Updates authorization for lint

* Force some form of authentication in order to access the lint
endpoint for unauthenticated users on GitLab instances with restrictions
* Adds a method for determining if registration on an instance is
limited based on the above
* Adds specs for all the cases mentioned above

Changelog: changed

config/feature_flags/development/security_ci_lint_authorization.yml

+---
+name: security_ci_lint_authorization
+introduced_by_url: https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/1279
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326708
+milestone: '14.0'
+type: development
+group: group::pipeline authoring
+default_enabled: false

doc/api/lint.md

@@ -13,7 +13,16 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 Checks if CI/CD YAML configuration is valid. This endpoint validates basic CI/CD
 configuration syntax. It doesn't have any namespace specific context.
 
-Access to this endpoint requires authentication.
+Access to this endpoint does not require authentication when the instance
+[allows new sign ups](../user/admin_area/settings/sign_up_restrictions.md#disable-new-sign-ups)
+and:
+
+- Does not have an [allowlist or denylist](../user/admin_area/settings/sign_up_restrictions.md#allow-or-deny-sign-ups-using-specific-email-domains).
+- Does not [require administrator approval for new sign ups](../user/admin_area/settings/sign_up_restrictions.md#require-administrator-approval-for-new-sign-ups).
+- Does not have additional [sign up
+  restrictions](../user/admin_area/settings/sign_up_restrictions.html#sign-up-restrictions).
+
+Otherwise, authentication is required.
 
 ```plaintext
 POST /ci/lint

lib/api/lint.rb

@@ -11,7 +11,11 @@ module API
         optional :include_merged_yaml, type: Boolean, desc: 'Whether or not to include merged CI config yaml in the response'
       end
       post '/lint' do
+        if Feature.enabled?(:security_ci_lint_authorization)
+          unauthorized! if (Gitlab::CurrentSettings.signup_disabled? || Gitlab::CurrentSettings.signup_limited?) && current_user.nil?
+        else
           unauthorized! if Gitlab::CurrentSettings.signup_disabled? && current_user.nil?
+        end
 
         result = Gitlab::Ci::YamlProcessor.new(params[:content], user: current_user).execute
 

lib/gitlab/current_settings.rb

@@ -7,6 +7,10 @@ module Gitlab
         !signup_enabled?
       end
 
+      def signup_limited?
+        domain_allowlist.present? || email_restrictions_enabled? || require_admin_approval_after_user_signup?
+      end
+
       def current_application_settings
         Gitlab::SafeRequestStore.fetch(:current_application_settings) { ensure_application_settings! }
       end

spec/lib/gitlab/current_settings_spec.rb

@@ -24,6 +24,42 @@ RSpec.describe Gitlab::CurrentSettings do
     end
   end
 
+  describe '.signup_limited?' do
+    subject { described_class.signup_limited? }
+
+    context 'when there are allowed domains' do
+      before do
+        create(:application_setting, domain_allowlist: ['www.gitlab.com'])
+      end
+
+      it { is_expected.to be_truthy }
+    end
+
+    context 'when there are email restrictions' do
+      before do
+        create(:application_setting, email_restrictions_enabled: true)
+      end
+
+      it { is_expected.to be_truthy }
+    end
+
+    context 'when the admin has to approve signups' do
+      before do
+        create(:application_setting, require_admin_approval_after_user_signup: true)
+      end
+
+      it { is_expected.to be_truthy }
+    end
+
+    context 'when there are no restrictions' do
+      before do
+        create(:application_setting, domain_allowlist: [], email_restrictions_enabled: false, require_admin_approval_after_user_signup: false)
+      end
+
+      it { is_expected.to be_falsey }
+    end
+  end
+
   describe '.signup_disabled?' do
     subject { described_class.signup_disabled? }
 

spec/requests/api/lint_spec.rb

@@ -27,9 +27,10 @@ RSpec.describe API::Lint do
       end
     end
 
-    context 'when signup settings are enabled' do
+    context 'when signup is enabled and not limited' do
       before do
         Gitlab::CurrentSettings.signup_enabled = true
+        stub_application_setting(domain_allowlist: [], email_restrictions_enabled: false, require_admin_approval_after_user_signup: false)
       end
 
       context 'when unauthenticated' do
@@ -50,6 +51,31 @@ RSpec.describe API::Lint do
       end
     end
 
+    context 'when limited signup is enabled' do
+      before do
+        stub_application_setting(domain_allowlist: ['www.gitlab.com'])
+        Gitlab::CurrentSettings.signup_enabled = true
+      end
+
+      context 'when unauthenticated' do
+        it 'returns unauthorized' do
+          post api('/ci/lint'), params: { content: 'content' }
+
+          expect(response).to have_gitlab_http_status(:unauthorized)
+        end
+      end
+
+      context 'when authenticated' do
+        let_it_be(:api_user) { create(:user) }
+
+        it 'returns authentication success' do
+          post api('/ci/lint', api_user), params: { content: 'content' }
+
+          expect(response).to have_gitlab_http_status(:ok)
+        end
+      end
+    end
+
     context 'when authenticated' do
       let_it_be(:api_user) { create(:user) }