linux-kernel - [PATCH] [netfilter]: Fix skb->csum calculation when netfilter manipulation for NF_NAT_MANIP

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Message-Id: <1572301675-5403-2-git-send-email-pchaudhary@linkedin.com>
Date:   Mon, 28 Oct 2019 15:27:55 -0700
From:   Praveen Chaudhary <praveen5582@...il.com>
To:     davem@...emloft.net, fw@...len.de, kadlec@...filter.org,
        pablo@...filter.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Zhenggen Xu <zxu@...kedin.com>,
        Andy Stracner <astracner@...kedin.com>
Subject: [PATCH] [netfilter]: Fix skb->csum calculation when netfilter manipulation for NF_NAT_MANIP_SRC\DST is done on IPV6 packet.

No need to update skb->csum in function inet_proto_csum_replace16(),
even if skb->ip_summed == CHECKSUM_COMPLETE, because change in L4
header checksum field and change in IPV6 header cancels each other
for skb->csum calculation.

Signed-off-by: Praveen Chaudhary <pchaudhary@...kedin.com>
Signed-off-by: Zhenggen Xu <zxu@...kedin.com>
Signed-off-by: Andy Stracner <astracner@...kedin.com>
---
Changes in V2.
1.) Updating diff as per email discussion with Florian Westphal.
    Since inet_proto_csum_replace16() does incorrect calculation
    for skb->csum in all cases.
2.) Change in Commmit logs.
---
---
 net/core/utils.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/core/utils.c b/net/core/utils.c
index 6b6e51d..cec9924 100644
--- a/net/core/utils.c
+++ b/net/core/utils.c
@@ -438,6 +438,12 @@ void inet_proto_csum_replace4(__sum16 *sum, struct sk_buff *skb,
 }
 EXPORT_SYMBOL(inet_proto_csum_replace4);
 
+/**
+ * No need to update skb->csum in this function, even if
+ * skb->ip_summed == CHECKSUM_COMPLETE, because change in
+ * L4 header checksum field and change in IPV6 header
+ * cancels each other for skb->csum calculation.
+ */
 void inet_proto_csum_replace16(__sum16 *sum, struct sk_buff *skb,
 			       const __be32 *from, const __be32 *to,
 			       bool pseudohdr)
@@ -449,9 +455,6 @@ void inet_proto_csum_replace16(__sum16 *sum, struct sk_buff *skb,
 	if (skb->ip_summed != CHECKSUM_PARTIAL) {
 		*sum = csum_fold(csum_partial(diff, sizeof(diff),
 				 ~csum_unfold(*sum)));
-		if (skb->ip_summed == CHECKSUM_COMPLETE && pseudohdr)
-			skb->csum = ~csum_partial(diff, sizeof(diff),
-						  ~skb->csum);
 	} else if (pseudohdr)
 		*sum = ~csum_fold(csum_partial(diff, sizeof(diff),
 				  csum_unfold(*sum)));
-- 
2.7.4