Allow to use untrusted Regexp via feature flag

This brings support for untrusted regexp for 'only:refs:' when
enabled via feature flag: alllow_unsafe_ruby_regexp.

This is by default disabled, and should not be used in production

4 files changed, 47 insertions, 14 deletions

diff --git a/lib/gitlab/ci/build/policy/refs.rb b/lib/gitlab/ci/build/policy/refs.rb
index 360424bec11..c3005303fd8 100644
--- a/lib/gitlab/ci/build/policy/refs.rb
+++ b/lib/gitlab/ci/build/policy/refs.rb
@@ -35,7 +35,7 @@ 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 regexp = Gitlab::UntrustedRegexp::RubySyntax.fabricate(pattern)
+              if regexp = Gitlab::UntrustedRegexp::RubySyntax.fabricate(pattern, fallback: true)
                 regexp.match?(pipeline.ref)
               else
                 pattern == pipeline.ref
diff --git a/lib/gitlab/ci/config/entry/policy.rb b/lib/gitlab/ci/config/entry/policy.rb
index adc3660d950..7b14218d3ea 100644
--- a/lib/gitlab/ci/config/entry/policy.rb
+++ b/lib/gitlab/ci/config/entry/policy.rb
@@ -17,7 +17,7 @@ module Gitlab
             include ::Gitlab::Config::Entry::Validatable
 
             validations do
-              validates :config, array_of_strings_or_regexps: true
+              validates :config, array_of_strings_or_regexps_with_fallback: true
             end
 
             def value
@@ -38,7 +38,7 @@ module Gitlab
               validate :variables_expressions_syntax
 
               with_options allow_nil: true do
-                validates :refs, array_of_strings_or_regexps: true
+                validates :refs, array_of_strings_or_regexps_with_fallback: true
                 validates :kubernetes, allowed_values: %w[active]
                 validates :variables, array_of_strings: true
                 validates :changes, array_of_strings: true
diff --git a/lib/gitlab/config/entry/validators.rb b/lib/gitlab/config/entry/validators.rb
index d0ee94370ba..746fe83f90f 100644
--- a/lib/gitlab/config/entry/validators.rb
+++ b/lib/gitlab/config/entry/validators.rb
@@ -129,6 +129,12 @@ module Gitlab
             end
           end
 
+          protected
+
+          def fallback
+            false
+          end
+
           private
 
           def matches_syntax?(value)
@@ -137,7 +143,7 @@ module Gitlab
 
           def validate_regexp(value)
             matches_syntax?(value) &&
-              Gitlab::UntrustedRegexp::RubySyntax.valid?(value)
+              Gitlab::UntrustedRegexp::RubySyntax.valid?(value, fallback: fallback)
           end
         end
 
@@ -162,6 +168,14 @@ module Gitlab
           end
         end
 
+        class ArrayOfStringsOrRegexpsWithFallbackValidator < ArrayOfStringsOrRegexpsValidator
+          protected
+
+          def fallback
+            true
+          end
+        end
+
         class ArrayOfStringsOrStringValidator < RegexpValidator
           def validate_each(record, attribute, value)
             unless validate_array_of_strings_or_string(value)
diff --git a/lib/gitlab/untrusted_regexp/ruby_syntax.rb b/lib/gitlab/untrusted_regexp/ruby_syntax.rb
index 91f300f97d0..6adf119aa75 100644
--- a/lib/gitlab/untrusted_regexp/ruby_syntax.rb
+++ b/lib/gitlab/untrusted_regexp/ruby_syntax.rb
@@ -6,7 +6,7 @@ module Gitlab
     # and converts that to RE2 representation:
     # /<regexp>/<flags>
     class RubySyntax
-      PATTERN = %r{^/(?<regexp>.+)/(?<flags>[ismU]*)$}.freeze
+      PATTERN = %r{^/(?<regexp>.*)/(?<flags>[ismU]*)$}.freeze
 
       # Checks if pattern matches a regexp pattern
       # but does not enforce it's validity
@@ -16,28 +16,47 @@ module Gitlab
 
       # 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)
+      def self.valid?(pattern, fallback: false)
+        !!self.fabricate(pattern, fallback: fallback)
       end
 
-      def self.fabricate(pattern)
-        self.fabricate!(pattern)
+      def self.fabricate(pattern, fallback: false)
+        self.fabricate!(pattern, fallback: fallback)
       rescue RegexpError
         nil
       end
 
-      def self.fabricate!(pattern)
+      def self.fabricate!(pattern, fallback: false)
         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?
+        begin
+          create_untrusted_regexp(matches[:regexp], matches[:flags])
+        rescue RegexpError
+          raise unless fallback &&
+              Feature.enabled?(:allow_unsafe_ruby_regexp, default_enabled: false)
 
-        UntrustedRegexp.new(expression, multiline: false)
+          create_ruby_regexp(matches[:regexp], matches[:flags])
+        end
       end
+
+      def self.create_untrusted_regexp(pattern, flags)
+        pattern.prepend("(?#{flags})") if flags.present?
+
+        UntrustedRegexp.new(pattern, multiline: false)
+      end
+      private_class_method :create_untrusted_regexp
+
+      def self.create_ruby_regexp(pattern, flags)
+        options = 0
+        options += Regexp::IGNORECASE if flags&.include?('i')
+        options += Regexp::MULTILINE if flags&.include?('m')
+
+        Regexp.new(pattern, options)
+      end
+      private_class_method :create_ruby_regexp
     end
   end
 end