Extract GitLab Pages using RubyZip

RubyZip allows us to perform strong validation of
expanded paths where we do extract file.

We introduce the following additional checks
to extract routines:

1. None of path components can be symlinked,
2. We drop privileges support for directories,
3. Symlink source needs to point within the target directory,
   like `public/`,
4. The symlink source needs to exist ahead of time.

3 files changed, 206 insertions, 0 deletions

diff --git a/lib/safe_zip/entry.rb b/lib/safe_zip/entry.rb
new file mode 100644
index 00000000000..664e2f52f91
--- /dev/null
+++ b/lib/safe_zip/entry.rb
@@ -0,0 +1,97 @@
+# frozen_string_literal: true
+
+module SafeZip
+  class Entry
+    attr_reader :zip_archive, :zip_entry
+    attr_reader :path, :params
+
+    def initialize(zip_archive, zip_entry, params)
+      @zip_archive = zip_archive
+      @zip_entry = zip_entry
+      @params = params
+      @path = ::File.expand_path(zip_entry.name, params.extract_path)
+    end
+
+    def path_dir
+      ::File.dirname(path)
+    end
+
+    def real_path_dir
+      ::File.realpath(path_dir)
+    end
+
+    def exist?
+      ::File.exist?(path)
+    end
+
+    def extract
+      # do not extract if file is not part of target directory
+      return false unless matching_target_directory
+
+      # do not overwrite existing file
+      raise SafeZip::Extract::AlreadyExistsError, "File already exists #{zip_entry.name}" if exist?
+
+      create_path_dir
+
+      if zip_entry.file?
+        extract_file
+      elsif zip_entry.directory?
+        extract_dir
+      elsif zip_entry.symlink?
+        extract_symlink
+      else
+        raise SafeZip::Extract::UnsupportedEntryError, "File #{zip_entry.name} cannot be extracted"
+      end
+    rescue SafeZip::Extract::Error
+      raise
+    rescue => e
+      raise SafeZip::Extract::ExtractError, e.message
+    end
+
+    private
+
+    def extract_file
+      zip_archive.extract(zip_entry, path)
+    end
+
+    def extract_dir
+      FileUtils.mkdir(path)
+    end
+
+    def extract_symlink
+      source_path = read_symlink
+      real_source_path = expand_symlink(source_path)
+
+      # ensure that source path of symlink is within target directories
+      unless real_source_path.start_with?(matching_target_directory)
+        raise SafeZip::Extract::PermissionDeniedError, "Symlink cannot be created targeting: #{source_path}"
+      end
+
+      ::File.symlink(source_path, path)
+    end
+
+    def create_path_dir
+      # Create all directories, but ignore permissions
+      FileUtils.mkdir_p(path_dir)
+
+      # disallow to make path dirs to point to another directories
+      unless path_dir == real_path_dir
+        raise SafeZip::Extract::PermissionDeniedError, "Directory of #{zip_entry.name} points to another directory"
+      end
+    end
+
+    def matching_target_directory
+      params.matching_target_directory(path)
+    end
+
+    def read_symlink
+      zip_archive.read(zip_entry)
+    end
+
+    def expand_symlink(source_path)
+      ::File.realpath(source_path, path_dir)
+    rescue
+      raise SafeZip::Extract::SymlinkSourceDoesNotExistError, "Symlink source #{source_path} does not exist"
+    end
+  end
+end
diff --git a/lib/safe_zip/extract.rb b/lib/safe_zip/extract.rb
new file mode 100644
index 00000000000..3bd4935ef34
--- /dev/null
+++ b/lib/safe_zip/extract.rb
@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+module SafeZip
+  class Extract
+    Error = Class.new(StandardError)
+    PermissionDeniedError = Class.new(Error)
+    SymlinkSourceDoesNotExistError = Class.new(Error)
+    UnsupportedEntryError = Class.new(Error)
+    AlreadyExistsError = Class.new(Error)
+    NoMatchingError = Class.new(Error)
+    ExtractError = Class.new(Error)
+
+    attr_reader :archive_path
+
+    def initialize(archive_file)
+      @archive_path = archive_file
+    end
+
+    def extract(opts = {})
+      params = SafeZip::ExtractParams.new(**opts)
+
+      if Feature.enabled?(:safezip_use_rubyzip, default_enabled: true)
+        extract_with_ruby_zip(params)
+      else
+        legacy_unsafe_extract_with_system_zip(params)
+      end
+    end
+
+    private
+
+    def extract_with_ruby_zip(params)
+      Zip::File.open(archive_path) do |zip_archive|
+        # Extract all files in the following order:
+        # 1. Directories first,
+        # 2. Files next,
+        # 3. Symlinks last (or anything else)
+        extracted = extract_all_entries(zip_archive, params,
+          zip_archive.lazy.select(&:directory?))
+
+        extracted += extract_all_entries(zip_archive, params,
+          zip_archive.lazy.select(&:file?))
+
+        extracted += extract_all_entries(zip_archive, params,
+          zip_archive.lazy.reject(&:directory?).reject(&:file?))
+
+        raise NoMatchingError, 'No entries extracted' unless extracted > 0
+      end
+    end
+
+    def extract_all_entries(zip_archive, params, entries)
+      entries.count do |zip_entry|
+        SafeZip::Entry.new(zip_archive, zip_entry, params)
+          .extract
+      end
+    end
+
+    def legacy_unsafe_extract_with_system_zip(params)
+      # Requires UnZip at least 6.00 Info-ZIP.
+      # -n  never overwrite existing files
+      args = %W(unzip -n -qq #{archive_path})
+
+      # We add * to end of directory, because we want to extract directory and all subdirectories
+      args += params.directories_wildcard
+
+      # Target directory where we extract
+      args += %W(-d #{params.extract_path})
+
+      unless system(*args)
+        raise Error, 'archive failed to extract'
+      end
+    end
+  end
+end
diff --git a/lib/safe_zip/extract_params.rb b/lib/safe_zip/extract_params.rb
new file mode 100644
index 00000000000..bd3b788bac9
--- /dev/null
+++ b/lib/safe_zip/extract_params.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module SafeZip
+  class ExtractParams
+    include Gitlab::Utils::StrongMemoize
+
+    attr_reader :directories, :extract_path
+
+    def initialize(directories:, to:)
+      @directories = directories
+      @extract_path = ::File.realpath(to)
+    end
+
+    def matching_target_directory(path)
+      target_directories.find do |directory|
+        path.start_with?(directory)
+      end
+    end
+
+    def target_directories
+      strong_memoize(:target_directories) do
+        directories.map do |directory|
+          ::File.join(::File.expand_path(directory, extract_path), '')
+        end
+      end
+    end
+
+    def directories_wildcard
+      strong_memoize(:directories_wildcard) do
+        directories.map do |directory|
+          ::File.join(directory, '*')
+        end
+      end
+    end
+  end
+end