Z, the user under whose identity the service will be granted.
This is the username returned by
pam_get_user() and also stored in the
Linux-PAM item,
PAM_USER.
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.
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.
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
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;
}