netdev - [PATCH net] netfilter: xt_TCPMSS: handle CHECKSUM_COMPLETE in tcpmss

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Message-ID: <1452874892.1223.175.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 15 Jan 2016 08:21:32 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Patrick McHardy <kaber@...sh.net>
Cc:	netfilter-devel@...r.kernel.org, netdev <netdev@...r.kernel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Tom Herbert <tom@...bertland.com>
Subject: [PATCH net] netfilter: xt_TCPMSS: handle CHECKSUM_COMPLETE in
 tcpmss_tg6()

From: Eric Dumazet <edumazet@...gle.com>

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@...gle.com>
---
 net/netfilter/xt_TCPMSS.c |    9 +++++++--
 1 file 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;
 }