InstAccess Tokens should prefer shard-local users

closes FOO-2368

TEST PLAN:
  1) have a user-merge occur x-shard
  2) visit an api-gateway mediated pathway
     like account notifications
  3) correct shard-local user should always be loaded
     as auth context

Change-Id: I2dfb86ec35499e9a00ebb8498c4eab9c6c95297e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/273451
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>

lib/authentication_methods.rb

@@ -42,34 +42,19 @@ module AuthenticationMethods
   end
 
   def load_pseudonym_from_inst_access_token(token_string)
-    return false unless InstAccess::Token.is_token?(token_string)
+    token = ::AuthenticationMethods::InstAccessToken.parse(token_string)
+    return false unless token
 
-    begin
-      token = InstAccess::Token.from_token_string(token_string)
-    rescue InstAccess::InvalidToken, # token didn't pass signature verification
-           InstAccess::TokenExpired # token passed signature verification, but is expired
-      raise AccessTokenError
-    rescue InstAccess::ConfigError => exception
-      # InstAccess isn't configured. A human should fix that, but this method
-      # should recover gracefully.
-      Canvas::Errors.capture_exception(:inst_access, exception, :warn)
-      return true
-    end
+    auth_context = ::AuthenticationMethods::InstAccessToken.load_user_and_pseudonym_context(token, @domain_root_account)
 
-    @current_user = User.find_by(uuid: token.user_uuid)
-    @current_pseudonym = SisPseudonym.for(
-      @current_user, @domain_root_account, type: :implicit, require_sis: false
-    )
+    @current_user = auth_context[:current_user]
+    @current_pseudonym = auth_context[:current_pseudonym]
     raise AccessTokenError unless @current_user && @current_pseudonym
 
-    if token.masquerading_user_uuid && token.masquerading_user_shard_id
-      Shard.lookup(token.masquerading_user_shard_id).activate do
-        @real_current_user = User.find_by!(uuid: token.masquerading_user_uuid)
-        @real_current_pseudonym = SisPseudonym.for(
-          @real_current_user, @domain_root_account, type: :implicit, require_sis: false
-        )
-        logger.warn "[AUTH] #{@real_current_user.name}(#{@real_current_user.id}) impersonating #{@current_user.name} on page #{request.url}"
-      end
+    if auth_context[:real_current_user]
+      @real_current_user = auth_context[:real_current_user]
+      @real_current_pseudonym = auth_context[:real_current_pseudonym]
+      logger.warn "[AUTH] #{@real_current_user.name}(#{@real_current_user.id}) impersonating #{@current_user.name} on page #{request.url}"
     end
     @authenticated_with_jwt = @authenticated_with_inst_access_token = true
   end

lib/authentication_methods/inst_access_token.rb

@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+#
+# Copyright (C) 2021 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+module AuthenticationMethods
+  # this module bridges the gap between the token
+  # defined in the canvas_security gem and the
+  # canvas domain itself (users, pseudonyms, accounts, etc)
+  module InstAccessToken
+
+    # given a POTENTIAL token string, this will validate
+    # it as being an InstAccess token and return
+    # the token ruby object.
+    #
+    # A 'false' indicates this is not a token at all and we can skip
+    # any other attempts at token processing.
+    #
+    # an error (AccessTokenError) means that this IS an inst access token, but
+    # not a valid one (expired or bad signature) and processing should only
+    # continue on the assumption that this is an invalid request.
+    def self.parse(token_string)
+      return false unless InstAccess::Token.is_token?(token_string)
+
+      begin
+        token = InstAccess::Token.from_token_string(token_string)
+        return token
+      rescue InstAccess::InvalidToken, # token didn't pass signature verification
+            InstAccess::TokenExpired # token passed signature verification, but is expired
+        raise AccessTokenError
+      rescue InstAccess::ConfigError => exception
+        # InstAccess isn't configured. A human should fix that, but this method
+        # should recover gracefully.
+        Canvas::Errors.capture_exception(:inst_access, exception, :warn)
+        return false
+      end
+    end
+
+    # functionally encapsulates mapping an InstAccess token and a domain root account
+    # to a user/pseudonym.  This is out on it's own because there are some db-state
+    # edge cases (like multiple users with the same UUID due to user merges, etc)
+    # that are convenient to test close to the implementation.
+    #
+    # the hash this method returns is defined up front with the intention
+    # that the masquerading keys will only have their values populated if the token contains these values
+    def self.load_user_and_pseudonym_context(token, domain_root_account)
+      auth_context = {
+        current_user: nil,
+        current_pseudonym: nil,
+        real_current_user: nil,
+        real_current_pseudonym: nil
+      }
+      auth_context[:current_user] = find_user_by_uuid_prefer_local(token.user_uuid)
+      auth_context[:current_pseudonym] = SisPseudonym.for(
+        auth_context[:current_user], domain_root_account, type: :implicit, require_sis: false
+      )
+      return auth_context unless auth_context[:current_user] && auth_context[:current_pseudonym]
+
+      if token.masquerading_user_uuid && token.masquerading_user_shard_id
+        Shard.lookup(token.masquerading_user_shard_id).activate do
+          real_user = find_user_by_uuid_prefer_local(token.masquerading_user_uuid)
+          raise AccessTokenError, "masquerading user not found" unless real_user
+          auth_context[:real_current_user] = real_user
+          auth_context[:real_current_pseudonym] = SisPseudonym.for(
+            real_user, domain_root_account, type: :implicit, require_sis: false
+          )
+        end
+      end
+      return auth_context
+    end
+
+    # generally users should not share uuids.
+    # this is just to make sure that when a shadow
+    # user or similar exists, the local user
+    # gets preferred.
+    def self.find_user_by_uuid_prefer_local(uuid)
+      User.where(uuid: uuid).order(:id).first
+    end
+    private_class_method :find_user_by_uuid_prefer_local
+  end
+end

spec/lib/authentication_methods/inst_access_token_spec.rb

@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+#
+# Copyright (C) 2021 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+
+require_relative '../../sharding_spec_helper'
+
+describe AuthenticationMethods::InstAccessToken do
+  let(:signing_keypair) { OpenSSL::PKey::RSA.new(2048) }
+  let(:encryption_keypair) { OpenSSL::PKey::RSA.new(2048) }
+  let(:signing_priv_key) { signing_keypair.to_s }
+  let(:signing_pub_key) { signing_keypair.public_key.to_s }
+  let(:encryption_priv_key) { encryption_keypair.to_s }
+  let(:encryption_pub_key) { encryption_keypair.public_key.to_s }
+
+  around(:each) do |example|
+    InstAccess.with_config(signing_key: signing_priv_key) do
+      example.run
+    end
+  end
+
+  describe ".parse" do
+    it "is false for bad tokens" do
+      result = ::AuthenticationMethods::InstAccessToken.parse("not-a-token")
+      expect(result).to be_falsey
+    end
+
+    it "returns a token object for good tokens" do
+      token_obj = ::InstAccess::Token.for_user(user_uuid: 'fake-user-uuid', account_uuid: 'fake-acct-uuid')
+      result = ::AuthenticationMethods::InstAccessToken.parse(token_obj.to_unencrypted_token_string)
+      expect(result.user_uuid).to eq('fake-user-uuid')
+    end
+  end
+
+  describe ".load_user_and_pseudonym_context" do
+    it "finds the user who created the token" do
+      account = Account.default
+      user_with_pseudonym(:active_all => true)
+      token_obj = ::InstAccess::Token.for_user(user_uuid: @user.uuid, account_uuid: account.uuid)
+      ctx = ::AuthenticationMethods::InstAccessToken.load_user_and_pseudonym_context(token_obj, account)
+      expect(ctx[:current_user]).to eq(@user)
+      expect(ctx[:current_pseudonym]).to eq(@pseudonym)
+    end
+
+    it "chooses the local user when a local and shadow user share the same UUID" do
+      user_model(id: (10_000_000_000_000 + (rand*10000000).to_i), uuid: 'a-shared-uuid-between-users')
+      account = Account.default
+      user_with_pseudonym(:active_all => true)
+      @user.uuid = 'a-shared-uuid-between-users'
+      @user.save!
+      token_obj = ::InstAccess::Token.for_user(user_uuid: 'a-shared-uuid-between-users', account_uuid: account.uuid)
+      ctx = ::AuthenticationMethods::InstAccessToken.load_user_and_pseudonym_context(token_obj, account)
+      expect(ctx[:current_user]).to eq(@user)
+      expect(ctx[:current_pseudonym]).to eq(@pseudonym)
+    end
+  end
+end