall aarrgguummeennttss 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 WWAARRNNIINNGG:: coding this functionality may cause the module writer to break _l_o_c_a_l 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 LLiinnuuxx-- PPAAMM. 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 _k_e_y to safely store/retrieve the authentication token for this module in/from a local file _e_t_c. . 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. 55.. PPrrooggrraammmmiinngg nnootteess Here we collect some pointers for the module writer to bear in mind when writing/developing a LLiinnuuxx--PPAAMM compatible module. 55..11.. SSeeccuurriittyy iissssuueess ffoorr mmoodduullee ccrreeaattiioonn 55..11..11.. SSuuffffiicciieenntt rreessoouurrcceess 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. 55..11..22.. WWhhoo''ss wwhhoo?? 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 _s_u program: user joe executes _s_u to become the user _j_a_n_e. 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 _e_f_f_e_c_t_i_v_e 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 LLiinnuuxx--PPAAMM item, PAM_USER. +o LLiinnuuxx--PPAAMM 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 LLiinnuuxx--PPAAMM library. 55..11..33.. UUssiinngg tthhee ccoonnvveerrssaattiioonn ffuunnccttiioonn 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. 55..11..44.. AAuutthheennttiiccaattiioonn ttookkeennss 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 LLiinnuuxx--PPAAMM 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; } 55..22.. UUssee ooff ssyysslloogg((33)) Only rarely should error information be directed to the user. Usually, this is to be limited to ``_s_o_r_r_y _y_o_u _c_a_n_n_o_t _l_o_g_i_n _n_o_w'' 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 _f_a_c_i_l_i_t_y_-_t_y_p_e 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 rreeqquuiirreedd 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. 55..33.. MMoodduulleess tthhaatt rreeqquuiirree ssyysstteemm lliibbrraarriieess 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_X_X_X arguments for the desired libraries when you create the shared module object. To make sure a module is linked to the lib_w_h_a_t_e_v_e_r.so library when it is dlopen()ed, try: % gcc -shared -Xlinker -x -o pam_module.so pam_module.o -lwhatever 55..44.. AAddddeedd rreeqquuiirreemmeennttss ffoorr ssttaattiiccaallllyy llooaaddeedd mmoodduulleess.. Modules may be statically linked into libpam. This should be true of all the modules distributed with the basic LLiinnuuxx--PPAAMM 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__m_o_d_n_a_m_e_modstruct, where _m_o_d_n_a_m_e is the name of the module aass uusseedd iinn tthhee ffiilleessyysstteemm 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 _s_t_a_t_i_c or _d_y_n_a_m_i_c: #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 _l_i_b_p_a_m, staticly-linked modules must be built from within the Linux-PAM-X.YY/modules/ subdirectory of the LLiinnuuxx--PPAAMM source directory as part of a normal build of the LLiinnuuxx--PPAAMM system. The _M_a_k_e_f_i_l_e, 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 _l_i_b_p_a_m. The ttwwoo 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 _M_a_k_e_f_i_l_e 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 LLiinnuuxx--PPAAMM distribution. 66.. AAnn eexxaammppllee mmoodduullee ffiillee 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 77.. FFiilleess /usr/lib/libpam.so.* the shared library providing applications with access to LLiinnuuxx-- PPAAMM. /etc/pam.conf the LLiinnuuxx--PPAAMM configuration file. /usr/lib/security/pam_*.so the primary location for LLiinnuuxx--PPAAMM dynamically loadable object files; the modules. 88.. SSeeee aallssoo +o The LLiinnuuxx--PPAAMM System Administrators' Guide. +o The LLiinnuuxx--PPAAMM 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. 99.. NNootteess 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 _a_p_p_l_i_c_a_t_i_o_n. The modules are permitted to add additional groups, however. +o Refinements/futher suggestions to syslog(3) usage by modules are needed. 1100.. AAuutthhoorr//aacckknnoowwlleeddggmmeennttss 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 LLiinnuuxx--PPAAMM, Sun graciously made the documentation for their implementation of PAM available. This act greatly accelerated the development of LLiinnuuxx--PPAAMM. 1111.. BBuuggss//oommiissssiioonnss 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. 1122.. CCooppyyrriigghhtt iinnffoorrmmaattiioonn ffoorr tthhiiss ddooccuummeenntt 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. AAlltteerrnnaattiivveellyy, 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 iinnsstteeaadd ooff 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.txt,v 1.15 2001/04/23 18:39:04 nalin Exp $