Fix synchronous recursive fetching of deeply-nested quoted posts (#35600)

app/lib/activitypub/activity/create.rb

@@ -206,8 +206,8 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     @quote.save
 
     embedded_quote = safe_prefetched_embed(@account, @status_parser.quoted_object, @json['context'])
-    ActivityPub::VerifyQuoteService.new.call(@quote, fetchable_quoted_uri: @quote_uri, prefetched_quoted_object: embedded_quote, request_id: @options[:request_id])
-  rescue Mastodon::UnexpectedResponseError, *Mastodon::HTTP_CONNECTION_ERRORS
+    ActivityPub::VerifyQuoteService.new.call(@quote, fetchable_quoted_uri: @quote_uri, prefetched_quoted_object: embedded_quote, request_id: @options[:request_id], depth: @options[:depth])
+  rescue Mastodon::RecursionLimitExceededError, Mastodon::UnexpectedResponseError, *Mastodon::HTTP_CONNECTION_ERRORS
     ActivityPub::RefetchAndVerifyQuoteWorker.perform_in(rand(30..600).seconds, @quote.id, @quote_uri, { 'request_id' => @options[:request_id] })
   end
 

app/services/activitypub/fetch_remote_status_service.rb

@@ -8,9 +8,10 @@ class ActivityPub::FetchRemoteStatusService < BaseService
   DISCOVERIES_PER_REQUEST = 1000
 
   # Should be called when uri has already been checked for locality
-  def call(uri, prefetched_body: nil, on_behalf_of: nil, expected_actor_uri: nil, request_id: nil)
+  def call(uri, prefetched_body: nil, on_behalf_of: nil, expected_actor_uri: nil, request_id: nil, depth: nil)
     return if domain_not_allowed?(uri)
 
+    @depth = depth || 0
     @request_id = request_id || "#{Time.now.utc.to_i}-status-#{uri}"
     @json = if prefetched_body.nil?
               fetch_status(uri, true, on_behalf_of)
@@ -52,7 +53,7 @@ class ActivityPub::FetchRemoteStatusService < BaseService
       return nil if discoveries > DISCOVERIES_PER_REQUEST
     end
 
-    ActivityPub::Activity.factory(activity_json, actor, request_id: @request_id).perform
+    ActivityPub::Activity.factory(activity_json, actor, request_id: @request_id, depth: @depth).perform
   end
 
   private

app/services/activitypub/verify_quote_service.rb

@@ -3,9 +3,12 @@
 class ActivityPub::VerifyQuoteService < BaseService
   include JsonLdHelper
 
+  MAX_SYNCHRONOUS_DEPTH = 2
+
   # Optionally fetch quoted post, and verify the quote is authorized
-  def call(quote, fetchable_quoted_uri: nil, prefetched_quoted_object: nil, prefetched_approval: nil, request_id: nil)
+  def call(quote, fetchable_quoted_uri: nil, prefetched_quoted_object: nil, prefetched_approval: nil, request_id: nil, depth: nil)
     @request_id = request_id
+    @depth = depth || 0
     @quote = quote
     @fetching_error = nil
 
@@ -72,10 +75,12 @@ class ActivityPub::VerifyQuoteService < BaseService
     return if uri.nil? || @quote.quoted_status.present?
 
     status = ActivityPub::TagManager.instance.uri_to_resource(uri, Status)
-    status ||= ActivityPub::FetchRemoteStatusService.new.call(uri, on_behalf_of: @quote.account.followers.local.first, prefetched_body:, request_id: @request_id)
+    raise Mastodon::RecursionLimitExceededError if @depth > MAX_SYNCHRONOUS_DEPTH && status.nil?
+
+    status ||= ActivityPub::FetchRemoteStatusService.new.call(uri, on_behalf_of: @quote.account.followers.local.first, prefetched_body:, request_id: @request_id, depth: @depth + 1)
 
     @quote.update(quoted_status: status) if status.present?
-  rescue Mastodon::UnexpectedResponseError, *Mastodon::HTTP_CONNECTION_ERRORS => e
+  rescue Mastodon::RecursionLimitExceededError, Mastodon::UnexpectedResponseError, *Mastodon::HTTP_CONNECTION_ERRORS => e
     @fetching_error = e
   end
 
@@ -90,7 +95,7 @@ class ActivityPub::VerifyQuoteService < BaseService
     # It's not safe to fetch if the inlined object is cross-origin or doesn't match expectations
     return if object['id'] != uri || non_matching_uri_hosts?(@quote.approval_uri, object['id'])
 
-    status = ActivityPub::FetchRemoteStatusService.new.call(object['id'], prefetched_body: object, on_behalf_of: @quote.account.followers.local.first, request_id: @request_id)
+    status = ActivityPub::FetchRemoteStatusService.new.call(object['id'], prefetched_body: object, on_behalf_of: @quote.account.followers.local.first, request_id: @request_id, depth: @depth)
 
     if status.present?
       @quote.update(quoted_status: status)

lib/exceptions.rb

@@ -14,6 +14,7 @@ module Mastodon
   class InvalidParameterError < Error; end
   class SignatureVerificationError < Error; end
   class MalformedHeaderError < Error; end
+  class RecursionLimitExceededError < Error; end
 
   class UnexpectedResponseError < Error
     attr_reader :response