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Make authentication prompting configurable

Problem statement

Although SSSD tries to use the most suitable prompting depending on which authentication options (password, two-factor authentication, smartcard authentication) are available for the user it does not fit in all cases. It should be possible with the help of configuration options to tune the prompting during authentication.

Use cases

General prompting

Sometimes just asking Password: is not sufficient because different authentication domains are used and something like Enter your AD Password: is more suitable.

Another example is SSSD’s default prompting for two-factor authentication if both plain password and two-factor authentication is available for the user. In this case an (optional) is shown with the prompt for the second factor. This might be irritating on systems where two-factor authentication is required.

Two-Factor Authentication Prompting

There are two ways how the two factors can be entered. Either each factor in a separate prompt and both chained together in a single prompt.

The PAM Conversation of OTP design page explains some details why using two separate prompts is useful from the SSSD point of view. However there are services which cannot handle PAM conversations and ask the user directly to enter the needed credentials. On the PAM level those application can put the user input in both prompts or only in the first one. Instead of adding heuristics to SSSD to figure out what is meant by which application it would be easier and more reliable to be able to configure the expected behavior for each service individually if needed.

What has to be noted here is that SSSD must know somehow if the user entered a single password or or two-factor credentials in a single string. The reason is that both types of authentication might be treated differently on the server side. This can e.g. be seen with Kerberos where a special pre-authentication scheme described in RFC6560 is used. If here the plain password is used authentication will fail and if the two-factor string is used with plain Kerberos authentication not using the OTP pre-auth method authentication will fail as well. Since a failed authentication might increment a bad-password count and cause a user lock, try-and-error is not an option here.

Alternative authentication principal

Typically during authentication the user name of the POSIX user trying to authenticate is entered and SSSD tries to figure out to which account in the authentication domain the POSIX user relates. If the account cannot be found, e.g. because the backend simply does not support such lookup, and there is a strict 1:1 mapping the krb5_map_user option might be used. But this might become cumbersome if many users have to be managed or if multiple users from an authentication domain should be allowed to authenticate as a single POSIX user.

To cover those cases and additional prompt for a user name which should be used for authentication would be helpful. Of course it has to be noted that a suitable access control scheme has to be used to make sure that only expected accounts from the authentication domain can login in as a given POSIX user.

Additionally this can only work properly with services which offer full PAM conversation functionality so that the prompt for the alternative name is shown to the user.

Asking for missing user name

There is also the case of gdm-smartcard where typically no user name is prompted at all because it is expected that the user name can be derived from the certificate found on the Smartcard. But there are cases where this is not possible as described in Smartcards and Multiple Identities. Here is would be useful to have a configurable extra prompt for a user name hint.

Using specific authentication methods

Currently SSSD tries to determine on its own what would be the most suitable prompting if multiple authentication methods are available. E.g. if a Smartcard with a certificate suitable to authenticate the current user is available SSSD will prompt for a Smartcard PIN even if password authentication would be available for the user as well. To see a password prompt the Smartcard has to be removed first. But there might be cases where password or two-factor authentication should be preferred and having to remove the Smartcard might be irritating.

Another use case would be to only ask for two-factor authentication even if password authentication would be available for the user as well.

An option where the prompting types can be ordered by preference would be useful here.

Overview of the solution

On one hand it can be seen from the use cases above that it should be possible to set the options for each PAM service individually. On the other hand it should be easy to set e.g. a global welcome/warning message for all services. For this a hierarchical configuration scheme, which SSSD already uses e.g. to configure sub-domains, would be suitable.

The hierarchy would start with a [prompting] section valid for all authentication types and services. The next level would be [prompting/password], [prompting/2fa] and [prompting/cert_auth] for password, two-factor and Smartcard authentication respectively. Finally there will be a level for the PAM service specific options like e.g. [prompting/2fa/sshd]. Lower level, i.e. more specific, options will of course overwrite higher level, i.e. more general, options.

All those options will be evaluated by SSSD’s PAM responder. It would be possible to have all those options in [pam/prompting/….] sections. But always having the pam/ prefix looks redundant and the section names are already long enough.

Currently the prompting for the different authentication schemes and the selection is basically hardcoded in the pam_sss.so PAM module. Since the PAM responder will control the prompting if any option is set below [prompting] the PAM responder must set /var/lib/sss/pubconf/pam_preauth_available in this case to make sure pam_sss.so tries to call the PAM responder before prompting the user.

One or more new PAM response_type should be added to send the needed data for prompting to pam_sss.so.

To handle the alternative authentication principal enum sss_authtok_type and the related methods for struct sss_auth_token should be extended.

Implementation details

_sssd.conf man page section

The details of the expected behavior of the new configuration option are best documented in a man page. For a start it might look like:

PROMPTING CONFIGURATION SECTION
   If a special file (/var/lib/sss/pubconf/pam_preauth_available) exists
   SSSD's PAM module pam_sss will ask SSSD to figure out which authentication
   methods are available for the user trying to log in. Based on the results
   pam_sss will prompt the user for appropriate credentials.

   With the growing number of authentication methods and the possibility
   that there are multiple ones for a single user the heuristic used by pam_sss to
   select the prompting might not be suitable for all use cases. To following
   options should provide a better flexibility here.

   Each supported authentication method has it's own configuration
   sub-section under “[prompting/...]”. Currently there are:

   [prompting/password]
       to configure password prompting, allowed options are:

       password_prompt
           to change the string of the password prompt

   [prompting/2fa]
       to configure two-factor authentication prompting, allowed options are:

       first_prompt
           to change the string of the prompt for the first factor

       second_prompt
           to change the string of the prompt for the second factor

       single_prompt
           boolean value, if True there will be only a single prompt using
           the value of first_prompt where it is expected that both factor are entered as
           a single string

   It is possible to add a sub-section for specific PAM services like e.g.
   “[prompting/password/sshd]” to individual change the prompting for this
   service.

Changes to pam_sss.so

If there is a SSS_PAM_PROMPT_CONFIG item during the pre-auth step in the response from the PAM responder SSSD’s PAM module pam_sss.so should act according to the received configuration. If there is no such item pam_sss.so should just show the current behavior.

The response currently has the following structure:

       
number of prompting configurations (uint32_t) prompting type (uint32_t) type specific data … additional type-data pairs …

where the type is defined by:

enum prompt_config_type {
    PC_TYPE_INVALID = 0,
    PC_TYPE_PASSWORD,
    PC_TYPE_2FA,
    PC_TYPE_2FA_SINGLE,
    PC_TYPE_SC_PIN,
    PC_TYPE_LAST
};

Where the data for PC_TYPE_PASSWORD is:

   
string length (uint32_t) string of given length

PC_TYPE_2FA obviously has 2 strings:

       
length of the first string (uint32_t) first string length of second string (uint32_t) second string

PC_TYPE_2FA_SINGLE uses a single prompt hence also only a single string

   
string length (uint32_t) string of given length

Finally PC_TYPE_SC_PIN currently has no specific data.

Changes to the PAM responder

First the PAM responder has to read the new configuration option from sssd.conf and if there are any it should create the pre-auth indicator file so that the PAM module pam_sss.so will run the pre-auth step. During the pre-auth step the PAM responder has to check if configured options apply to the given request and create an appropriate SSS_PAM_PROMPT_CONFIG responses if needed.

Configuration changes

See sssd.conf()

How To Test

Please see sssd.conf() what options are currently available.

In general testing can start with changing the password prompt for all services by adding:

[prompting/password]
password_prompt = My Password Prompt

If now su is called as non-root user for an SSSD user:

$ su sssd_user
My Password Prompt

$ su - sssd_user
My Password Prompt

Next step can be to add a specific prompt for `su-l`:

[prompting/password/su-l]
password_prompt = My su-l Prompt

and now calling su should show:

$ su sssd_user
My Password Prompt

$ su - sssd_user
My su-l Prompt

If a FreeIPA setup with users configured for 2-Factor Authentication you can use first_prompt and second_prompt in the [prompting/2fa] section to change the 2FA prompting strings for all services. With:

[prompting/2fa/my_service]
single_prompt = True
first_prompt = Please enter password + OTP token value

you can enable single 2FA prompting for the PAM service my_service where only one prompt will be show where the long term password and the one-time password have to be entered in a single string.

How To Debug

Setting debug_level in the [pam] section of sssd.conf to a sufficiently high value will show error and debugging information for the processing of the prompting option.

If you compile pam_sss.so with -DPAM_DEBUG in the C-compiler flags the PAM module will write some debug information to /var/run/pam-debug.log (or a similar file depending on the platform). Since this is typically not the case using gdb might help to identify issues in processing the prompting configuration. Suitable break points are pc_list_from_response() where the response form the PAM responder is translated and prompt_by_config() where the right prompting based on the received configuration is selected.

Authors

Sumit Bose <<sbose@redhat.com>>