Fix symlink vulnerability in project import

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49133

changelogs/unreleased/sh-fix-issue-49133.yml

+---
+title: Fix symlink vulnerability in project import
+merge_request:
+author:
+type: security

lib/gitlab/import_export/file_importer.rb

@@ -4,6 +4,7 @@ module Gitlab
       include Gitlab::ImportExport::CommandLineUtil
 
       MAX_RETRIES = 8
+      WHITELISTED_FILENAMES = %w(. ..).freeze
 
       def self.import(*args)
         new(*args).import
@@ -59,7 +60,7 @@ module Gitlab
       end
 
       def extracted_files
-        Dir.glob("#{@shared.export_path}/**/*", File::FNM_DOTMATCH).reject { |f| f =~ %r{.*/\.{1,2}$} }
+        Dir.glob("#{@shared.export_path}/**/*", File::FNM_DOTMATCH).reject { |f| WHITELISTED_FILENAMES.include?(File.basename(f)) }
       end
     end
   end

spec/lib/gitlab/import_export/file_importer_spec.rb

@@ -7,6 +7,7 @@ describe Gitlab::ImportExport::FileImporter do
   let(:symlink_file) { "#{shared.export_path}/invalid.json" }
   let(:hidden_symlink_file) { "#{shared.export_path}/.hidden" }
   let(:subfolder_symlink_file) { "#{shared.export_path}/subfolder/invalid.json" }
+  let(:evil_symlink_file) { "#{shared.export_path}/.\nevil" }
 
   before do
     stub_const('Gitlab::ImportExport::FileImporter::MAX_RETRIES', 0)
@@ -34,6 +35,10 @@ describe Gitlab::ImportExport::FileImporter do
       expect(File.exist?(hidden_symlink_file)).to be false
     end
 
+    it 'removes evil symlinks in root folder' do
+      expect(File.exist?(evil_symlink_file)).to be false
+    end
+
     it 'removes symlinks in subfolders' do
       expect(File.exist?(subfolder_symlink_file)).to be false
     end
@@ -75,5 +80,7 @@ describe Gitlab::ImportExport::FileImporter do
     FileUtils.touch(valid_file)
     FileUtils.ln_s(valid_file, symlink_file)
     FileUtils.ln_s(valid_file, subfolder_symlink_file)
+    FileUtils.ln_s(valid_file, hidden_symlink_file)
+    FileUtils.ln_s(valid_file, evil_symlink_file)
   end
 end