Add support for Content-Security-Policy

A nonce-based Content-Security-Policy thwarts XSS attacks by allowing
inline JavaScript to execute if the script nonce matches the header
value. Rails 5.2 supports nonce-based Content-Security-Policy headers,
so provide configuration to enable this and make it work.

To support this, we need to change all `:javascript` HAML filters to the
following form:

```
= javascript_tag nonce: true do
  :plain
    ...
```

We use `%script` throughout our HAML to store JSON and other text, but
since this doesn't execute, browsers don't appear to block this content
from being used and require the nonce value to be present.

1 files changed, 43 insertions, 0 deletions

diff --git a/lib/gitlab/content_security_policy/config_loader.rb b/lib/gitlab/content_security_policy/config_loader.rb
new file mode 100644
index 00000000000..b2f3345d33a
--- /dev/null
+++ b/lib/gitlab/content_security_policy/config_loader.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module ContentSecurityPolicy
+    class ConfigLoader
+      DIRECTIVES = %w(base_uri child_src connect_src default_src font_src
+                      form_action frame_ancestors frame_src img_src manifest_src
+                      media_src object_src script_src style_src worker_src).freeze
+
+      def self.default_settings_hash
+        {
+          'enabled' => false,
+          'report_only' => false,
+          'directives' => DIRECTIVES.each_with_object({}) { |directive, hash| hash[directive] = nil }
+        }
+      end
+
+      def initialize(csp_directives)
+        @csp_directives = HashWithIndifferentAccess.new(csp_directives)
+      end
+
+      def load(policy)
+        DIRECTIVES.each do |directive|
+          arguments = arguments_for(directive)
+
+          next unless arguments.present?
+
+          policy.public_send(directive, *arguments) # rubocop:disable GitlabSecurity/PublicSend
+        end
+      end
+
+      private
+
+      def arguments_for(directive)
+        arguments = @csp_directives[directive.to_s]
+
+        return unless arguments.present? && arguments.is_a?(String)
+
+        arguments.strip.split(' ').map(&:strip)
+      end
+    end
+  end
+end