linux-kernel - [PATCH 3.16 096/204] udp: fix bcast packet reception

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Date:   Thu, 28 Dec 2017 17:05:44 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "David S. Miller" <davem@...emloft.net>,
        "Hannes Frederic Sowa" <hannes@...essinduktion.org>,
        "Paolo Abeni" <pabeni@...hat.com>
Subject: [PATCH 3.16 096/204] udp: fix bcast packet reception

3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Paolo Abeni <pabeni@...hat.com>

commit 996b44fcef8f216ea0b6b6e74468c5a77b5e341f upstream.

The commit bc044e8db796 ("udp: perform source validation for
mcast early demux") does not take into account that broadcast packets
lands in the same code path and they need different checks for the
source address - notably, zero source address are valid for bcast
and invalid for mcast.

As a result, 2nd and later broadcast packets with 0 source address
landing to the same socket are dropped. This breaks dhcp servers.

Since we don't have stringent performance requirements for ingress
broadcast traffic, fix it by disabling UDP early demux such traffic.

Reported-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
Fixes: bc044e8db796 ("udp: perform source validation for mcast early demux")
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 net/ipv4/udp.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1968,20 +1968,16 @@ int udp_v4_early_demux(struct sk_buff *s
 	iph = ip_hdr(skb);
 	uh = udp_hdr(skb);
 
-	if (skb->pkt_type == PACKET_BROADCAST ||
-	    skb->pkt_type == PACKET_MULTICAST) {
+	if (skb->pkt_type == PACKET_MULTICAST) {
 		in_dev = __in_dev_get_rcu(skb->dev);
 
 		if (!in_dev)
 			return 0;
 
-		/* we are supposed to accept bcast packets */
-		if (skb->pkt_type == PACKET_MULTICAST) {
-			ours = ip_check_mc_rcu(in_dev, iph->daddr, iph->saddr,
-					       iph->protocol);
-			if (!ours)
-				return 0;
-		}
+		ours = ip_check_mc_rcu(in_dev, iph->daddr, iph->saddr,
+				       iph->protocol);
+		if (!ours)
+			return 0;
 
 		sk = __udp4_lib_mcast_demux_lookup(net, uh->dest, iph->daddr,
 						   uh->source, iph->saddr, dif);