From 6aba56ae89cde535fcc6fbee0366c843cdf47845 Mon Sep 17 00:00:00 2001
From: Evan Hunt <each@isc.org>
Date: Thu, 23 Jan 2025 17:16:30 -0800
Subject: [PATCH] Check whether a rejected rrset is different

Add a new dns_rdataset_equals() function to check whether two
rdatasets are equal in DNSSEC terms.

When an rdataset being cached is rejected because its trust
level is lower than the existing rdataset, we now check to see
whether the rejected data was identical to the existing data.
This allows us to cache a potentially useful RRSIG when handling
CD=1 queries, while still rejecting RRSIGs that would definitely
have resulted in a validation failure.
---
 lib/dns/include/dns/rdataset.h | 13 +++++++++++++
 lib/dns/rdataset.c             | 15 +++++++++++++++
 lib/dns/rdataslab.c            | 20 ++++++++++++++++++++
 lib/dns/resolver.c             | 34 ++++++++++++++++++++++------------
 4 files changed, 70 insertions(+), 12 deletions(-)

diff --git a/lib/dns/include/dns/rdataset.h b/lib/dns/include/dns/rdataset.h
index 289cfa6dd6..e54fbc6b6b 100644
--- a/lib/dns/include/dns/rdataset.h
+++ b/lib/dns/include/dns/rdataset.h
@@ -100,6 +100,8 @@ struct dns_rdatasetmethods {
 	isc_result_t (*addglue)(dns_rdataset_t	*rdataset,
 				dns_dbversion_t *version, dns_message_t *msg);
 	dns_slabheader_t *(*getheader)(const dns_rdataset_t *rdataset);
+	bool (*equals)(const dns_rdataset_t *rdataset1,
+		       const dns_rdataset_t *rdataset2);
 };
 
 #define DNS_RDATASET_MAGIC	ISC_MAGIC('D', 'N', 'S', 'R')
@@ -676,3 +678,14 @@ dns_rdataset_getheader(const dns_rdataset_t *rdataset);
  * Requires:
  * \li	'rdataset' is a valid rdataset.
  */
+
+bool
+dns_rdataset_equals(const dns_rdataset_t *rdataset1,
+		    const dns_rdataset_t *rdataset2);
+/*%<
+ * Returns true if the rdata in the rdataset is equal.
+ *
+ * Requires:
+ * \li	'rdataset1' is a valid rdataset.
+ * \li	'rdataset2' is a valid rdataset.
+ */
diff --git a/lib/dns/rdataset.c b/lib/dns/rdataset.c
index ceef803ee5..d0962fe95e 100644
--- a/lib/dns/rdataset.c
+++ b/lib/dns/rdataset.c
@@ -659,3 +659,18 @@ dns_rdataset_getheader(const dns_rdataset_t *rdataset) {
 
 	return NULL;
 }
+
+bool
+dns_rdataset_equals(const dns_rdataset_t *rdataset1,
+		    const dns_rdataset_t *rdataset2) {
+	REQUIRE(DNS_RDATASET_VALID(rdataset1));
+	REQUIRE(DNS_RDATASET_VALID(rdataset2));
+
+	if (rdataset1->methods->equals != NULL &&
+	    rdataset1->methods->equals == rdataset2->methods->equals)
+	{
+		return (rdataset1->methods->equals)(rdataset1, rdataset2);
+	}
+
+	return false;
+}
diff --git a/lib/dns/rdataslab.c b/lib/dns/rdataslab.c
index 65b803d162..e4fce867ba 100644
--- a/lib/dns/rdataslab.c
+++ b/lib/dns/rdataslab.c
@@ -115,6 +115,9 @@ static void
 rdataset_getownercase(const dns_rdataset_t *rdataset, dns_name_t *name);
 static dns_slabheader_t *
 rdataset_getheader(const dns_rdataset_t *rdataset);
+static bool
+rdataset_equals(const dns_rdataset_t *rdataset1,
+		const dns_rdataset_t *rdataset2);
 
 /*% Note: the "const void *" are just to make qsort happy.  */
 static int
@@ -943,6 +946,7 @@ dns_rdatasetmethods_t dns_rdataslab_rdatasetmethods = {
 	.setownercase = rdataset_setownercase,
 	.getownercase = rdataset_getownercase,
 	.getheader = rdataset_getheader,
+	.equals = rdataset_equals,
 };
 
 /* Fixed RRSet helper macros */
@@ -1234,3 +1238,19 @@ rdataset_getheader(const dns_rdataset_t *rdataset) {
 	dns_slabheader_t *header = (dns_slabheader_t *)rdataset->slab.raw;
 	return header - 1;
 }
+
+static bool
+rdataset_equals(const dns_rdataset_t *rdataset1,
+		const dns_rdataset_t *rdataset2) {
+	if (rdataset1->rdclass != rdataset2->rdclass ||
+	    rdataset1->type != rdataset2->type)
+	{
+		return false;
+	}
+
+	dns_slabheader_t *header1 = (dns_slabheader_t *)rdataset1->slab.raw - 1;
+	dns_slabheader_t *header2 = (dns_slabheader_t *)rdataset2->slab.raw - 1;
+
+	return dns_rdataslab_equalx(header1, header2, rdataset1->rdclass,
+				    rdataset2->type);
+}
diff --git a/lib/dns/resolver.c b/lib/dns/resolver.c
index ec23f3b185..e99e21c763 100644
--- a/lib/dns/resolver.c
+++ b/lib/dns/resolver.c
@@ -6096,33 +6096,43 @@ cache_name(fetchctx_t *fctx, dns_name_t *name, dns_message_t *message,
 				if (result == DNS_R_UNCHANGED) {
 					result = ISC_R_SUCCESS;
 					if (!need_validation &&
-					    ardataset != NULL &&
-					    NEGATIVE(ardataset))
+					    ardataset != NULL)
 					{
 						/*
 						 * The answer in the
 						 * cache is better than
-						 * the answer we found,
-						 * and is a negative
-						 * cache entry, so we
+						 * the answer we found.
+						 * If it's a negative
+						 * cache entry, we
 						 * must set eresult
 						 * appropriately.
 						 */
 						if (NXDOMAIN(ardataset)) {
 							eresult =
 								DNS_R_NCACHENXDOMAIN;
-						} else {
+						} else if (NEGATIVE(ardataset))
+						{
 							eresult =
 								DNS_R_NCACHENXRRSET;
 						}
+
 						/*
-						 * We have a negative
-						 * response from the
-						 * cache so don't
-						 * attempt to add the
-						 * RRSIG rrset.
+						 * The cache wasn't updated
+						 * because something was
+						 * already there. If the
+						 * data was the same as what
+						 * we were trying to add,
+						 * then sigrdataset might
+						 * still be useful. If
+						 * not, move on.
 						 */
-						continue;
+						if (sigrdataset != NULL &&
+						    !dns_rdataset_equals(
+							    rdataset,
+							    addedrdataset))
+						{
+							continue;
+						}
 					}
 				}
 				if (result != ISC_R_SUCCESS) {