Add DOS safeguard in replay retry algorithm

daphne/src/roles/helper.rs

@@ -218,7 +218,16 @@ pub trait DapHelper<S>: DapAggregator<S> {
  // won't happen often enough that it matters.
  let (out_shares_count, agg_job_resp) = {
  let mut replayed_reports = HashSet::new();
+ let mut retry_count = 3;
  loop {
+ if retry_count < 1 {
+ // we need to prevent an attacker from keeping this loop running for too long,
+ // potentialy enabling an DOS attack.
+ return Err(DapAbort::BadRequest(
+ "AggregationJobContinueReq contained too many replays".into(),
+ ));
+ }
+ retry_count -= 1;
  let (agg_share_span, agg_job_resp) = task_config.vdaf.handle_agg_job_cont_req(
  task_id,
  task_config,