Improve logging when Origin header doesn't match

I came up against this while dealing with a misconfigured server. The
browser was setting the Origin header to "https://example.com", but the
Rails app returned "http://example.com" from request.base_url (because
it was failing to detect that HTTPS was used).

This caused verify_authenticity_token to fail, but the message in the
log was "Can't verify CSRF token", which is confusing because the
failure had nothing to do with the CSRF token sent in the request. This
made it very hard to identify the issue, so hopefully this will make it
more obvious for the next person.

actionpack/lib/action_controller/metal/request_forgery_protection.rb

@@ -213,7 +213,11 @@ def verify_authenticity_token # :doc:
 
         if !verified_request?
           if logger && log_warning_on_csrf_failure
-            logger.warn "Can't verify CSRF token authenticity."
+            if valid_request_origin?
+              logger.warn "Can't verify CSRF token authenticity."
+            else
+              logger.warn "HTTP Origin header (#{request.origin}) didn't match request.base_url (#{request.base_url})"
+            end
           end
           handle_unverified_request
         end

actionpack/test/controller/request_forgery_protection_test.rb

@@ -347,6 +347,10 @@ def test_should_allow_post_with_origin_checking_and_no_origin
   end
 
   def test_should_block_post_with_origin_checking_and_wrong_origin
+    old_logger = ActionController::Base.logger
+    logger = ActiveSupport::LogSubscriber::TestHelper::MockLogger.new
+    ActionController::Base.logger = logger
+
     forgery_protection_origin_check do
       session[:_csrf_token] = @token
       @controller.stub :form_authenticity_token, @token do
@@ -356,6 +360,13 @@ def test_should_block_post_with_origin_checking_and_wrong_origin
         end
       end
     end
+
+    assert_match(
+      "HTTP Origin header (http://bad.host) didn't match request.base_url (http://test.host)",
+      logger.logged(:warn).last
+    )
+  ensure
+    ActionController::Base.logger = old_logger
   end
 
   def test_should_warn_on_missing_csrf_token