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.

1 files changed, 24 insertions, 0 deletions

diff --git a/app/services/concerns/merge_requests/assigns_merge_params.rb b/app/services/concerns/merge_requests/assigns_merge_params.rb
new file mode 100644
index 00000000000..bd870d9a1e7
--- /dev/null
+++ b/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