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Support for non-POSIX users and groups

https://github.com/SSSD/sssd/issues/4343

Problem statement

SSSD, with its D-Bus interface is appealing to applications as a gateway to an LDAP directory where users and groups are stored. However, contrary to the traditional SSSD deployment where all users and groups either have POSIX attributes or those attributes can be inferred from the Windows SIDs, in many cases the users and groups in the application support scenario have no POSIX attributes. At the same time, SSSD especially on the back end (LDAP provider) level currently requires the presence of either an UID number or a Windows SID.

This feature tracks adding support for user and group resolution through the D-Bus interface and authn/authz through the PAM interface even for setups without UIDs or Windows SIDs present on the LDAP directory side.

Use cases

  • As an application developer, I want to fetch user attributes for my application from an LDAP server that only uses a fairly generic LDAP objectclasses such as groupOfNames for groups or person for users. The application can either issue D-Bus calls itself or rely on Apache modules such as mod_lookup_identity
  • As an application developer, I want to authenticate users to my application using the mod_authnz_pam module even if the users lack PAM attributes.

Overview of the solution

On a high level, the biggest change is tweaking the internal LDAP searches so that they are less restrictive and match users and groups based on just the requested key (typically a name or a UPN) and the configured objectclass.

Whether SSSD will restrict the lookups to only POSIX accounts or allow even non-POSIX lookups will be controlled at the domain level. This is important for several reasons:

  • To avoid “leaking” the non-POSIX accounts or, perhaps more importantly, their non-POSIX group memberships to interfaces such as NSS that otherwise require a complete POSIX entry. While these checks could be implemented by the POSIX interfaces as well, making the distinction at the domain level is much safer.
  • To minimize the number of inconsistencies. These include:
    • One interface requesting a non-POSIX user through a D-Bus call, then later a POSIX interface searching for the same user. The latter call, having a more restrictive filter, would not match the object in the domain and because we tend to treat the domain reply as authoritative, remove the cached non-POSIX users.
    • Mixed POSIX and non-POSIX group memberships would require a large amount of special-casing and very careful testing to make sure requests for such hybrid user would return the correct group memberships but at the same time not remove the non-POSIX parts in all cases.
    • Interfaces that serve both the POSIX and non-POSIX case such as the D-Bus interface need to know whether to return only the list of POSIX group membership if a user that has POSIX attributes is requested. The interfaces cannot assume the intent of the caller and forcing the caller to decide based on the target domain helps here.

In short, by forcing to make the distinction on the domain level, the SSSD code would be made more robust and error-prone while limiting the complexity.

Configuration-wise, this change would allow to define a new section type in SSSD called [application] in addition to the current [domain]. Internally, this would create an SSSD domain as well, just marked as a non-POSIX one.

An application domain section can be either defined as a standalone domain, listing all the needed options or can be used in conjunction with a traditional POSIX domain in mixed setups. In this case, the application domain can use the inherit_from option to inherit all options from the “sibling” POSIX domain. Any options specified in the application domain override the POSIX domain options. Examples of both are given in the “How to test” section below.

An application domain would be skipped during lookups by interfaces that require POSIX attributes, such as sudo or NSS. Better distinction must be made for interfaces that support both POSIX and non-POSIX users, though and the details are discussed below.

In the simplest case, only a single domain will be defined. The tricky part are setups that must serve data to both the application from a non-POSIX domain and for the OS. For these setups, two [domain/] sections pointing to the same remote server must be defined, one marked as posix and one marked as application type (see below).

To distinguish between accounts with the same name that exist in both these domains by interfaces that support both POSIX and application domains, a qualified name is the safest way.

In addition, the PAM responder will keep a list of application PAM services so that if an unqualified name arrives from an OS-level PAM service (such as ssh), the PAM responder will know it is safe to skip the application domains and route the PAM request to POSIX domains only. This would make life easier for accounts that both use the application and OS-level PAM services in the sense that the OS-level PAM interface will be usable even without qualified names.

For distinguishing accounts between a POSIX and an application domain from the D-Bus interface, a fully qualified name must be used. But since the D-Bus interface is normally used only by the application, putting the application domain first in the domains list should be enough to avoid using qualified names from the application while still making it possible to use the POSIX domain defined as second in the list.

Configuration changes

A new SSSD domain option will be added. The option will be called domain_type and would support two values - posix and application. The default value will be posix, the non-POSIX support will be enabled by setting the domain_type to application. This option will, however, not be typically used. Instead, the application domain (see below) would be defined.

A new application section will be read. The application section will internally unroll into a domain section with domain_type set to application. Additionally, the application section will enable the administrator to link the application domain with a POSIX domain through the inherit_from option described above.

The pam responder will gain a new option pam_app_services. This option will be empty by default. Only PAM services listed in this option will be allowed to contact the application domains. This option will be added to make sure that unqualified names can still be used in PAM conversations in case both posix and applications domains are configured for SSSD. In future, we might add a similar option (with UID of the caller of the interface as the key) to the ifp responder as well, but such option will not be added in the first iteration.

Implementation details

When the domain_type is set to application, SSSD responders that are only usable on a POSIX system (currently nss, ssh, sudo, kcm and secrets) will skip over application type domains. This logic will be implemented at the cache_req level and set by the caller, which is the responder, by adding a cache_req option. In particular, the cache_req requests will include an enum with allowed values CACHE_REQ_POSIX and CACHE_REQ_APP where the POSIX responders will only use the CACHE_REQ_POSIX value. The logic that determines which value the ifp and pam responders use is described below.

The responders that are usable by both applications and the OS as well (that’s pam and ifp) will set a flag for the cache_req request that will instruct the cache_req domain-loop to also consider application domains.

The ifp responder will consider application domains for all requests. Therefore in order to resolve a POSIX user with only POSIX membership, the request towards the ifp interface must be qualified with the domain name.

The pam responder will route requests only to POSIX domains by default. Requests coming from PAM services listed in the pam_app_services option will on the other hand only be routed to application domains.

The last change needed is for Kerberos authentication. Since the Kerberos ticket returned cannot be owned by a POSIX owner in application domains, especially in cases the system would otherwise use a FILE-based ccache, we will use a temporary MEMORY-based ccache in the krb5_child process destroy the ticket when krb5_child finishes and only return the resulting error code from the child process to the Data Provider.

How To Test

The best way of testing this feature is to test the “full stack” together with an application and the Apache modules. However, at the moment there is a bug in mod_lookup_identity that prevents the non-POSIX lookups from working.

However, isolated testing can be performed as well. All tests expect that the LDAP server is populated with entries that do not contain POSIX attributes or might contain POSIX attributes in the case of mixed setup tests. For testing PAM authn and authz, the pam_test_client binary built from SSSD sources can be used until sssctl provides a better way by fixing https://github.com/SSSD/sssd/issues/4325.

The users in the test setup should be members of non-POSIX groups, or a mix of POSIX and non-POSIX groups in the mixed scenario setups.

  1. SSSD with only the application domain can be reached through the D-Bus and PAM interfaces
    • Add a domain as follows:

        [sssd]
        domains = appdomain.test
        services = ifp, pam, nss
              
        [pam]
        pam_app_services = sss_test
              
        [ifp]
        user_attributes = +phone
              
        [application/appdomain.test]
        id_provider = ldap
        ldap_uri = ldap://server.test
        ldap_search_base = dc=test
        ldap_user_extra_attrs = phone:telephoneNumber
      
    • Note that the nss responder is added for testing purposes only to prove that the non-POSIX users cannot be resolved. The nss service can be removed in production. Also note that on a modern distribution, all the services can be socket-activated, so the services line is not required at all.

    • Make sure that a D-Bus call towards the SSSD interface can resolve the phone attribute of $user:

        dbus-send --print-reply \
                --system \
                --dest=org.freedesktop.sssd.infopipe \
                  /org/freedesktop/sssd/infopipe \
                  org.freedesktop.sssd.infopipe.GetUserAttr \
                  string:$user array:string:phone
      
    • Test that calling a PAM application (for example pam_test_client auth $user) succeeds with the correct password. Please note that testing auth_provider=krb5 is also important here to make sure the krb5 provider can deal with ccaches that cannot be owned by a POSIX owner. Also note that it is important that the PAM application doesn’t try to canonicalize the user with NSS calls like getpwnam() itself.

  2. No users or groups from the application domain can be resolved through the system interfaces
  • With the setup above, make sure that getent passwd $user doesn’t return the user and even doesn’t contact the remote server.
  • Same test should be performed for the ssh, sudo and other responders.
  1. A mixed setup with two domains, one posix and one application reports the proper results from all interfaces

    • Modify the test domain to look like:

        [sssd]
        domains = appdomain.test, posixdomain.test
        services = ifp, pam, nss
              
        [pam]
        pam_app_services = sss_test
              
        [ifp]
        user_attributes = +phone
              
        [domain/appdomain.test]
        inherit_from = posixdomain.test
              
        [domain/posixdomain.test]
        id_provider = ldap
        ldap_uri = ldap://server.test
      
    • Make sure the LDAP server contains a mix of POSIX and non-POSIX users and groups. The POSIX users should be members of a mix of both POSIX and non-POSIX groups.

    • The tests from the first test case against a non-POSIX user should function as before

    • However, retrieving the POSIX users should now hit the domain posixdomain.test, so the following should work:

      • getent passwd $posix_user should return the POSIX user
      • id $posix_user should return only POSIX groups the user is a member of and omit non-POSIX groups
      • getent group $posix_group should list all POSIX members of this group.

      In all the above tests, the NSS responder should skip the appdomain.test domain completely.

    • Since the appdomain.test domain comes first in the domain list, running GetUserAttr with an unqualified name should return non-POSIX users

    • Also invoking the GetUserGroups function should list their non-POSIX groups. Requests qualified to reach the posixdomain.test domain should only list POSIX groups.

  2. A mixed setup with two domains, one posix and one application can be used for OS-level authentication and authorization

    • Use the setup from the previous test
    • Make sure that running a PAM application on the OS level (su or ssh are good tests) allows the user to log in using shortname as the NSS responder would skip the appdomain.test completely when the PAM application calls initgroups and the PAM responder would skip the appdomain.test domain because the PAM service is not listed in the pam_app_services option

How To Debug

Debug messages listing the domain type must be added to the cache_req code. Then, the regular method of issuing a request and watching the logs should work. Expiring the cache and using the qualified names is recommended.

Authors

  • Sumit Bose
  • Jakub Hrozek
  • Simo Sorce