netfilter: rpfilter: bypass ipv4 lbcast packets with zeronet source

Otherwise, DHCP Discover packets(0.0.0.0->255.255.255.255) may be
dropped incorrectly.
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/ipt_rpfilter.c

@@ -83,10 +83,12 @@ static bool rpfilter_mt(const struct sk_buff *skb, struct xt_action_param *par)
 		return true ^ invert;
 
 	iph = ip_hdr(skb);
-	if (ipv4_is_multicast(iph->daddr)) {
-		if (ipv4_is_zeronet(iph->saddr))
-			return ipv4_is_local_multicast(iph->daddr) ^ invert;
+	if (ipv4_is_zeronet(iph->saddr)) {
+		if (ipv4_is_lbcast(iph->daddr) ||
+		    ipv4_is_local_multicast(iph->daddr))
+			return true ^ invert;
 	}
+
 	flow.flowi4_iif = LOOPBACK_IFINDEX;
 	flow.daddr = iph->saddr;
 	flow.saddr = rpfilter_get_saddr(iph->daddr);

net/ipv4/netfilter/nft_fib_ipv4.c

@@ -101,13 +101,14 @@ void nft_fib4_eval(const struct nft_expr *expr, struct nft_regs *regs,
 	}
 
 	iph = ip_hdr(pkt->skb);
-	if (ipv4_is_multicast(iph->daddr) &&
-	    ipv4_is_zeronet(iph->saddr) &&
+	if (ipv4_is_zeronet(iph->saddr)) {
+		if (ipv4_is_lbcast(iph->daddr) ||
 		    ipv4_is_local_multicast(iph->daddr)) {
 			nft_fib_store_result(dest, priv->result, pkt,
 					     get_ifindex(pkt->skb->dev));
 			return;
 		}
+	}
 
 	if (priv->flags & NFTA_FIB_F_MARK)
 		fl4.flowi4_mark = pkt->skb->mark;