Only assign merge params when allowed

When a user updates a merge request coming from a fork, they should
not be able to set `force_remove_source_branch` if they cannot push
code to the source project.

Otherwise developers of the target project could remove the source
branch of the source project by setting this flag through the API.

app/models/merge_request.rb

@@ -69,6 +69,14 @@ class MergeRequest < ApplicationRecord
   has_many :merge_request_assignees
   has_many :assignees, class_name: "User", through: :merge_request_assignees
 
+  KNOWN_MERGE_PARAMS = [
+    :auto_merge_strategy,
+    :should_remove_source_branch,
+    :force_remove_source_branch,
+    :commit_message,
+    :squash_commit_message,
+    :sha
+  ].freeze
   serialize :merge_params, Hash # rubocop:disable Cop/ActiveRecordSerialize
 
   after_create :ensure_merge_request_diff

app/services/auto_merge/base_service.rb

@@ -3,12 +3,13 @@
 module AutoMerge
   class BaseService < ::BaseService
     include Gitlab::Utils::StrongMemoize
+    include MergeRequests::AssignsMergeParams
 
     def execute(merge_request)
-      merge_request.merge_params.merge!(params)
+      assign_allowed_merge_params(merge_request, params.merge(auto_merge_strategy: strategy))
+
       merge_request.auto_merge_enabled = true
       merge_request.merge_user = current_user
-      merge_request.auto_merge_strategy = strategy
 
       return :failed unless merge_request.save
 
@@ -21,7 +22,7 @@ module AutoMerge
     end
 
     def update(merge_request)
-      merge_request.merge_params.merge!(params)
+      assign_allowed_merge_params(merge_request, params.merge(auto_merge_strategy: strategy))
 
       return :failed unless merge_request.save
 

app/services/concerns/merge_requests/assigns_merge_params.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module MergeRequests
+  module AssignsMergeParams
+    def self.included(klass)
+      raise "#{self} can not be included in #{klass} without implementing #current_user" unless klass.method_defined?(:current_user)
+    end
+
+    def assign_allowed_merge_params(merge_request, merge_params)
+      known_merge_params = merge_params.to_h.with_indifferent_access.slice(*MergeRequest::KNOWN_MERGE_PARAMS)
+
+      # Not checking `MergeRequest#can_remove_source_branch` as that includes
+      # other checks that aren't needed here.
+      known_merge_params.delete(:force_remove_source_branch) unless current_user.can?(:push_code, merge_request.source_project)
+
+      merge_request.merge_params.merge!(known_merge_params)
+
+      # Delete the known params now that they're assigned, so we don't try to
+      # assign them through an `#assign_attributes` later.
+      # They could be coming in as strings or symbols
+      merge_params.to_h.with_indifferent_access.except!(*MergeRequest::KNOWN_MERGE_PARAMS)
+    end
+  end
+end

app/services/merge_requests/base_service.rb

@@ -2,6 +2,8 @@
 
 module MergeRequests
   class BaseService < ::IssuableBaseService
+    include MergeRequests::AssignsMergeParams
+
     def create_note(merge_request, state = merge_request.state)
       SystemNoteService.change_status(merge_request, merge_request.target_project, current_user, state, nil)
     end
@@ -29,6 +31,18 @@ module MergeRequests
 
     private
 
+    def create(merge_request)
+      self.params = assign_allowed_merge_params(merge_request, params)
+
+      super
+    end
+
+    def update(merge_request)
+      self.params = assign_allowed_merge_params(merge_request, params)
+
+      super
+    end
+
     def handle_wip_event(merge_request)
       if wip_event = params.delete(:wip_event)
         # We update the title that is provided in the params or we use the mr title

app/services/merge_requests/build_service.rb

@@ -24,6 +24,8 @@ module MergeRequests
           merge_request.source_project.remove_source_branch_after_merge?
         end
 
+      self.params = assign_allowed_merge_params(merge_request, params)
+
       filter_params(merge_request)
 
       # merge_request.assign_attributes(...) below is a Rails

app/services/merge_requests/create_service.rb

@@ -9,7 +9,6 @@ module MergeRequests
       merge_request.target_project = @project
       merge_request.source_project = @source_project
       merge_request.source_branch = params[:source_branch]
-      merge_request.merge_params['force_remove_source_branch'] = params.delete(:force_remove_source_branch)
 
       create(merge_request)
     end

app/services/merge_requests/update_service.rb

@@ -16,10 +16,6 @@ module MergeRequests
         params.delete(:force_remove_source_branch)
       end
 
-      if params.has_key?(:force_remove_source_branch)
-        merge_request.merge_params['force_remove_source_branch'] = params.delete(:force_remove_source_branch)
-      end
-
       handle_wip_event(merge_request)
       update_task_event(merge_request) || update(merge_request)
     end

changelogs/unreleased/security-bvl-validate-force-remove-branch-on-mrs.yml

@@ -0,0 +1,6 @@
+---
+title: Don't allow maintainers of a target project to delete the source branch of
+  a merge request from a fork
+merge_request:
+author:
+type: security

spec/requests/api/merge_requests_spec.rb

@@ -1737,6 +1737,38 @@ describe API::MergeRequests do
         expect(json_response['state']).to eq('closed')
         expect(json_response['force_remove_source_branch']).to be_truthy
       end
+
+      context 'with a merge request across forks' do
+        let(:fork_owner) { create(:user) }
+        let(:source_project) { fork_project(project, fork_owner) }
+        let(:target_project) { project }
+
+        let(:merge_request) do
+          create(:merge_request,
+                 source_project: source_project,
+                 target_project: target_project,
+                 source_branch: 'fixes',
+                 merge_params: { 'force_remove_source_branch' => false })
+        end
+
+        it 'is true for an authorized user' do
+          put api("/projects/#{target_project.id}/merge_requests/#{merge_request.iid}", fork_owner), params: { state_event: 'close', remove_source_branch: true }
+
+          expect(response).to have_gitlab_http_status(200)
+          expect(json_response['state']).to eq('closed')
+          expect(json_response['force_remove_source_branch']).to be true
+        end
+
+        it 'is false for an unauthorized user' do
+          expect do
+            put api("/projects/#{target_project.id}/merge_requests/#{merge_request.iid}", target_project.owner), params: { state_event: 'close', remove_source_branch: true }
+          end.not_to change { merge_request.reload.merge_params }
+
+          expect(response).to have_gitlab_http_status(200)
+          expect(json_response['state']).to eq('closed')
+          expect(json_response['force_remove_source_branch']).to be false
+        end
+      end
     end
 
     context "to close a MR" do

spec/services/auto_merge/base_service_spec.rb

@@ -51,7 +51,7 @@ describe AutoMerge::BaseService do
         expect(merge_request.merge_params['commit_message']).to eq("Merge branch 'patch-12' into 'master'")
         expect(merge_request.merge_params['sha']).to eq('200fcc9c260f7219eaf0daba87d818f0922c5b18')
         expect(merge_request.merge_params['should_remove_source_branch']).to eq(false)
-        expect(merge_request.merge_params['squash']).to eq(false)
+        expect(merge_request.squash).to eq(false)
         expect(merge_request.merge_params['squash_commit_message']).to eq('Update README.md')
       end
     end
@@ -108,7 +108,6 @@ describe AutoMerge::BaseService do
           'commit_message' => "Merge branch 'patch-12' into 'master'",
           'sha' => "200fcc9c260f7219eaf0daba87d818f0922c5b18",
           'should_remove_source_branch' => false,
-          'squash' => false,
           'squash_commit_message' => "Update README.md"
         }
       end

spec/services/auto_merge/merge_when_pipeline_succeeds_service_spec.rb

@@ -59,8 +59,9 @@ describe AutoMerge::MergeWhenPipelineSucceedsService do
       it 'sets the params, merge_user, and flag' do
         expect(merge_request).to be_valid
         expect(merge_request.merge_when_pipeline_succeeds).to be_truthy
-        expect(merge_request.merge_params).to include commit_message: 'Awesome message'
+        expect(merge_request.merge_params).to include 'commit_message' => 'Awesome message'
         expect(merge_request.merge_user).to be user
+        expect(merge_request.auto_merge_strategy).to eq AutoMergeService::STRATEGY_MERGE_WHEN_PIPELINE_SUCCEEDS
       end
 
       it 'creates a system note' do
@@ -73,7 +74,7 @@ describe AutoMerge::MergeWhenPipelineSucceedsService do
     end
 
     context 'already approved' do
-      let(:service) { described_class.new(project, user, new_key: true) }
+      let(:service) { described_class.new(project, user, should_remove_source_branch: true) }
       let(:build)   { create(:ci_build, ref: mr_merge_if_green_enabled.source_branch) }
 
       before do
@@ -90,7 +91,7 @@ describe AutoMerge::MergeWhenPipelineSucceedsService do
         expect(SystemNoteService).not_to receive(:merge_when_pipeline_succeeds)
 
         service.execute(mr_merge_if_green_enabled)
-        expect(mr_merge_if_green_enabled.merge_params).to have_key(:new_key)
+        expect(mr_merge_if_green_enabled.merge_params).to have_key('should_remove_source_branch')
       end
     end
   end

spec/services/concerns/merge_requests/assigns_merge_params_spec.rb

@@ -0,0 +1,70 @@
+require 'spec_helper'
+
+describe MergeRequests::AssignsMergeParams do
+  it 'raises an error when used from an instance that does not respond to #current_user' do
+    define_class = -> { Class.new { include MergeRequests::AssignsMergeParams }.new }
+
+    expect { define_class.call }.to raise_error %r{can not be included in (.*) without implementing #current_user}
+  end
+
+  describe '#assign_allowed_merge_params' do
+    let(:merge_request) { build(:merge_request) }
+
+    let(:params) do
+      { commit_message: 'Commit Message',
+        'should_remove_source_branch' => true,
+        unknown_symbol: 'Unknown symbol',
+        'unknown_string' => 'Unknown String' }
+    end
+
+    subject(:merge_request_service) do
+      Class.new do
+        attr_accessor :current_user
+
+        include MergeRequests::AssignsMergeParams
+
+        def initialize(user)
+          @current_user = user
+        end
+      end
+    end
+
+    it 'only assigns known parameters to the merge request' do
+      service = merge_request_service.new(merge_request.author)
+
+      service.assign_allowed_merge_params(merge_request, params)
+
+      expect(merge_request.merge_params).to eq('commit_message' => 'Commit Message', 'should_remove_source_branch' => true)
+    end
+
+    it 'returns a hash without the known merge params' do
+      service = merge_request_service.new(merge_request.author)
+
+      result = service.assign_allowed_merge_params(merge_request, params)
+
+      expect(result).to eq({ 'unknown_symbol' => 'Unknown symbol', 'unknown_string' => 'Unknown String' })
+    end
+
+    context 'the force_remove_source_branch param' do
+      let(:params) { { force_remove_source_branch: true } }
+
+      it 'assigns the param if the user is allowed to do that' do
+        service = merge_request_service.new(merge_request.author)
+
+        result = service.assign_allowed_merge_params(merge_request, params)
+
+        expect(merge_request.force_remove_source_branch?).to be true
+        expect(result).to be_empty
+      end
+
+      it 'only removes the param if the user is not allowed to do that' do
+        service = merge_request_service.new(build(:user))
+
+        result = service.assign_allowed_merge_params(merge_request, params)
+
+        expect(merge_request.force_remove_source_branch?).to be_falsy
+        expect(result).to be_empty
+      end
+    end
+  end
+end

spec/services/merge_requests/build_service_spec.rb

@@ -83,8 +83,9 @@ describe MergeRequests::BuildService do
       expect(merge_request.force_remove_source_branch?).to be_truthy
     end
 
-    context 'with force_remove_source_branch parameter' do
+    context 'with force_remove_source_branch parameter when the user is authorized' do
       let(:mr_params) { params.merge(force_remove_source_branch: '1') }
+      let(:source_project) { fork_project(project, user) }
       let(:merge_request) { described_class.new(project, user, mr_params).execute }
 
       it 'assigns force_remove_source_branch' do

spec/services/merge_requests/update_service_spec.rb

@@ -646,5 +646,29 @@ describe MergeRequests::UpdateService, :mailer do
         expect(merge_request.allow_collaboration).to be_truthy
       end
     end
+
+    context 'updating `force_remove_source_branch`' do
+      let(:target_project) { create(:project, :repository, :public) }
+      let(:source_project) { fork_project(target_project, nil, repository: true) }
+      let(:user) { target_project.owner }
+      let(:merge_request) do
+        create(:merge_request,
+               source_project: source_project,
+               source_branch: 'fixes',
+               target_project: target_project)
+      end
+
+      it "cannot be done by members of the target project when they don't have access" do
+        expect { update_merge_request(force_remove_source_branch: true) }
+          .not_to change { merge_request.reload.force_remove_source_branch? }.from(nil)
+      end
+
+      it 'can be done by members of the target project if they can push to the source project' do
+        source_project.add_developer(user)
+
+        expect { update_merge_request(force_remove_source_branch: true) }
+          .to change { merge_request.reload.force_remove_source_branch? }.from(nil).to(true)
+      end
+    end
   end
 end