Prevent access to Group SAML metadata/SLO endpoints

These have not been tested with the unique requirements of per-group SAML
and have not yet been verified for security

In particular, these will likely allow someone to determine
if a group exists or not by guessing the name.

ee/lib/omni_auth/strategies/group_saml.rb

@@ -21,6 +21,12 @@ module OmniAuth
         super
       end
 
+      # Prevent access to SLO and metadata endpoints
+      # These will need addtional work to securely support
+      def other_phase
+        call_app!
+      end
+
       def self.callback?(env)
         env['PATH_INFO'] =~ Gitlab::PathRegex.saml_callback_regex
       end

ee/spec/lib/omni_auth/strategies/group_saml_spec.rb

@@ -60,7 +60,7 @@ describe OmniAuth::Strategies::GroupSaml, type: :strategy do
       end
     end
 
-    it 'returns 404 when if group is not found' do
+    it 'returns 404 when the group is not found' do
       expect do
         post "/groups/not-a-group/-/saml/callback", SAMLResponse: saml_response
       end.to raise_error(ActionController::RoutingError)
@@ -92,7 +92,7 @@ describe OmniAuth::Strategies::GroupSaml, type: :strategy do
       end.to raise_error(ActionController::RoutingError)
     end
 
-    it 'returns 404 when if group is not found' do
+    it 'returns 404 when the group is not found' do
       expect do
         post '/users/auth/group_saml', group_path: 'not-a-group'
       end.to raise_error(ActionController::RoutingError)
@@ -104,4 +104,34 @@ describe OmniAuth::Strategies::GroupSaml, type: :strategy do
       end.to raise_error(ActionController::RoutingError)
     end
   end
+
+  describe 'POST /users/auth/group_saml/metadata' do
+    it 'returns 404 when the group is not found' do
+      post '/users/auth/group_saml/metadata', group_path: 'not-a-group'
+
+      expect(last_response).to be_not_found
+    end
+
+    it 'returns 404 to avoid disclosing group existence' do
+      post '/users/auth/group_saml/metadata', group_path: 'my-group'
+
+      expect(last_response).to be_not_found
+    end
+  end
+
+  describe 'POST /users/auth/group_saml/slo' do
+    it 'returns 404 to avoid disclosing group existence' do
+      post '/users/auth/group_saml/slo', group_path: 'my-group'
+
+      expect(last_response).to be_not_found
+    end
+  end
+
+  describe 'POST /users/auth/group_saml/spslo' do
+    it 'returns 404 to avoid disclosing group existence' do
+      post '/users/auth/group_saml/spslo', group_path: 'my-group'
+
+      expect(last_response).to be_not_found
+    end
+  end
 end