netfilter: nft_synproxy: use the family-aware checksum helper

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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: ad49d86e07 ("netfilter: nf_tables: Add synproxy support")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

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Karl Mehltretter 2026-09-10 22:02:28 +02:00 • committed by Pablo Neira Ayuso

parent 82313c169e

commit a311a89817

1 changed files with 2 additions and 1 deletions

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net/netfilter/nft_synproxy.c

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