[PATCH] netfilter: nft_synproxy: use the family-aware checksum helper

nft_synproxy_do_eval() verifies the TCP checksum before it switches on
skb->protocol.  It uses nf_ip_checksum(), which constructs an IPv4
pseudo header and relies on the IPv4 header checksum when folding the
whole skb.  Neither operation is valid for an IPv6 packet.

A correctly checksummed IPv6 segment can therefore fail verification
when it reaches the hook as CHECKSUM_NONE or, at NF_INET_LOCAL_IN,
CHECKSUM_COMPLETE.  nft_synproxy_do_eval() returns NF_DROP before
nft_synproxy_eval_v6() can send a SYN-ACK.

nft_synproxy_validate() deliberately admits NFPROTO_IPV6 and
NFPROTO_INET, and the xtables counterpart ip6t_SYNPROXY.c already calls
nf_ip6_checksum().

Use nf_checksum() with nft_pf() so the checksum helper dispatches to the
packet family's implementation.

Fixes: ad49d86e07a4 ("netfilter: nf_tables: Add synproxy support")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <redacted>
---

Notes (testing):
    Testing:
    - Raspberry Pi 400 with bcmgenet and 6.18.39+rpt-rpi-v8: the base kernel
      received IPv6 SYNs but sent no reply.  IPv4 with the same synproxy rule
      and IPv6 without synproxy completed normally.
    - Two x86_64 QEMU guests with pcnet CHECKSUM_NONE receive packets: the
      current-mainline base completed the IPv4 handshake but dropped six
      IPv6 SYNs.  With this patch, both families completed SYN, SYN-ACK and
      ACK.  ICMP controls passed and the invalid-drop counters stayed zero.

 net/netfilter/nft_synproxy.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nft_synproxy.c b/net/netfilter/nft_synproxy.c
index 9ed288c9d168..554a96a000f4 100644
--- a/net/netfilter/nft_synproxy.c
+++ b/net/netfilter/nft_synproxy.c

@@ -118,7 +118,8 @@ static void nft_synproxy_do_eval(const struct nft_synproxy *priv,
 		return;
 	}
 
-	if (nf_ip_checksum(skb, nft_hook(pkt), thoff, IPPROTO_TCP)) {
+	if (nf_checksum(skb, nft_hook(pkt), thoff, IPPROTO_TCP,
+			nft_pf(pkt))) {
 		regs->verdict.code = NF_DROP;
 		return;
 	}

base-commit: 50d05c7c76c96b90462f24debacca971d2e86713
-- 
2.53.0