Disallow bare pages domain as custom domain

app/models/pages_domain.rb

@@ -245,8 +245,8 @@ class PagesDomain < ApplicationRecord
   def validate_pages_domain
     return unless domain
 
-    if domain.downcase.ends_with?(".#{Settings.pages.host.downcase}")
-      self.errors.add(:domain, "*.#{Settings.pages.host} is restricted. Please compare our documentation at https://docs.gitlab.com/ee/administration/pages/#advanced-configuration against your configuration.")
+    if domain.downcase.ends_with?(".#{Settings.pages.host.downcase}") || domain.casecmp(Settings.pages.host) == 0
+      self.errors.add(:domain, "#{Settings.pages.host} and its subdomains cannot be used as custom pages domains. Please compare our documentation at https://docs.gitlab.com/ee/administration/pages/#advanced-configuration against your configuration.")
     end
   end
 

spec/models/pages_domain_spec.rb

@@ -36,9 +36,9 @@ RSpec.describe PagesDomain do
         '123.456.789'      => true,
         '0x12345.com'      => true,
         '0123123'          => true,
-        'reserved.com'     => true,
         'a-reserved.com'   => true,
         'a.b-reserved.com' => true,
+        'reserved.com'     => false,
         '_foo.com'         => false,
         'a.reserved.com'   => false,
         'a.b.reserved.com' => false,