net: Fix skb->csum update in inet_proto_csum_replace16().

BugLink: https://bugs.launchpad.net/bugs/1864774

[ Upstream commit 189c9b1e ]

skb->csum is updated incorrectly, when manipulation for
NF_NAT_MANIP_SRC\DST is done on IPV6 packet.

Fix:
There is no need to update skb->csum in inet_proto_csum_replace16(),
because update in two fields a.) IPv6 src/dst address and b.) L4 header
checksum cancels each other for skb->csum calculation. Whereas
inet_proto_csum_replace4 function needs to update skb->csum, because
update in 3 fields a.) IPv4 src/dst address, b.) IPv4 Header checksum
and c.) L4 header checksum results in same diff as L4 Header checksum
for skb->csum calculation.

[ pablo@netfilter.org: a few comestic documentation edits ]
Signed-off-by: Praveen Chaudhary <pchaudhary@linkedin.com>
Signed-off-by: Zhenggen Xu <zxu@linkedin.com>
Signed-off-by: Andy Stracner <astracner@linkedin.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/core/utils.c

@@ -316,6 +316,23 @@ void inet_proto_csum_replace4(__sum16 *sum, struct sk_buff *skb,
 }
 EXPORT_SYMBOL(inet_proto_csum_replace4);
 
+/**
+ * inet_proto_csum_replace16 - update layer 4 header checksum field
+ * @sum: Layer 4 header checksum field
+ * @skb: sk_buff for the packet
+ * @from: old IPv6 address
+ * @to: new IPv6 address
+ * @pseudohdr: True if layer 4 header checksum includes pseudoheader
+ *
+ * Update layer 4 header as per the update in IPv6 src/dst address.
+ *
+ * There is no need to update skb->csum in this function, because update in two
+ * fields a.) IPv6 src/dst address and b.) L4 header checksum cancels each other
+ * for skb->csum calculation. Whereas inet_proto_csum_replace4 function needs to
+ * update skb->csum, because update in 3 fields a.) IPv4 src/dst address,
+ * b.) IPv4 Header checksum and c.) L4 header checksum results in same diff as
+ * L4 Header checksum for skb->csum calculation.
+ */
 void inet_proto_csum_replace16(__sum16 *sum, struct sk_buff *skb,
 			       const __be32 *from, const __be32 *to,
 			       bool pseudohdr)
@@ -327,9 +344,6 @@ 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)));