Use MergeRequest#source_project as permissions reference for MergeRequest#all_pipelines

MergeRequest#all_pipelines fetches Ci::Pipeline records from the source
project, so we should specifically check that project for permissions.
This was already happening for intra-project merge requests, but in the
event that the target and source projects both have private builds, we
should ensure that the project permissions are respected.

app/controllers/projects/merge_requests/application_controller.rb

@@ -41,7 +41,7 @@ class Projects::MergeRequests::ApplicationController < Projects::ApplicationCont
   def set_pipeline_variables
     @pipelines =
-      if can?(current_user, :read_pipeline, @project)
+      if can?(current_user, :read_pipeline, @merge_request.source_project)
         @merge_request.all_pipelines
       else
         Ci::Pipeline.none

app/controllers/projects/merge_requests_controller.rb

@@ -82,7 +82,8 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   end
   def pipelines
-    @pipelines = @merge_request.all_pipelines.page(params[:page]).per(30)
+    set_pipeline_variables
+    @pipelines = @pipelines.page(params[:page]).per(30)
     Gitlab::PollingInterval.set_header(response, interval: 10_000)

changelogs/unreleased/security-mr-pipeline-permissions.yml

+---
+title: Use source project as permissions reference for MergeRequestsController#pipelines
+merge_request:
+author:
+type: security

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -598,10 +598,100 @@ describe Projects::MergeRequestsController do
           format: :json
     end
-    it 'responds with serialized pipelines' do
-      expect(json_response['pipelines']).not_to be_empty
-      expect(json_response['count']['all']).to eq 1
-      expect(response).to include_pagination_headers
+    context 'with "enabled" builds on a public project' do
+      let(:project) { create(:project, :repository, :public) }
+
+      context 'for a project owner' do
+        it 'responds with serialized pipelines' do
+          expect(json_response['pipelines']).to be_present
+          expect(json_response['count']['all']).to eq(1)
+          expect(response).to include_pagination_headers
+        end
+      end
+
+      context 'for an unassociated user' do
+        let(:user) { create :user }
+
+        it 'responds with no pipelines' do
+          expect(json_response['pipelines']).to be_present
+          expect(json_response['count']['all']).to eq(1)
+          expect(response).to include_pagination_headers
+        end
+      end
+    end
+
+    context 'with private builds on a public project' do
+      let(:project) { create(:project, :repository, :public, :builds_private) }
+
+      context 'for a project owner' do
+        it 'responds with serialized pipelines' do
+          expect(json_response['pipelines']).to be_present
+          expect(json_response['count']['all']).to eq(1)
+          expect(response).to include_pagination_headers
+        end
+      end
+
+      context 'for an unassociated user' do
+        let(:user) { create :user }
+
+        it 'responds with no pipelines' do
+          expect(json_response['pipelines']).to be_empty
+          expect(json_response['count']['all']).to eq(0)
+          expect(response).to include_pagination_headers
+        end
+      end
+
+      context 'from a project fork' do
+        let(:fork_user)      { create :user }
+        let(:forked_project) { fork_project(project, fork_user, repository: true) } # Forked project carries over :builds_private
+        let(:merge_request)  { create(:merge_request_with_diffs, target_project: project, source_project: forked_project) }
+
+        context 'with private builds' do
+          context 'for the target project member' do
+            it 'does not respond with serialized pipelines' do
+              expect(json_response['pipelines']).to be_empty
+              expect(json_response['count']['all']).to eq(0)
+              expect(response).to include_pagination_headers
+            end
+          end
+
+          context 'for the source project member' do
+            let(:user) { fork_user }
+
+            it 'responds with serialized pipelines' do
+              expect(json_response['pipelines']).to be_present
+              expect(json_response['count']['all']).to eq(1)
+              expect(response).to include_pagination_headers
+            end
+          end
+        end
+
+        context 'with public builds' do
+          let(:forked_project) do
+            fork_project(project, fork_user, repository: true).tap do |new_project|
+              new_project.project_feature.update(builds_access_level: ProjectFeature::ENABLED)
+            end
+          end
+
+          context 'for the target project member' do
+            it 'does not respond with serialized pipelines' do
+              expect(json_response['pipelines']).to be_present
+              expect(json_response['count']['all']).to eq(1)
+              expect(response).to include_pagination_headers
+            end
+          end
+
+          context 'for the source project member' do
+            let(:user) { fork_user }
+
+            it 'responds with serialized pipelines' do
+              expect(json_response['pipelines']).to be_present
+              expect(json_response['count']['all']).to eq(1)
+              expect(response).to include_pagination_headers
+            end
+          end
+        end
+      end
     end
   end