Make CI refs matching to to use UntrustedRegexp

This makes ref validation to use always `UntrustedRegexp`.
This also splits the existing RubySyntax into separate
class.

changelogs/unreleased/use-untrusted-regexp.yml

+---
+title: Use UntrustedRegexp for matching refs policy
+merge_request:
+author:
+type: security

doc/ci/yaml/README.md

@@ -255,6 +255,19 @@ job:
     - branches
 ```
 
+Pattern matching is case-sensitive by default. Use `i` flag modifier, like
+`/pattern/i` to make a pattern case-insensitive:
+
+```yaml
+job:
+  # use regexp
+  only:
+    - /^issue-.*$/i
+  # use special keyword
+  except:
+    - branches
+```
+
 In this example, `job` will run only for refs that are tagged, or if a build is
 explicitly requested via an API trigger or a [Pipeline Schedule][schedules]:
 

lib/gitlab/ci/build/policy/refs.rb

@@ -35,8 +35,8 @@ module Gitlab
             # patterns can be matched only when branch or tag is used
             # the pattern matching does not work for merge requests pipelines
             if pipeline.branch? || pipeline.tag?
-              if pattern.first == "/" && pattern.last == "/"
-                Regexp.new(pattern[1...-1]) =~ pipeline.ref
+              if regexp = Gitlab::UntrustedRegexp::RubySyntax.fabricate(pattern)
+                regexp.match?(pipeline.ref)
               else
                 pattern == pipeline.ref
               end

lib/gitlab/ci/pipeline/expression/lexeme/pattern.rb

@@ -13,13 +13,13 @@ module Gitlab
             def initialize(regexp)
               @value = regexp
 
-              unless Gitlab::UntrustedRegexp.valid?(@value)
+              unless Gitlab::UntrustedRegexp::RubySyntax.valid?(@value)
                 raise Lexer::SyntaxError, 'Invalid regular expression!'
               end
             end
 
             def evaluate(variables = {})
-              Gitlab::UntrustedRegexp.fabricate(@value)
+              Gitlab::UntrustedRegexp::RubySyntax.fabricate!(@value)
             rescue RegexpError
               raise Expression::RuntimeError, 'Invalid regular expression!'
             end

lib/gitlab/config/entry/legacy_validation_helpers.rb

@@ -45,17 +45,15 @@ module Gitlab
         end
 
         def validate_regexp(value)
-          !value.nil? && Regexp.new(value.to_s) && true
-        rescue RegexpError, TypeError
-          false
+          Gitlab::UntrustedRegexp::RubySyntax.valid?(value)
         end
 
         def validate_string_or_regexp(value)
           return true if value.is_a?(Symbol)
           return false unless value.is_a?(String)
 
-          if value.first == '/' && value.last == '/'
-            validate_regexp(value[1...-1])
+          if Gitlab::UntrustedRegexp::RubySyntax.matches_syntax?(value)
+            validate_regexp(value)
           else
             true
           end

lib/gitlab/config/entry/validators.rb

@@ -120,17 +120,13 @@ module Gitlab
 
           private
 
-          def look_like_regexp?(value)
-            value.is_a?(String) && value.start_with?('/') &&
-              value.end_with?('/')
+          def matches_syntax?(value)
+            Gitlab::UntrustedRegexp::RubySyntax.matches_syntax?(value)
           end
 
           def validate_regexp(value)
-            look_like_regexp?(value) &&
-              Regexp.new(value.to_s[1...-1]) &&
-              true
-          rescue RegexpError
-            false
+            matches_syntax?(value) &&
+              Gitlab::UntrustedRegexp::RubySyntax.valid?(value)
           end
         end
 
@@ -149,7 +145,7 @@ module Gitlab
 
           def validate_string_or_regexp(value)
             return false unless value.is_a?(String)
-            return validate_regexp(value) if look_like_regexp?(value)
+            return validate_regexp(value) if matches_syntax?(value)
 
             true
           end

lib/gitlab/untrusted_regexp.rb

@@ -35,6 +35,10 @@ module Gitlab
       matches
     end
 
+    def match?(text)
+      text.present? && scan(text).present?
+    end
+
     def replace(text, rewrite)
       RE2.Replace(text, regexp, rewrite)
     end
@@ -43,37 +47,6 @@ module Gitlab
       self.source == other.source
     end
 
-    # Handles regular expressions with the preferred RE2 library where possible
-    # via UntustedRegex. Falls back to Ruby's built-in regular expression library
-    # when the syntax would be invalid in RE2.
-    #
-    # One difference between these is `(?m)` multi-line mode. Ruby regex enables
-    # this by default, but also handles `^` and `$` differently.
-    # See: https://www.regular-expressions.info/modifiers.html
-    def self.with_fallback(pattern, multiline: false)
-      UntrustedRegexp.new(pattern, multiline: multiline)
-    rescue RegexpError
-      Regexp.new(pattern)
-    end
-
-    def self.valid?(pattern)
-      !!self.fabricate(pattern)
-    rescue RegexpError
-      false
-    end
-
-    def self.fabricate(pattern)
-      matches = pattern.match(%r{^/(?<regexp>.+)/(?<flags>[ismU]*)$})
-
-      raise RegexpError, 'Invalid regular expression!' if matches.nil?
-
-      expression = matches[:regexp]
-      flags = matches[:flags]
-      expression.prepend("(?#{flags})") if flags.present?
-
-      self.new(expression, multiline: false)
-    end
-
     private
 
     attr_reader :regexp

lib/gitlab/untrusted_regexp/ruby_syntax.rb

+# frozen_string_literal: true
+
+module Gitlab
+  class UntrustedRegexp
+    # This class implements support for Ruby syntax of regexps
+    # and converts that to RE2 representation:
+    # /<regexp>/<flags>
+    class RubySyntax
+      PATTERN = %r{^/(?<regexp>.+)/(?<flags>[ismU]*)$}.freeze
+
+      # Checks if pattern matches a regexp pattern
+      # but does not enforce it's validity
+      def self.matches_syntax?(pattern)
+        pattern.is_a?(String) && pattern.match(PATTERN).present?
+      end
+
+      # The regexp can match the pattern `/.../`, but may not be fabricatable:
+      # it can be invalid or incomplete: `/match ( string/`
+      def self.valid?(pattern)
+        !!self.fabricate(pattern)
+      end
+
+      def self.fabricate(pattern)
+        self.fabricate!(pattern)
+      rescue RegexpError
+        nil
+      end
+
+      def self.fabricate!(pattern)
+        raise RegexpError, 'Pattern is not string!' unless pattern.is_a?(String)
+
+        matches = pattern.match(PATTERN)
+        raise RegexpError, 'Invalid regular expression!' if matches.nil?
+
+        expression = matches[:regexp]
+        flags = matches[:flags]
+        expression.prepend("(?#{flags})") if flags.present?
+
+        UntrustedRegexp.new(expression, multiline: false)
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/build/policy/refs_spec.rb

@@ -92,10 +92,23 @@ describe Gitlab::Ci::Build::Policy::Refs do
           .to be_satisfied_by(pipeline)
       end
 
+      it 'is satisfied when case-insensitive regexp matches pipeline ref' do
+        expect(described_class.new(['/DOCS-.*/i']))
+          .to be_satisfied_by(pipeline)
+      end
+
       it 'is not satisfied when regexp does not match pipeline ref' do
         expect(described_class.new(['/fix-.*/']))
           .not_to be_satisfied_by(pipeline)
       end
     end
+
+    context 'malicious regexp' do
+      let(:pipeline) { build_stubbed(:ci_pipeline, ref: malicious_text) }
+
+      subject { described_class.new([malicious_regexp_ruby]) }
+
+      include_examples 'malicious regexp'
+    end
   end
 end

spec/lib/gitlab/ci/pipeline/expression/lexeme/pattern_spec.rb

@@ -85,7 +85,7 @@ describe Gitlab::Ci::Pipeline::Expression::Lexeme::Pattern do
     end
 
     it 'raises error if evaluated regexp is not valid' do
-      allow(Gitlab::UntrustedRegexp).to receive(:valid?).and_return(true)
+      allow(Gitlab::UntrustedRegexp::RubySyntax).to receive(:valid?).and_return(true)
 
       regexp = described_class.new('/invalid ( .*/')
 

spec/lib/gitlab/ci/trace/stream_spec.rb

@@ -414,7 +414,7 @@ describe Gitlab::Ci::Trace::Stream, :clean_gitlab_redis_cache do
 
       context 'malicious regexp' do
         let(:data) { malicious_text }
-        let(:regex) { malicious_regexp }
+        let(:regex) { malicious_regexp_re2 }
 
         include_examples 'malicious regexp'
       end

spec/lib/gitlab/route_map_spec.rb

@@ -60,7 +60,7 @@ describe Gitlab::RouteMap do
 
       subject do
         map = described_class.new(<<-"MAP".strip_heredoc)
-        - source: '#{malicious_regexp}'
+        - source: '#{malicious_regexp_re2}'
           public: '/'
         MAP
 

spec/lib/gitlab/untrusted_regexp/ruby_syntax_spec.rb

+require 'fast_spec_helper'
+require 'support/shared_examples/malicious_regexp_shared_examples'
+
+describe Gitlab::UntrustedRegexp::RubySyntax do
+  describe '.matches_syntax?' do
+    it 'returns true if regexp is valid' do
+      expect(described_class.matches_syntax?('/some .* thing/'))
+        .to be true
+    end
+
+    it 'returns true if regexp is invalid, but resembles regexp' do
+      expect(described_class.matches_syntax?('/some ( thing/'))
+        .to be true
+    end
+  end
+
+  describe '.valid?' do
+    it 'returns true if regexp is valid' do
+      expect(described_class.valid?('/some .* thing/'))
+        .to be true
+    end
+
+    it 'returns false if regexp is invalid' do
+      expect(described_class.valid?('/some ( thing/'))
+        .to be false
+    end
+  end
+
+  describe '.fabricate' do
+    context 'when regexp is valid' do
+      it 'fabricates regexp without flags' do
+        expect(described_class.fabricate('/some .* thing/')).not_to be_nil
+      end
+    end
+
+    context 'when regexp is a raw pattern' do
+      it 'returns error' do
+        expect(described_class.fabricate('some .* thing')).to be_nil
+      end
+    end
+  end
+
+  describe '.fabricate!' do
+    context 'when regexp is using /regexp/ scheme with flags' do
+      it 'fabricates regexp with a single flag' do
+        regexp = described_class.fabricate!('/something/i')
+
+        expect(regexp).to eq Gitlab::UntrustedRegexp.new('(?i)something')
+        expect(regexp.scan('SOMETHING')).to be_one
+      end
+
+      it 'fabricates regexp with multiple flags' do
+        regexp = described_class.fabricate!('/something/im')
+
+        expect(regexp).to eq Gitlab::UntrustedRegexp.new('(?im)something')
+      end
+
+      it 'fabricates regexp without flags' do
+        regexp = described_class.fabricate!('/something/')
+
+        expect(regexp).to eq Gitlab::UntrustedRegexp.new('something')
+      end
+    end
+
+    context 'when regexp is a raw pattern' do
+      it 'raises an error' do
+        expect { described_class.fabricate!('some .* thing') }
+          .to raise_error(RegexpError)
+      end
+    end
+  end
+end

spec/lib/gitlab/untrusted_regexp_spec.rb

@@ -2,48 +2,6 @@ require 'fast_spec_helper'
 require 'support/shared_examples/malicious_regexp_shared_examples'
 
 describe Gitlab::UntrustedRegexp do
-  describe '.valid?' do
-    it 'returns true if regexp is valid' do
-      expect(described_class.valid?('/some ( thing/'))
-        .to be false
-    end
-
-    it 'returns true if regexp is invalid' do
-      expect(described_class.valid?('/some .* thing/'))
-        .to be true
-    end
-  end
-
-  describe '.fabricate' do
-    context 'when regexp is using /regexp/ scheme with flags' do
-      it 'fabricates regexp with a single flag' do
-        regexp = described_class.fabricate('/something/i')
-
-        expect(regexp).to eq described_class.new('(?i)something')
-        expect(regexp.scan('SOMETHING')).to be_one
-      end
-
-      it 'fabricates regexp with multiple flags' do
-        regexp = described_class.fabricate('/something/im')
-
-        expect(regexp).to eq described_class.new('(?im)something')
-      end
-
-      it 'fabricates regexp without flags' do
-        regexp = described_class.fabricate('/something/')
-
-        expect(regexp).to eq described_class.new('something')
-      end
-    end
-
-    context 'when regexp is a raw pattern' do
-      it 'raises an error' do
-        expect { described_class.fabricate('some .* thing') }
-          .to raise_error(RegexpError)
-      end
-    end
-  end
-
   describe '#initialize' do
     subject { described_class.new(pattern) }
 
@@ -92,11 +50,41 @@ describe Gitlab::UntrustedRegexp do
     end
   end
 
+  describe '#match?' do
+    subject { described_class.new(regexp).match?(text) }
+
+    context 'malicious regexp' do
+      let(:text) { malicious_text }
+      let(:regexp) { malicious_regexp_re2 }
+
+      include_examples 'malicious regexp'
+    end
+
+    context 'matching regexp' do
+      let(:regexp) { 'foo' }
+      let(:text) { 'foo' }
+
+      it 'returns an array of nil matches' do
+        is_expected.to eq(true)
+      end
+    end
+
+    context 'non-matching regexp' do
+      let(:regexp) { 'boo' }
+      let(:text) { 'foo' }
+
+      it 'returns an array of nil matches' do
+        is_expected.to eq(false)
+      end
+    end
+  end
+
   describe '#scan' do
     subject { described_class.new(regexp).scan(text) }
+
     context 'malicious regexp' do
       let(:text) { malicious_text }
-      let(:regexp) { malicious_regexp }
+      let(:regexp) { malicious_regexp_re2 }
 
       include_examples 'malicious regexp'
     end

spec/support/shared_examples/malicious_regexp_shared_examples.rb

@@ -2,7 +2,8 @@ require 'timeout'
 
 shared_examples 'malicious regexp' do
   let(:malicious_text) { 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa!' }
-  let(:malicious_regexp) { '(?i)^(([a-z])+.)+[A-Z]([a-z])+$' }
+  let(:malicious_regexp_re2) { '(?i)^(([a-z])+.)+[A-Z]([a-z])+$' }
+  let(:malicious_regexp_ruby) { '/^(([a-z])+.)+[A-Z]([a-z])+$/i' }
 
   it 'takes under a second' do
     expect { Timeout.timeout(1) { subject } }.not_to raise_error