netfilter: nf_conntrack_sip: fix use of uninitialized rtp_addr in process_sdp

process_sdp() declares union nf_inet_addr rtp_addr on the stack and
passes it to the nf_nat_sip sdp_session hook after walking the SDP
media descriptions. However rtp_addr is only initialized inside the
media loop when a recognized media type with a non-zero port is found.

If the SDP body contains no m= lines, only inactive media sections
(m=audio 0 ...) or only unrecognized media types, rtp_addr is never
assigned. Despite that, the function still calls hooks->sdp_session()
with &rtp_addr, causing nf_nat_sdp_session() to format the stale stack
value as an IP address and rewrite the SDP session owner and connection
lines with it.

With CONFIG_INIT_STACK_ALL_ZERO (default on most distributions) this
results in the session-level o= and c= addresses being rewritten to
0.0.0.0 for inactive SDP sessions. Without stack auto-init the
rewritten address is whatever happened to be on the stack.

Fix this by pre-initializing rtp_addr from the session-level connection
address (caddr) when available, and tracking via a have_rtp_addr flag
whether any valid address was established. Skip the sdp_session hook
entirely when no valid address exists.

Fixes: 4ab9e64e5e3c ("[NETFILTER]: nf_nat_sip: split up SDP mangling")
Reported-by: Xiang Mei <[e-mail]>
Signed-off-by: Weiming Shi <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_conntrack_sip.c

···
 	unsigned int port;
 	const struct sdp_media_type *t;
 	int ret = NF_ACCEPT;
+	bool have_rtp_addr = false;
 
 	hooks = rcu_dereference(nf_nat_sip_hooks);
 
···
 	caddr_len = 0;
 	if (ct_sip_parse_sdp_addr(ct, *dptr, sdpoff, *datalen,
 				  SDP_HDR_CONNECTION, SDP_HDR_MEDIA,
-				  &matchoff, &matchlen, &caddr) > 0)
+				  &matchoff, &matchlen, &caddr) > 0) {
 		caddr_len = matchlen;
+		memcpy(&rtp_addr, &caddr, sizeof(rtp_addr));
+		have_rtp_addr = true;
+	}
 
 	mediaoff = sdpoff;
 	for (i = 0; i < ARRAY_SIZE(sdp_media_types); ) {
···
 					  &matchoff, &matchlen, &maddr) > 0) {
 			maddr_len = matchlen;
 			memcpy(&rtp_addr, &maddr, sizeof(rtp_addr));
-		} else if (caddr_len)
+			have_rtp_addr = true;
+		} else if (caddr_len) {
 			memcpy(&rtp_addr, &caddr, sizeof(rtp_addr));
-		else {
+			have_rtp_addr = true;
+		} else {
 			nf_ct_helper_log(skb, ct, "cannot parse SDP message");
 			return NF_DROP;
 		}
···
 
 	/* Update session connection and owner addresses */
 	hooks = rcu_dereference(nf_nat_sip_hooks);
-	if (hooks && ct->status & IPS_NAT_MASK)
+	if (hooks && ct->status & IPS_NAT_MASK && have_rtp_addr)
 		ret = hooks->sdp_session(skb, protoff, dataoff,
 					 dptr, datalen, sdpoff,
 					 &rtp_addr);