Validate Wiki attachments are valid temporary files

A malicious attacker could craft a request to read arbitrary files on
the system. This change adds a Grape validation to ensure that the
tempfile parameter delivered by the Rack multipart uploader is a
Tempfile type to prevent users from being able to specify arbitrary
filenames.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53072

changelogs/unreleased/sh-fix-wiki-security-issue-53072.yml

+---
+title: Validate Wiki attachments are valid temporary files
+merge_request:
+author:
+type: security

lib/api/validations/types/safe_file.rb

+# frozen_string_literal: true
+
+# This module overrides the Grape type validator defined in
+# https://github.com/ruby-grape/grape/blob/master/lib/grape/validations/types/file.rb
+module API
+  module Validations
+    module Types
+      class SafeFile < ::Grape::Validations::Types::File
+        def value_coerced?(value)
+          super && value[:tempfile].is_a?(Tempfile)
+        end
+      end
+    end
+  end
+end

lib/api/wikis.rb

@@ -6,7 +6,7 @@ module API
       def commit_params(attrs)
         {
           file_name: attrs[:file][:filename],
-          file_content: File.read(attrs[:file][:tempfile]),
+          file_content: attrs[:file][:tempfile].read,
           branch_name: attrs[:branch]
         }
       end
@@ -100,7 +100,7 @@ module API
         success Entities::WikiAttachment
       end
       params do
-        requires :file, type: File, desc: 'The attachment file to be uploaded'
+        requires :file, type: ::API::Validations::Types::SafeFile, desc: 'The attachment file to be uploaded'
         optional :branch, type: String, desc: 'The name of the branch'
       end
       post ":id/wikis/attachments", requirements: API::PROJECT_ENDPOINT_REQUIREMENTS do

spec/requests/api/wikis_spec.rb

@@ -158,6 +158,16 @@ describe API::Wikis do
       expect(json_response.size).to eq(1)
       expect(json_response['error']).to eq('file is missing')
     end
+
+    it 'responds with validation error on invalid temp file' do
+      payload[:file] = { tempfile: '/etc/hosts' }
+
+      post(api(url, user), payload)
+
+      expect(response).to have_gitlab_http_status(400)
+      expect(json_response.size).to eq(1)
+      expect(json_response['error']).to eq('file is invalid')
+    end
   end
 
   describe 'GET /projects/:id/wikis' do