Validate bundle files before unpacking them

changelogs/unreleased/security-2770-verify-bundle-import-files.yml

+---
+title: Validate bundle files before unpacking them
+merge_request:
+author:
+type: security

lib/gitlab/git/bundle_file.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Git
+    class BundleFile
+      # All git bundle files start with this string
+      #
+      # https://github.com/git/git/blob/v2.20.1/bundle.c#L15
+      MAGIC = "# v2 git bundle\n"
+
+      InvalidBundleError = Class.new(StandardError)
+
+      attr_reader :filename
+
+      def self.check!(filename)
+        new(filename).check!
+      end
+
+      def initialize(filename)
+        @filename = filename
+      end
+
+      def check!
+        data = File.open(filename, 'r') { |f| f.read(MAGIC.size) }
+
+        raise InvalidBundleError, 'Invalid bundle file' unless data == MAGIC
+      end
+    end
+  end
+end

lib/gitlab/git/repository.rb

@@ -789,6 +789,11 @@ module Gitlab
       end
 
       def create_from_bundle(bundle_path)
+        # It's important to check that the linked-to file is actually a valid
+        # .bundle file as it is passed to `git clone`, which may otherwise
+        # interpret it as a pointer to another repository
+        ::Gitlab::Git::BundleFile.check!(bundle_path)
+
         gitaly_repository_client.create_from_bundle(bundle_path)
       end
 

spec/fixtures/malicious.bundle

+gitdir: foo.git

spec/lib/gitlab/git/bundle_file_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Git::BundleFile do
+  describe '.check!' do
+    let(:valid_bundle) { Tempfile.new }
+    let(:valid_bundle_path) { valid_bundle.path }
+    let(:invalid_bundle_path) { Rails.root.join('spec/fixtures/malicious.bundle') }
+
+    after do
+      valid_bundle.close!
+    end
+
+    it 'returns nil for a valid bundle' do
+      valid_bundle.write("# v2 git bundle\nfoo bar baz\n")
+      valid_bundle.close
+
+      expect(described_class.check!(valid_bundle_path)).to be_nil
+    end
+
+    it 'raises an exception for an invalid bundle' do
+      expect do
+        described_class.check!(invalid_bundle_path)
+      end.to raise_error(described_class::InvalidBundleError)
+    end
+  end
+end

spec/lib/gitlab/git/repository_spec.rb

@@ -1753,22 +1753,23 @@ describe Gitlab::Git::Repository, :seed_helper do
   end
 
   describe '#create_from_bundle' do
-    let(:bundle_path) { File.join(Dir.tmpdir, "repo-#{SecureRandom.hex}.bundle") }
+    let(:valid_bundle_path) { File.join(Dir.tmpdir, "repo-#{SecureRandom.hex}.bundle") }
+    let(:malicious_bundle_path) { Rails.root.join('spec/fixtures/malicious.bundle') }
     let(:project) { create(:project) }
     let(:imported_repo) { project.repository.raw }
 
     before do
-      expect(repository.bundle_to_disk(bundle_path)).to be_truthy
+      expect(repository.bundle_to_disk(valid_bundle_path)).to be_truthy
     end
 
     after do
-      FileUtils.rm_rf(bundle_path)
+      FileUtils.rm_rf(valid_bundle_path)
     end
 
     it 'creates a repo from a bundle file' do
       expect(imported_repo).not_to exist
 
-      result = imported_repo.create_from_bundle(bundle_path)
+      result = imported_repo.create_from_bundle(valid_bundle_path)
 
       expect(result).to be_truthy
       expect(imported_repo).to exist
@@ -1776,11 +1777,17 @@ describe Gitlab::Git::Repository, :seed_helper do
     end
 
     it 'creates a symlink to the global hooks dir' do
-      imported_repo.create_from_bundle(bundle_path)
+      imported_repo.create_from_bundle(valid_bundle_path)
       hooks_path = Gitlab::GitalyClient::StorageSettings.allow_disk_access { File.join(imported_repo.path, 'hooks') }
 
       expect(File.readlink(hooks_path)).to eq(Gitlab.config.gitlab_shell.hooks_path)
     end
+
+    it 'raises an error if the bundle is an attempted malicious payload' do
+      expect do
+        imported_repo.create_from_bundle(malicious_bundle_path)
+      end.to raise_error(::Gitlab::Git::BundleFile::InvalidBundleError)
+    end
   end
 
   describe '#checksum' do