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.

1 files changed, 73 insertions, 0 deletions

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