Check that an NSEC signer is at or above the name to be validated

Add a check that an NSEC record being used as a proof of nonexistence
for a given name is not signed by a name lower in the DNS hierarchy than
the one in question.

Fixes: isc-projects/bind9#5876

(cherry picked from commit c5f99f3508)

lib/dns/nsec.c

@@ -441,10 +441,23 @@ dns_nsec_noexistnodata(dns_rdatatype_t type, const dns_name_t *name,
 		return DNS_R_DNAME;
 	}
 
+	if (relation != dns_namereln_subdomain &&
+	    dns_nsec_typepresent(&rdata, dns_rdatatype_soa))
+	{
+		/*
+		 * An NSEC with an SOA in the bitmap can only cover
+		 * names that are subdomains of the owner.
+		 */
+		(*logit)(arg, ISC_LOG_DEBUG(3),
+			 "ignoring nsec with SOA covering non-subdomain");
+		return ISC_R_IGNORE;
+	}
+
 	result = dns_rdata_tostruct(&rdata, &nsec, NULL);
 	if (result != ISC_R_SUCCESS) {
 		return result;
 	}
+
 	relation = dns_name_fullcompare(&nsec.next, name, &order, &nlabels);
 	if (order == 0) {
 		dns_rdata_freestruct(&nsec);

lib/dns/validator.c

@@ -846,6 +846,8 @@ validator_callback_nsec(void *arg) {
 		    rdataset->trust == dns_trust_secure &&
 		    (NEEDNODATA(val) || NEEDNOQNAME(val)) &&
 		    !FOUNDNODATA(val) && !FOUNDNOQNAME(val) &&
+		    dns_name_issubdomain(val->name,
+					 &subvalidator->siginfo->signer) &&
 		    dns_nsec_noexistnodata(val->type, val->name,
 					   subvalidator->name, rdataset,
 					   &exists, &data, wild, validator_log,

lib/ns/query.c

@@ -9959,6 +9959,13 @@ query_coveringnsec(query_ctx_t *qctx) {
 		goto cleanup;
 	}
 
+	/*
+	 * The query name can't be above the signer of the NSEC.
+	 */
+	if (!dns_name_issubdomain(qctx->client->query.qname, signer)) {
+		goto cleanup;
+	}
+
 	/*
 	 * If NSEC or RRSIG are missing from the type map
 	 * reject the NSEC RRset.