CVE-2018-14629 dns: fix CNAME loop prevention using counter regression

The loop prevention should only be done for CNAME records!

Otherwise we truncate the answer records for A, AAAA or
SRV queries, which is a bad idea if you have more than 20 DCs.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13600

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Tue Dec  4 08:52:29 CET 2018 on sn-devel-144

(cherry picked from commit 34f4491d79b47b2fe2457b8882f11644cf773bc4)

1 files changed, 20 insertions, 9 deletions

diff --git a/source4/dns_server/dns_query.c b/source4/dns_server/dns_query.c
index 07cde80a258..ef3ece248fe 100644
--- a/source4/dns_server/dns_query.c
+++ b/source4/dns_server/dns_query.c
@@ -388,7 +388,8 @@ static struct tevent_req *handle_authoritative_send(
 	TALLOC_CTX *mem_ctx, struct tevent_context *ev,
 	struct dns_server *dns, const char *forwarder,
 	struct dns_name_question *question,
-	struct dns_res_rec **answers, struct dns_res_rec **nsrecs);
+	struct dns_res_rec **answers, struct dns_res_rec **nsrecs,
+	size_t cname_depth);
 static WERROR handle_authoritative_recv(struct tevent_req *req);
 
 struct handle_dnsrpcrec_state {
@@ -404,7 +405,8 @@ static struct tevent_req *handle_dnsrpcrec_send(
 	struct dns_server *dns, const char *forwarder,
 	const struct dns_name_question *question,
 	struct dnsp_DnssrvRpcRecord *rec,
-	struct dns_res_rec **answers, struct dns_res_rec **nsrecs)
+	struct dns_res_rec **answers, struct dns_res_rec **nsrecs,
+	size_t cname_depth)
 {
 	struct tevent_req *req, *subreq;
 	struct handle_dnsrpcrec_state *state;
@@ -420,7 +422,7 @@ static struct tevent_req *handle_dnsrpcrec_send(
 	state->answers = answers;
 	state->nsrecs = nsrecs;
 
-	if (talloc_array_length(*answers) >= MAX_Q_RECURSION_DEPTH) {
+	if (cname_depth >= MAX_Q_RECURSION_DEPTH) {
 		tevent_req_done(req);
 		return tevent_req_post(req, ev);
 	}
@@ -465,7 +467,8 @@ static struct tevent_req *handle_dnsrpcrec_send(
 	if (dns_authoritative_for_zone(dns, new_q->name)) {
 		subreq = handle_authoritative_send(
 			state, ev, dns, forwarder, new_q,
-			state->answers, state->nsrecs);
+			state->answers, state->nsrecs,
+			cname_depth + 1);
 		if (tevent_req_nomem(subreq, req)) {
 			return tevent_req_post(req, ev);
 		}
@@ -549,6 +552,8 @@ struct handle_authoritative_state {
 
 	struct dns_res_rec **answers;
 	struct dns_res_rec **nsrecs;
+
+	size_t cname_depth;
 };
 
 static void handle_authoritative_done(struct tevent_req *subreq);
@@ -557,7 +562,8 @@ static struct tevent_req *handle_authoritative_send(
 	TALLOC_CTX *mem_ctx, struct tevent_context *ev,
 	struct dns_server *dns, const char *forwarder,
 	struct dns_name_question *question,
-	struct dns_res_rec **answers, struct dns_res_rec **nsrecs)
+	struct dns_res_rec **answers, struct dns_res_rec **nsrecs,
+	size_t cname_depth)
 {
 	struct tevent_req *req, *subreq;
 	struct handle_authoritative_state *state;
@@ -575,6 +581,7 @@ static struct tevent_req *handle_authoritative_send(
 	state->forwarder = forwarder;
 	state->answers = answers;
 	state->nsrecs = nsrecs;
+	state->cname_depth = cname_depth;
 
 	werr = dns_name2dn(dns, state, question->name, &dn);
 	if (tevent_req_werror(req, werr)) {
@@ -595,7 +602,8 @@ static struct tevent_req *handle_authoritative_send(
 	subreq = handle_dnsrpcrec_send(
 		state, state->ev, state->dns, state->forwarder,
 		state->question, &state->recs[state->recs_done],
-		state->answers, state->nsrecs);
+		state->answers, state->nsrecs,
+		state->cname_depth);
 	if (tevent_req_nomem(subreq, req)) {
 		return tevent_req_post(req, ev);
 	}
@@ -627,7 +635,8 @@ static void handle_authoritative_done(struct tevent_req *subreq)
 	subreq = handle_dnsrpcrec_send(
 		state, state->ev, state->dns, state->forwarder,
 		state->question, &state->recs[state->recs_done],
-		state->answers, state->nsrecs);
+		state->answers, state->nsrecs,
+		state->cname_depth);
 	if (tevent_req_nomem(subreq, req)) {
 		return;
 	}
@@ -999,7 +1008,8 @@ struct tevent_req *dns_server_process_query_send(
 
 		subreq = handle_authoritative_send(
 			state, ev, dns, (forwarders == NULL ? NULL : forwarders[0]),
-			&in->questions[0], &state->answers, &state->nsrecs);
+			&in->questions[0], &state->answers, &state->nsrecs,
+			0); /* cname_depth */
 		if (tevent_req_nomem(subreq, req)) {
 			return tevent_req_post(req, ev);
 		}
@@ -1101,7 +1111,8 @@ static void dns_server_process_query_got_auth(struct tevent_req *subreq)
 		subreq = handle_authoritative_send(state, state->ev, state->dns,
 						   state->forwarders->forwarder,
 						   state->question, &state->answers,
-						   &state->nsrecs);
+						   &state->nsrecs,
+						   0); /* cname_depth */
 
 		if (tevent_req_nomem(subreq, req)) {
 			return;