netfilter: fix regression in looped (broad|multi)cast's MAC handling

In commit 5648b5e1 ("netfilter: nfnetlink_queue: fix OOB when mac
header was cleared"), the test for non-empty MAC header introduced in
commit 2c38de4c ("netfilter: fix looped (broad|multi)cast's MAC
handling") has been replaced with a test for a set MAC header.

This breaks the case when the MAC header has been reset (using
skb_reset_mac_header), as is the case with looped-back multicast
packets.  As a result, the packets ending up in NFQUEUE get a bogus
hwaddr interpreted from the first bytes of the IP header.

This patch adds a test for a non-empty MAC header in addition to the
test for a set MAC header.  The same two tests are also implemented in
nfnetlink_log.c, where the initial code of commit 2c38de4c
("netfilter: fix looped (broad|multi)cast's MAC handling") has not been
touched, but where supposedly the same situation may happen.

Fixes: 5648b5e1 ("netfilter: nfnetlink_queue: fix OOB when mac header was cleared")
Signed-off-by: Ignacy Gawędzki <ignacy.gawedzki@green-communications.fr>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nfnetlink_log.c

@@ -556,7 +556,8 @@ __build_packet_message(struct nfnl_log_net *log,
 		goto nla_put_failure;
 
 	if (indev && skb->dev &&
-	    skb->mac_header != skb->network_header) {
+	    skb_mac_header_was_set(skb) &&
+	    skb_mac_header_len(skb) != 0) {
 		struct nfulnl_msg_packet_hw phw;
 		int len;
 

net/netfilter/nfnetlink_queue.c

@@ -560,7 +560,8 @@ nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 		goto nla_put_failure;
 
 	if (indev && entskb->dev &&
-	    skb_mac_header_was_set(entskb)) {
+	    skb_mac_header_was_set(entskb) &&
+	    skb_mac_header_len(entskb) != 0) {
 		struct nfqnl_msg_packet_hw phw;
 		int len;