netfilter: xt_TCPMSS: handle CHECKSUM_COMPLETE in tcpmss_tg6()

In case MSS option is added in TCP options, skb length increases by 4.
IPv6 needs to update skb->csum if skb has CHECKSUM_COMPLETE,
otherwise kernel complains loudly in netdev_rx_csum_fault() with a
stack dump.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
index b7c43def0dc6..e118397254af 100644
--- a/net/netfilter/xt_TCPMSS.c
+++ b/net/netfilter/xt_TCPMSS.c
@@ -228,7 +228,7 @@ tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
 {
         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
         u8 nexthdr;
-        __be16 frag_off;
+        __be16 frag_off, oldlen, newlen;
         int tcphoff;
         int ret;
 
@@ -244,7 +244,12 @@ tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
                 return NF_DROP;
         if (ret > 0) {
                 ipv6h = ipv6_hdr(skb);
-                ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
+                oldlen = ipv6h->payload_len;
+                newlen = htons(ntohs(oldlen) + ret);
+                if (skb->ip_summed == CHECKSUM_COMPLETE)
+                        skb->csum = csum_add(csum_sub(skb->csum, oldlen),
+                                             newlen);
+                ipv6h->payload_len = newlen;
         }
         return XT_CONTINUE;
 }