From 240caa32b9cab90a38ab863fd64e6becf5d1393c Mon Sep 17 00:00:00 2001 From: Evan Hunt Date: Thu, 25 May 2023 23:53:50 -0700 Subject: [PATCH] Stale answer lookups could loop when over recursion quota When a query was aborted because of the recursion quota being exceeded, but triggered a stale answer response and a stale data refresh query, it could cause named to loop back where we are iterating and following a delegation. Having no good answer in cache, we would fall back to using serve-stale again, use the stale data, try to refresh the RRset, and loop back again, without ever terminating until crashing due to stack overflow. This happens because in the functions 'query_notfound()' and 'query_delegation_recurse()', we check whether we can fall back to serving stale data. We shouldn't do so if we are already refreshing an RRset due to having prioritized stale data in cache. In other words, we need to add an extra check to 'query_usestale()' to disallow serving stale data if we are currently refreshing a stale RRset. As an additional mitigation to prevent looping, we now use the result code ISC_R_ALREADYRUNNING rather than ISC_R_FAILURE when a recursion loop is encountered, and we check for that condition in 'query_usestale()' as well. --- lib/ns/query.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-) diff --git a/lib/ns/query.c b/lib/ns/query.c index 1fab263a35..9fa8ee0f9a 100644 --- a/lib/ns/query.c +++ b/lib/ns/query.c @@ -6533,7 +6533,7 @@ ns_query_recurse(ns_client_t *client, dns_rdatatype_t qtype, dns_name_t *qname, if (recparam_match(&client->query.recparam, qtype, qname, qdomain)) { ns_client_log(client, NS_LOGCATEGORY_CLIENT, NS_LOGMODULE_QUERY, ISC_LOG_INFO, "recursion loop detected"); - return (ISC_R_FAILURE); + return (ISC_R_ALREADYRUNNING); } recparam_update(&client->query.recparam, qtype, qname, qdomain); @@ -7650,10 +7650,21 @@ query_usestale(query_ctx_t *qctx, isc_result_t result) { return (false); } - if (result == DNS_R_DUPLICATE || result == DNS_R_DROP) { + if (qctx->refresh_rrset) { + /* + * This is a refreshing query, we have already prioritized + * stale data, so don't enable serve-stale again. + */ + return (false); + } + + if (result == DNS_R_DUPLICATE || result == DNS_R_DROP || + result == ISC_R_ALREADYRUNNING) + { /* * Don't enable serve-stale if the result signals a duplicate - * query or query that is being dropped. + * query or a query that is being dropped or can't proceed + * because of a recursion loop. */ return (false); }