The Linux-PAM Module Writers' Guide: What can be expected by the module

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## 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.

### 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.

(*This paragraph is currently under advisement with Sun*) 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 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.

### 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`.

### 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_type`s. The reader is urged to read the entry for this function in the Linux-PAM application developers' manual.

In addition to the `item`s listed there, the module can set the following two `item_type`s:

`PAM_AUTHTOK`

  The authentication token (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.

### 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:

- 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).
- The module is additionally privileged to read the authentication tokens, `PAM_AUTHTOK` and `PAM_OLDAUTHTOK` (see the section above on `pam_set_data()`).
- 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()`.

### The conversation mechanism

Following the call `pam_get_item(pamh,PAM_CONV,&item)`, the pointer `item` points 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 `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 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.

The return values for this function are listed in the Linux-PAM Application Developers' Guide.

### 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:

- checks what `pam_get_item(pamh, PAM_USER, ... );` would have returned. If this is not `NULL` this is what it returns. Otherwise,
- 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:
  - The `prompt` argument passed to the function
  - What is returned by `pam_get_item(pamh,PAM_USER_PROMPT, ... );`
  - 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.

### Setting a Linux-PAM environment variable

Synopsis:

> `

>     extern int pam_putenv(pam_handle_t *pamh, const char *name_value);

> `

Linux-PAM (0.54+) 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.

- 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 `""`.)
- 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.

### 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.

### 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`

### Understanding errors

- `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.

### Planning for delays

- `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 HAVE_PAM_FAIL_DELAY
  >         ....
  >     #endif /* HAVE_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 calls each of the stacked authentication modules listed in the `/etc/pam.conf` 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 (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.

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