[PATCH v2] openvswitch: Fix L4 checksum handling when dealing with IP fragments

[Glenn Griffin <ggriffin.kernel-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

openvswitch modifies the L4 checksum of a packet when modifying
the ip address. When an IP packet is fragmented only the first
fragment contains an L4 header and checksum. Prior to this change
openvswitch would modify all fragments, modifying application data
in non-first fragments, causing checksum failures in the
reassembled packet.

Signed-off-by: Glenn Griffin <ggriffin.kernel@gmail.com>
---
Changes in v2:
  - Compare frag_off in network byte order rather than host byte order

 net/openvswitch/actions.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
index 8a8c0b8..ee34f47 100644
--- a/net/openvswitch/actions.c
+++ b/net/openvswitch/actions.c
@@ -273,28 +273,36 @@ static int set_eth_addr(struct sk_buff *skb, struct sw_flow_key *flow_key,
 	return 0;
 }
 
-static void set_ip_addr(struct sk_buff *skb, struct iphdr *nh,
-			__be32 *addr, __be32 new_addr)
+static void update_ip_l4_checksum(struct sk_buff *skb, struct iphdr *nh,
+				  __be32 addr, __be32 new_addr)
 {
 	int transport_len = skb->len - skb_transport_offset(skb);
 
+	if (nh->frag_off & htons(IP_OFFSET))
+		return;
+
 	if (nh->protocol == IPPROTO_TCP) {
 		if (likely(transport_len >= sizeof(struct tcphdr)))
 			inet_proto_csum_replace4(&tcp_hdr(skb)->check, skb,
-						 *addr, new_addr, 1);
+						 addr, new_addr, 1);
 	} else if (nh->protocol == IPPROTO_UDP) {
 		if (likely(transport_len >= sizeof(struct udphdr))) {
 			struct udphdr *uh = udp_hdr(skb);
 
 			if (uh->check || skb->ip_summed == CHECKSUM_PARTIAL) {
 				inet_proto_csum_replace4(&uh->check, skb,
-							 *addr, new_addr, 1);
+							 addr, new_addr, 1);
 				if (!uh->check)
 					uh->check = CSUM_MANGLED_0;
 			}
 		}
 	}
+}
 
+static void set_ip_addr(struct sk_buff *skb, struct iphdr *nh,
+			__be32 *addr, __be32 new_addr)
+{
+	update_ip_l4_checksum(skb, nh, *addr, new_addr);
 	csum_replace4(&nh->check, *addr, new_addr);
 	skb_clear_hash(skb);
 	*addr = new_addr;
-- 
2.1.4

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