[PATCH nf v3 2/2] ipvs: return the csum validation for forward hook

Sashiko notes that playing games with the skb dst and rt
flags instead of providing hooknum is not a good idea
when validating the checksums.

Also, skipping checksum validation for FORWARD packets
risk silent data corruption, even if the only user is
the FTP-CMD packets coming from the real server.

Sashiko also noticed that by using common checksum
helper in the previous commit we actually fixed old bug
where the TCP/UDP checksum for IPv6 on CHECKSUM_COMPLETE
was not validated correctly.

Fixes: e876b75b9020 ("ipvs: fix the checksum validations")
Link: https://sashiko.dev/#/patchset/20260722211420.153933-1-pablo%40netfilter.org
Link: https://sashiko.dev/#/patchset/20260727185024.67534-1-ja%40ssi.bg
Link: https://sashiko.dev/#/patchset/20260728202520.59179-1-ja%40ssi.bg
Signed-off-by: Julian Anastasov <[email protected]>
---
 include/net/ip_vs.h                   | 19 +++++--------------
 net/netfilter/ipvs/ip_vs_proto_sctp.c |  2 +-
 2 files changed, 6 insertions(+), 15 deletions(-)

diff --git a/include/net/ip_vs.h b/include/net/ip_vs.h
index 1235f1934e94..d2813eb795be 100644
--- a/include/net/ip_vs.h
+++ b/include/net/ip_vs.h
@@ -25,9 +25,7 @@
 #include <linux/netfilter.h>		/* for union nf_inet_addr */
 #include <linux/ip.h>
 #include <linux/ipv6.h>			/* for struct ipv6hdr */
-#include <net/route.h>
 #include <net/ipv6.h>
-#include <net/ip6_fib.h>
 #if IS_ENABLED(CONFIG_NF_CONNTRACK)
 #include <net/netfilter/nf_conntrack.h>
 #endif
@@ -2095,30 +2093,23 @@ static inline __wsum ip_vs_check_diff2(__be16 old, __be16 new, __wsum oldsum)
 	return csum_partial(diff, sizeof(diff), oldsum);
 }
 
-static inline bool ip_vs_checksum_needed(struct sk_buff *skb, int af)
+static inline bool ip_vs_checksum_needed(struct sk_buff *skb)
 {
 	/* Checksum unnecessary or already validated? */
 	if (skb_csum_unnecessary(skb))
 		return false;
-	/* LOCAL_OUT ? */
-	if (!skb->dev || skb->dev->flags & IFF_LOOPBACK)
+	/* Locally generated ? */
+	if (!skb->dev)
 		return false;
-	/* !LOCAL_IN (FORWARD) ? */
-	if (af == AF_INET6) {
-		if (!(dst_rt6_info(skb_dst(skb))->rt6i_flags & RTF_LOCAL))
-			return false;
-	} else {
-		if (!(skb_rtable(skb)->rt_flags & RTCF_LOCAL))
-			return false;
-	}
 	return true;
 }
 
 static inline bool ip_vs_checksum_common_check(struct sk_buff *skb,
 					       int offset, int proto, int af)
 {
-	if (!ip_vs_checksum_needed(skb, af))
+	if (!ip_vs_checksum_needed(skb))
 		return true;
+	/* Validate csum even for FORWARD */
 	return !nf_checksum(skb, NF_INET_LOCAL_IN, offset, proto, af);
 }
 
diff --git a/net/netfilter/ipvs/ip_vs_proto_sctp.c b/net/netfilter/ipvs/ip_vs_proto_sctp.c
index 3dbd3096e163..c80567c73469 100644
--- a/net/netfilter/ipvs/ip_vs_proto_sctp.c
+++ b/net/netfilter/ipvs/ip_vs_proto_sctp.c
@@ -193,7 +193,7 @@ sctp_csum_check(int af, struct sk_buff *skb, struct ip_vs_protocol *pp,
 	struct sctphdr *sh;
 	__le32 cmp, val;
 
-	if (!ip_vs_checksum_needed(skb, af))
+	if (!ip_vs_checksum_needed(skb))
 		return 1;
 	sh = (struct sctphdr *)(skb->data + sctphoff);
 	cmp = sh->checksum;
-- 
2.55.0