[PATCH net] udp: ipv4: manipulate network header of NATed UDP GRO fraglist

[Dongseok Yi]

UDP/IP header of UDP GROed frag_skbs are not updated even after NAT
forwarding. Only the header of head_skb from ip_finish_output_gso ->
skb_gso_segment is updated but following frag_skbs are not updated.

A call path skb_mac_gso_segment -> inet_gso_segment ->
udp4_ufo_fragment -> __udp_gso_segment -> __udp_gso_segment_list
does not try to update UDP/IP header of the segment list.

Update dport, daddr and checksums of each skb of the segment list
after __udp_gso_segment.

Fixes: 9fd1ff5d2ac7 (udp: Support UDP fraglist GRO/GSO.)
Signed-off-by: Dongseok Yi <dseok.yi@xxxxxxxxxxx>
---
Steffen Klassert said, there could be 2 options.
https://lore.kernel.org/patchwork/patch/1362257/

I was trying to write a quick fix, but it was not easy to forward
segmented list. Currently, assuming DNAT only. Should we consider
SNAT too?

 net/ipv4/udp_offload.c | 45 +++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 41 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index ff39e94..7e24928 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -309,10 +309,12 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
 					 netdev_features_t features)
 {
 	struct sk_buff *segs = ERR_PTR(-EINVAL);
+	struct sk_buff *seg;
 	unsigned int mss;
 	__wsum csum;
-	struct udphdr *uh;
-	struct iphdr *iph;
+	struct udphdr *uh, *uh2;
+	struct iphdr *iph, *iph2;
+	bool is_fraglist = false;
 
 	if (skb->encapsulation &&
 	    (skb_shinfo(skb)->gso_type &
@@ -327,8 +329,43 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
 	if (!pskb_may_pull(skb, sizeof(struct udphdr)))
 		goto out;
 
-	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4)
-		return __udp_gso_segment(skb, features);
+	if (skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) {
+		if (skb_shinfo(skb)->gso_type & SKB_GSO_FRAGLIST)
+			is_fraglist = true;
+
+		segs = __udp_gso_segment(skb, features);
+		if (IS_ERR_OR_NULL(segs) || !is_fraglist)
+			return segs;
+
+		seg = segs;
+		uh = udp_hdr(seg);
+		iph = ip_hdr(seg);
+
+		while ((seg = seg->next)) {
+			uh2 = udp_hdr(seg);
+			iph2 = ip_hdr(seg);
+
+			if (uh->dest == uh2->dest && iph->daddr == iph2->daddr)
+				continue;
+
+			if (uh2->check) {
+				inet_proto_csum_replace4(&uh2->check, seg,
+							 iph2->daddr,
+							 iph->daddr, true);
+				inet_proto_csum_replace2(&uh2->check, seg,
+							 uh2->dest, uh->dest,
+							 false);
+				if (!uh2->check)
+					uh2->check = CSUM_MANGLED_0;
+			}
+			uh2->dest = uh->dest;
+
+			csum_replace4(&iph2->check, iph2->daddr, iph->daddr);
+			iph2->daddr = iph->daddr;
+		}
+
+		return segs;
+	}
 
 	mss = skb_shinfo(skb)->gso_size;
 	if (unlikely(skb->len <= mss))
-- 
2.7.4