[PATCH net v3] net: ipv6: fix TCP GSO segmentation with NAT

[Felix Fietkau <nbd@nbd.name>]

When updating the source/destination address, the TCP/UDP checksum needs to
be updated as well.

Fixes: bee88cd5bd83 ("net: add support for segmenting TCP fraglist GSO packets")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
---
v2: move code to make it similar to __tcpv4_gso_segment_list_csum
v3: fix uninitialized variable

 net/ipv6/tcpv6_offload.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/net/ipv6/tcpv6_offload.c b/net/ipv6/tcpv6_offload.c
index a45bf17cb2a1..ae2da28f9dfb 100644
--- a/net/ipv6/tcpv6_offload.c
+++ b/net/ipv6/tcpv6_offload.c
@@ -94,14 +94,23 @@ INDIRECT_CALLABLE_SCOPE int tcp6_gro_complete(struct sk_buff *skb, int thoff)
 }
 
 static void __tcpv6_gso_segment_csum(struct sk_buff *seg,
+				     struct in6_addr *oldip,
+				     const struct in6_addr *newip,
 				     __be16 *oldport, __be16 newport)
 {
-	struct tcphdr *th;
+	struct tcphdr *th = tcp_hdr(seg);
+
+	if (!ipv6_addr_equal(oldip, newip)) {
+		inet_proto_csum_replace16(&th->check, seg,
+					  oldip->s6_addr32,
+					  newip->s6_addr32,
+					  true);
+		*oldip = *newip;
+	}
 
 	if (*oldport == newport)
 		return;
 
-	th = tcp_hdr(seg);
 	inet_proto_csum_replace2(&th->check, seg, *oldport, newport, false);
 	*oldport = newport;
 }
@@ -129,10 +138,10 @@ static struct sk_buff *__tcpv6_gso_segment_list_csum(struct sk_buff *segs)
 		th2 = tcp_hdr(seg);
 		iph2 = ipv6_hdr(seg);
 
-		iph2->saddr = iph->saddr;
-		iph2->daddr = iph->daddr;
-		__tcpv6_gso_segment_csum(seg, &th2->source, th->source);
-		__tcpv6_gso_segment_csum(seg, &th2->dest, th->dest);
+		__tcpv6_gso_segment_csum(seg, &iph2->saddr, &iph->saddr,
+					 &th2->source, th->source);
+		__tcpv6_gso_segment_csum(seg, &iph2->daddr, &iph->daddr,
+					 &th2->dest, th->dest);
 	}
 
 	return segs;
-- 
2.47.1