[PATCH] net: fix csum calculation for encapsulated packets

["沈安琪(凛玥)" <amy.saq@antgroup.com>]

This commit fixes the issue that when a packet is encapsulated, such as
sending through a UDP tunnel, the outer TCP/UDP checksum is not
correctly recalculated if (1) checksum has been offloaded to hardware
and (2) encapsulated packet has been NAT-ed again, which causes the
packet being dropped due to the invalid outer checksum.

Previously, when an encapsulated packet met some NAT rules and its
src/dst ip and/or src/dst port has been modified,
inet_proto_csum_replace4 will be invoked to recalculated the outer
checksum. However, if the packet is under the following condition: (1)
checksum offloaded to hardware and (2) NAT rule has changed the src/dst
port, its outer checksum will not be recalculated, since (1)
skb->ip_summed is set to CHECKSUM_PARTIAL due to csum offload and (2)
pseudohdr is set to false since port number is not part of pseudo
header. This leads to the outer TCP/UDP checksum invalid since it does
not change along with the port number change.

In this commit, another condition has been added to recalculate outer
checksum: if (1) the packet is encapsulated, (2) checksum has been
offloaded, (3) the encapsulated packet has been NAT-ed to change port
number and (4) outer checksum is needed, the outer checksum for
encapsulated packet will be recalculated to make sure it is valid.

Signed-off-by: Anqi Shen <amy.saq@antgroup.com>
---
 net/core/utils.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/core/utils.c b/net/core/utils.c
index c994e95172ac..d9de60e9b347 100644
--- a/net/core/utils.c
+++ b/net/core/utils.c
@@ -435,6 +435,8 @@ void inet_proto_csum_replace4(__sum16 *sum, struct sk_buff *skb,
 		*sum = ~csum_fold(csum_add(csum_sub(csum_unfold(*sum),
 						    (__force __wsum)from),
 					   (__force __wsum)to));
+	else if (skb->encapsulation && !!(*sum))
+		csum_replace4(sum, from, to);
 }
 EXPORT_SYMBOL(inet_proto_csum_replace4);
 
-- 
2.19.1.6.gb485710b