From 240caa32b9cab90a38ab863fd64e6becf5d1393c Mon Sep 17 00:00:00 2001
From: Evan Hunt <each@isc.org>
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 | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/lib/ns/query.c b/lib/ns/query.c
index 1444de5..3ce6742 100644
--- a/lib/ns/query.c
+++ b/lib/ns/query.c
@@ -5696,6 +5696,7 @@ query_refresh_rrset(query_ctx_t *orig_qctx) {
 	qctx.client->query.dboptions &= ~(DNS_DBFIND_STALETIMEOUT |
 					  DNS_DBFIND_STALEOK |
 					  DNS_DBFIND_STALEENABLED);
+	qctx.client->nodetach = false;
 
 	/*
 	 * We'll need some resources...
@@ -5920,7 +5921,14 @@ query_lookup(query_ctx_t *qctx) {
 					"%s stale answer used, an attempt to "
 					"refresh the RRset will still be made",
 					namebuf);
+
 				qctx->refresh_rrset = STALE(qctx->rdataset);
+
+				/*
+				 * If we are refreshing the RRSet, we must not
+				 * detach from the client in query_send().
+				 */
+				qctx->client->nodetach = qctx->refresh_rrset;
 			}
 		} else {
 			/*
@@ -6272,7 +6280,7 @@ 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);
@@ -7235,10 +7243,21 @@ 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);
 	}
@@ -11490,12 +11509,7 @@ ns_query_done(query_ctx_t *qctx) {
 	/*
 	 * Client may have been detached after query_send(), so
 	 * we test and store the flag state here, for safety.
-	 * If we are refreshing the RRSet, we must not detach from the client
-	 * in the query_send(), so we need to override the flag.
 	 */
-	if (qctx->refresh_rrset) {
-		qctx->client->nodetach = true;
-	}
 	nodetach = qctx->client->nodetach;
 	query_send(qctx->client);
 
-- 
2.33.0

