Merge branch '2540-confidential-issue-existing-dname' into 'security-main'

[CVE-2021-25215] Properly answer queries for DNAME records that require the DNAME to be processed to resolve itself

See merge request isc-private/bind9!253

CHANGES

@@ -43,7 +43,9 @@
 
 5617.	[placeholder]
 
-5616.	[placeholder]
+5616.	[security]	named crashed when a DNAME record placed in the ANSWER
+			section during DNAME chasing turned out to be the final
+			answer to a client query. (CVE-2021-25215) [GL #2540]
 
 5615.	[security]	Insufficient IXFR checks could result in named serving a
 			zone without an SOA record at the apex, leading to a

doc/notes/notes-current.rst

@@ -20,6 +20,15 @@ Security Fixes
   ISC would like to thank Greg Kuechle of SaskTel for bringing this
   vulnerability to our attention. [GL #2467]
 
+- ``named`` crashed when a DNAME record placed in the ANSWER section
+  during DNAME chasing turned out to be the final answer to a client
+  query. (CVE-2021-25215)
+
+  ISC would like to thank `Siva Kakarla`_ for bringing this
+  vulnerability to our attention. [GL #2540]
+
+.. _Siva Kakarla: https://github.com/sivakesava1
+
 Known Issues
 ~~~~~~~~~~~~
 

lib/ns/query.c

@@ -8173,10 +8173,16 @@ query_respond(query_ctx_t *qctx) {
 	query_addnoqnameproof(qctx);
 
 	/*
-	 * We shouldn't ever fail to add 'rdataset'
-	 * because it's already in the answer.
+	 * 'qctx->rdataset' will only be non-NULL here if the ANSWER section of
+	 * the message to be sent to the client already contains an RRset with
+	 * the same owner name and the same type as 'qctx->rdataset'.  This
+	 * should never happen, with one exception: when chasing DNAME records,
+	 * one of the DNAME records placed in the ANSWER section may turn out
+	 * to be the final answer to the client's query, but we have no way of
+	 * knowing that until now.  In such a case, 'qctx->rdataset' will be
+	 * freed later, so we do not need to free it here.
 	 */
-	INSIST(qctx->rdataset == NULL);
+	INSIST(qctx->rdataset == NULL || qctx->qtype == dns_rdatatype_dname);
 
 	query_addauth(qctx);