Prevent Billion Laughs attack

It keeps track of the memory being used when loading the YAML file
as well as the depth of nesting.
Track exception when YAML is too big

changelogs/unreleased/security-fp-prevent-billion-laughs-attack.yml

+---
+title: Prevent Billion Laughs attack
+merge_request:
+author:
+type: security

lib/gitlab/ci/config.rb

@@ -15,6 +15,9 @@ module Gitlab
         @global = Entry::Global.new(@config)
         @global.compose!
+      rescue Gitlab::Config::Loader::Yaml::DataTooLargeError => e
+        Gitlab::Sentry.track_exception(e, extra: { user: user.inspect, project: project.inspect })
+        raise Config::ConfigError, e.message
       rescue Gitlab::Config::Loader::FormatError,
              Extendable::ExtensionError,
              External::Processor::IncludeError => e

lib/gitlab/config/loader/yaml.rb

@@ -4,6 +4,13 @@ module Gitlab
   module Config
     module Loader
       class Yaml
+        DataTooLargeError = Class.new(Loader::FormatError)
+
+        include Gitlab::Utils::StrongMemoize
+
+        MAX_YAML_SIZE = 1.megabyte
+        MAX_YAML_DEPTH = 100
+
         def initialize(config)
           @config = YAML.safe_load(config, [Symbol], [], true)
         rescue Psych::Exception => e
@@ -11,16 +18,35 @@ module Gitlab
         end
         def valid?
-          @config.is_a?(Hash)
+          hash? && !too_big?
         end
         def load!
-          unless valid?
-            raise Loader::FormatError, 'Invalid configuration format'
-          end
+          raise DataTooLargeError, 'The parsed YAML is too big' if too_big?
+          raise Loader::FormatError, 'Invalid configuration format' unless hash?
           @config.deep_symbolize_keys
         end
+
+        private
+
+        def hash?
+          @config.is_a?(Hash)
+        end
+
+        def too_big?
+          return false unless Feature.enabled?(:ci_yaml_limit_size, default_enabled: true)
+
+          !deep_size.valid?
+        end
+
+        def deep_size
+          strong_memoize(:deep_size) do
+            Gitlab::Utils::DeepSize.new(@config,
+              max_size: MAX_YAML_SIZE,
+              max_depth: MAX_YAML_DEPTH)
+          end
+        end
       end
     end
   end

lib/gitlab/utils/deep_size.rb

+# frozen_string_literal: true
+
+require 'objspace'
+
+module Gitlab
+  module Utils
+    class DeepSize
+      Error = Class.new(StandardError)
+      TooMuchDataError = Class.new(Error)
+
+      DEFAULT_MAX_SIZE = 1.megabyte
+      DEFAULT_MAX_DEPTH = 100
+
+      def initialize(root, max_size: DEFAULT_MAX_SIZE, max_depth: DEFAULT_MAX_DEPTH)
+        @root = root
+        @max_size = max_size
+        @max_depth = max_depth
+        @size = 0
+        @depth = 0
+
+        evaluate
+      end
+
+      def valid?
+        !too_big? && !too_deep?
+      end
+
+      private
+
+      def evaluate
+        add_object(@root)
+      rescue Error
+        # NOOP
+      end
+
+      def too_big?
+        @size > @max_size
+      end
+
+      def too_deep?
+        @depth > @max_depth
+      end
+
+      def add_object(object)
+        @size += ObjectSpace.memsize_of(object)
+        raise TooMuchDataError if @size > @max_size
+
+        add_array(object) if object.is_a?(Array)
+        add_hash(object) if object.is_a?(Hash)
+      end
+
+      def add_array(object)
+        with_nesting do
+          object.each do |n|
+            add_object(n)
+          end
+        end
+      end
+
+      def add_hash(object)
+        with_nesting do
+          object.each do |key, value|
+            add_object(key)
+            add_object(value)
+          end
+        end
+      end
+
+      def with_nesting
+        @depth += 1
+        raise TooMuchDataError if too_deep?
+
+        yield
+
+        @depth -= 1
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/config_spec.rb

@@ -90,6 +90,27 @@ describe Gitlab::Ci::Config do
       end
     end
+    context 'when yml is too big' do
+      let(:yml) do
+        <<~YAML
+          --- &1
+          - hi
+          - *1
+        YAML
+      end
+
+      describe '.new' do
+        it 'raises error' do
+          expect(Gitlab::Sentry).to receive(:track_exception)
+
+          expect { config }.to raise_error(
+            described_class::ConfigError,
+            /The parsed YAML is too big/
+          )
+        end
+      end
+    end
+
     context 'when config logic is incorrect' do
       let(:yml) { 'before_script: "ls"' }

spec/lib/gitlab/config/loader/yaml_spec.rb

@@ -8,7 +8,7 @@ describe Gitlab::Config::Loader::Yaml do
     describe '#valid?' do
       it 'returns true' do
-        expect(loader.valid?).to be true
+        expect(loader).to be_valid
       end
     end
@@ -24,7 +24,7 @@ describe Gitlab::Config::Loader::Yaml do
     describe '#valid?' do
       it 'returns false' do
-        expect(loader.valid?).to be false
+        expect(loader).not_to be_valid
       end
     end
@@ -43,7 +43,10 @@ describe Gitlab::Config::Loader::Yaml do
     describe '#initialize' do
       it 'raises FormatError' do
-        expect { loader }.to raise_error(Gitlab::Config::Loader::FormatError, 'Unknown alias: bad_alias')
+        expect { loader }.to raise_error(
+          Gitlab::Config::Loader::FormatError,
+          'Unknown alias: bad_alias'
+        )
       end
     end
   end
@@ -53,7 +56,68 @@ describe Gitlab::Config::Loader::Yaml do
     describe '#valid?' do
       it 'returns false' do
-        expect(loader.valid?).to be false
+        expect(loader).not_to be_valid
+      end
+    end
+  end
+
+  # Prevent Billion Laughs attack: https://gitlab.com/gitlab-org/gitlab-ce/issues/56018
+  context 'when yaml size is too large' do
+    let(:yml) do
+      <<~YAML
+        a: &a ["lol","lol","lol","lol","lol","lol","lol","lol","lol"]
+        b: &b [*a,*a,*a,*a,*a,*a,*a,*a,*a]
+        c: &c [*b,*b,*b,*b,*b,*b,*b,*b,*b]
+        d: &d [*c,*c,*c,*c,*c,*c,*c,*c,*c]
+        e: &e [*d,*d,*d,*d,*d,*d,*d,*d,*d]
+        f: &f [*e,*e,*e,*e,*e,*e,*e,*e,*e]
+        g: &g [*f,*f,*f,*f,*f,*f,*f,*f,*f]
+        h: &h [*g,*g,*g,*g,*g,*g,*g,*g,*g]
+        i: &i [*h,*h,*h,*h,*h,*h,*h,*h,*h]
+      YAML
+    end
+
+    describe '#valid?' do
+      it 'returns false' do
+        expect(loader).not_to be_valid
+      end
+
+      it 'returns true if "ci_yaml_limit_size" feature flag is disabled' do
+        stub_feature_flags(ci_yaml_limit_size: false)
+
+        expect(loader).to be_valid
+      end
+    end
+
+    describe '#load!' do
+      it 'raises FormatError' do
+        expect { loader.load! }.to raise_error(
+          Gitlab::Config::Loader::FormatError,
+          'The parsed YAML is too big'
+        )
+      end
+    end
+  end
+
+  # Prevent Billion Laughs attack: https://gitlab.com/gitlab-org/gitlab-ce/issues/56018
+  context 'when yaml has cyclic data structure' do
+    let(:yml) do
+      <<~YAML
+        --- &1
+        - hi
+        - *1
+      YAML
+    end
+
+    describe '#valid?' do
+      it 'returns false' do
+        expect(loader.valid?).to be(false)
+      end
+    end
+
+    describe '#load!' do
+      it 'raises FormatError' do
+        expect { loader.load! }.to raise_error(Gitlab::Config::Loader::FormatError, 'The parsed YAML is too big')
       end
     end
   end

spec/lib/gitlab/utils/deep_size_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Utils::DeepSize do
+  let(:data) do
+    {
+      a: [1, 2, 3],
+      b: {
+        c: [4, 5],
+        d: [
+          { e: [[6], [7]] }
+        ]
+      }
+    }
+  end
+
+  let(:max_size) { 1.kilobyte }
+  let(:max_depth) { 10 }
+  let(:deep_size) { described_class.new(data, max_size: max_size, max_depth: max_depth) }
+
+  describe '#evaluate' do
+    context 'when data within size and depth limits' do
+      it 'returns true' do
+        expect(deep_size).to be_valid
+      end
+    end
+
+    context 'when data not within size limit' do
+      let(:max_size) { 200.bytes }
+
+      it 'returns false' do
+        expect(deep_size).not_to be_valid
+      end
+    end
+
+    context 'when data not within depth limit' do
+      let(:max_depth) { 2 }
+
+      it 'returns false' do
+        expect(deep_size).not_to be_valid
+      end
+    end
+  end
+end