Merge branch '2906-make-kerberos-mapping-configurable' into 'master'

Resolve "Make the mapping between Kerberos and LDAP configurable"

See merge request gitlab-org/gitlab!9962

config/initializers/1_settings.rb

@@ -731,6 +731,7 @@ Gitlab.ee do
   Settings['kerberos'] ||= Settingslogic.new({})
   Settings.kerberos['enabled'] = false if Settings.kerberos['enabled'].nil?
   Settings.kerberos['keytab'] = nil if Settings.kerberos['keytab'].blank? # nil means use default keytab
+  Settings.kerberos['simple_ldap_linking_allowed_realms'] = [] if Settings.kerberos['simple_ldap_linking_allowed_realms'].blank?
   Settings.kerberos['service_principal_name'] = nil if Settings.kerberos['service_principal_name'].blank? # nil means any SPN in keytab
   Settings.kerberos['use_dedicated_port'] = false if Settings.kerberos['use_dedicated_port'].nil?
   Settings.kerberos['https'] = Settings.gitlab.https if Settings.kerberos['https'].nil?

ee/changelogs/unreleased/2906-make-kerberos-mapping-configurable.yml

+---
+title: Make mapping between LDAP and Kerberos configurable
+merge_request: 9962
+author: Christopher Schenk
+type: added

ee/lib/ee/gitlab/auth/ldap/person.rb

@@ -30,15 +30,25 @@ module EE
             def find_by_kerberos_principal(principal, adapter)
               uid, domain = principal.split('@', 2)
               return unless uid && domain
-
-              # In multi-forest setups, there may be several users with matching
-              # uids but differing DNs, so skip adapters configured to connect to
-              # non-matching domains
-              return unless domain.casecmp(domain_from_dn(adapter.config.base)) == 0
+              return unless allowed_realm?(domain, adapter)
 
               find_by_uid(uid, adapter)
             end
 
+            def allowed_realm?(domain, adapter)
+              return domain.casecmp(domain_from_dn(adapter.config.base)) == 0 unless simple_ldap_linking?
+
+              simple_ldap_linking_allowed_realms.select { |realm| domain.casecmp(realm) == 0 }.any?
+            end
+
+            def simple_ldap_linking_allowed_realms
+              ::Gitlab.config.kerberos.simple_ldap_linking_allowed_realms
+            end
+
+            def simple_ldap_linking?
+              simple_ldap_linking_allowed_realms.present?
+            end
+
             # Extracts the rightmost unbroken set of domain components from an
             # LDAP DN and constructs a domain name from them
             def domain_from_dn(dn)

ee/spec/lib/gitlab/auth/ldap/person_spec.rb

@@ -60,15 +60,17 @@ RSpec.describe Gitlab::Auth::Ldap::Person do
   describe '.find_by_kerberos_principal' do
     let(:adapter) { ldap_adapter }
     let(:username) { 'foo' }
-    let(:principal) { username + '@' + kerberos_realm }
     let(:ldap_server) { 'ad.example.com' }
 
-    subject { described_class.find_by_kerberos_principal(principal, adapter) }
+    subject(:ldap_person) { described_class.find_by_kerberos_principal(principal, adapter) }
 
     before do
       stub_ldap_config(uid: 'sAMAccountName', base: 'ou=foo,dc=' + ldap_server.gsub('.', ',dc='))
     end
 
+    context 'when simple LDAP linking is not configured' do
+      let(:principal) { username + '@' + kerberos_realm }
+
       context 'LDAP server is not for kerberos realm' do
         let(:kerberos_realm) { 'kerberos.example.com' }
 
@@ -90,6 +92,35 @@ RSpec.describe Gitlab::Auth::Ldap::Person do
       end
     end
 
+    context 'when simple LDAP linking is enabled' do
+      let(:allowed_realms) { ['kerberos.example.com', ldap_server] }
+
+      before do
+        stub_config(kerberos: { simple_ldap_linking_allowed_realms: allowed_realms })
+      end
+
+      context 'principal domain matches an allowed realm' do
+        let(:principal) { "#{username}@#{allowed_realms[0]}" }
+
+        it 'searches by configured uid attribute' do
+          expect(adapter).to receive(:user).with('sAMAccountName', username).and_return(:fake_user)
+
+          expect(ldap_person).to eq(:fake_user)
+        end
+      end
+
+      context 'principal domain does not match an allowed realm' do
+        let(:principal) { "#{username}@alternate.example.com" }
+
+        it 'returns nil without searching' do
+          expect(adapter).not_to receive(:user)
+
+          is_expected.to be_nil
+        end
+      end
+    end
+  end
+
   describe '.ldap_attributes' do
     def stub_sync_ssh_keys(value)
       stub_ldap_config(