Validate session key when authorizing with GCP to create a cluster

It was previously possible to link a GCP account to another
user's GitLab account by having them visit the callback URL,
as there was no check that they were the initiator of the
request.

We now reject the callback unless the state parameter
matches the one added to the initiating user's session.

1 files changed, 21 insertions, 11 deletions

diff --git a/app/controllers/google_api/authorizations_controller.rb b/app/controllers/google_api/authorizations_controller.rb
index dd9f5af61b3..ed0995e7ffd 100644
--- a/app/controllers/google_api/authorizations_controller.rb
+++ b/app/controllers/google_api/authorizations_controller.rb
@@ -2,6 +2,10 @@
 
 module GoogleApi
   class AuthorizationsController < ApplicationController
+    include Gitlab::Utils::StrongMemoize
+
+    before_action :validate_session_key!
+
     def callback
       token, expires_at = GoogleApi::CloudPlatform::Client
         .new(nil, callback_google_api_auth_url)
@@ -11,21 +15,27 @@ module GoogleApi
       session[GoogleApi::CloudPlatform::Client.session_key_for_expires_at] =
         expires_at.to_s
 
-      state_redirect_uri = redirect_uri_from_session_key(params[:state])
-
-      if state_redirect_uri
-        redirect_to state_redirect_uri
-      else
-        redirect_to root_path
-      end
+      redirect_to redirect_uri_from_session
     end
 
     private
 
-    def redirect_uri_from_session_key(state)
-      key = GoogleApi::CloudPlatform::Client
-        .session_key_for_redirect_uri(params[:state])
-      session[key] if key
+    def validate_session_key!
+      access_denied! unless redirect_uri_from_session.present?
+    end
+
+    def redirect_uri_from_session
+      strong_memoize(:redirect_uri_from_session) do
+        if params[:state].present?
+          session[session_key_for_redirect_uri(params[:state])]
+        else
+          nil
+        end
+      end
+    end
+
+    def session_key_for_redirect_uri(state)
+      GoogleApi::CloudPlatform::Client.session_key_for_redirect_uri(state)
     end
   end
 end