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[2a02:3100:b3d9:b901:601c:58ef:b3f5:74f5]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49e62227071sm638505e9.3.2026.09.10.13.02.31 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Thu, 10 Sep 2026 13:02:32 -0700 (PDT) From: Karl Mehltretter To: Pablo Neira Ayuso , Florian Westphal Cc: Karl Mehltretter , Phil Sutter , Fernando Fernandez Mancera , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , netfilter-devel@vger.kernel.org, coreteam@netfilter.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] netfilter: nft_synproxy: use the family-aware checksum helper Date: Thu, 10 Sep 2026 22:02:28 +0200 Message-Id: <20260910200228.5127-1-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 --- 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 ru= le 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_synpr= oxy *priv, return; } =20 - 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 =3D NF_DROP; return; } base-commit: 50d05c7c76c96b90462f24debacca971d2e86713 --=20 2.53.0