bridge: ebtables: don't crash when using dnat target in output chains

xt_in() returns NULL in the output hook, skip the pkt_type change for
that case, redirection only makes sense in broute/prerouting hooks.
Reported-by: Tom Yan <tom.ty89@gmail.com>
Cc: Linus Lüssing <linus.luessing@c0d3.blue>
Fixes: cf3cb246 ("bridge: ebtables: fix reception of frames DNAT-ed to bridge device/port")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/bridge/netfilter/ebt_dnat.c

@@ -20,7 +20,6 @@ static unsigned int
 ebt_dnat_tg(struct sk_buff *skb, const struct xt_action_param *par)
 {
 	const struct ebt_nat_info *info = par->targinfo;
-	struct net_device *dev;
 
 	if (skb_ensure_writable(skb, ETH_ALEN))
 		return EBT_DROP;
@@ -33,10 +32,22 @@ ebt_dnat_tg(struct sk_buff *skb, const struct xt_action_param *par)
 		else
 			skb->pkt_type = PACKET_MULTICAST;
 	} else {
-		if (xt_hooknum(par) != NF_BR_BROUTING)
-			dev = br_port_get_rcu(xt_in(par))->br->dev;
-		else
+		const struct net_device *dev;
+
+		switch (xt_hooknum(par)) {
+		case NF_BR_BROUTING:
 			dev = xt_in(par);
+			break;
+		case NF_BR_PRE_ROUTING:
+			dev = br_port_get_rcu(xt_in(par))->br->dev;
+			break;
+		default:
+			dev = NULL;
+			break;
+		}
+
+		if (!dev) /* NF_BR_LOCAL_OUT */
+			return info->target;
 
 		if (ether_addr_equal(info->mac, dev->dev_addr))
 			skb->pkt_type = PACKET_HOST;