[PATCH net 07/12] netfilter: nft_payload: restrict checksum offsets to known values
From: Pablo Neira Ayuso <pablo@netfilter.org>
Date: 2026-09-03 00:42:06
Also in:
netfilter-devel
Subsystem:
netfilter, networking [general], the rest · Maintainers:
Pablo Neira Ayuso, Florian Westphal, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
From: Florian Westphal <fw@strlen.de>
We need to prevent userspace from corrupting e.g. tcp->doff, because
many locations in conntrack and conntrack helpers rely on
nf_conntrack_in() having validated the packet headers.
nft_payload allows to alter headers later which invalidates this
assumption.
The 'Fixes' commit restricts writes to safe fields, but there is
another side channel: the checksum location.
Restrict this too. Reported via sashiko/gemini.
Fixes: 112e447d17f7 ("netfilter: validate L4 headers after userspace packet writes")
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
net/netfilter/nft_payload.c | 36 +++++++++++++++++++++++++++++-------
1 file changed, 29 insertions(+), 7 deletions(-)
diff --git a/net/netfilter/nft_payload.c b/net/netfilter/nft_payload.c
index e315d35f73d4..70f70a65e327 100644
--- a/net/netfilter/nft_payload.c
+++ b/net/netfilter/nft_payload.c@@ -1008,11 +1008,13 @@ static bool nft_payload_validate_inet_csum_offset(const struct nft_ctx *ctx, if (priv->csum_flags) /* makes no sense, asks for "re-update" of L4 checksum */ return false; - /* no further check here; offset can't be negative so bogus - * offsets can corrupt L4 or payload but not l3 headers. - * We already allow arbitrary l4/inner payload writes. - */ - return true; + /* Validate csum_offset is one of the supported transport header checksums */ + if (priv->csum_offset == offsetof(struct tcphdr, check) || + priv->csum_offset == offsetof(struct udphdr, check) || + priv->csum_offset == offsetof(struct icmp6hdr, icmp6_cksum)) + return true; + + return false; case NFT_PAYLOAD_INNER_HEADER: return true; case NFT_PAYLOAD_TUN_HEADER:
@@ -1046,6 +1048,25 @@ static bool nft_payload_csum_nh_write_ok(const struct nft_payload_set *priv, return false; } +static bool nft_payload_csum_th_write_ok(const struct nft_payload_set *priv, + const struct nft_pktinfo *pkt) +{ + if (!(pkt->flags & NFT_PKTINFO_L4PROTO)) + return false; + + switch (pkt->tprot) { + case IPPROTO_TCP: + return priv->csum_offset == offsetof(struct tcphdr, check); + case IPPROTO_UDP: + case IPPROTO_UDPLITE: + return priv->csum_offset == offsetof(struct udphdr, check); + case IPPROTO_ICMPV6: + return priv->csum_offset == offsetof(struct icmp6hdr, icmp6_cksum); + } + + return false; +} + static bool nft_payload_csum_write_ok(const struct nft_pktinfo *pkt, const struct nft_payload_set *priv) {
@@ -1055,9 +1076,10 @@ static bool nft_payload_csum_write_ok(const struct nft_pktinfo *pkt, case NFT_PAYLOAD_NETWORK_HEADER: return nft_payload_csum_nh_write_ok(priv, pkt); case NFT_PAYLOAD_TRANSPORT_HEADER: + return nft_payload_csum_th_write_ok(priv, pkt); case NFT_PAYLOAD_INNER_HEADER: - /* neither offsets are validated, offsets cannot be - * negative so real l3 headers cannot be mangled. + /* offset is not validated, offset cannot be + * negative so real l3/l4 headers cannot be mangled. */ return true; case NFT_PAYLOAD_TUN_HEADER:
--
2.47.3