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. v2: add missing IPPROTO_ICMP handling, else 'icmp code set 42' won't update icmp checksum. Fixes: 112e447d17f7 ("netfilter: validate L4 headers after userspace packet writes") Assisted-by: Claude:claude-opus-4-6 Signed-off-by: Florian Westphal Signed-off-by: Pablo Neira Ayuso --- I pushed an update to stateless_nat to catch this. net/netfilter/nft_payload.c | 39 ++++++++++++++++++++++++++++++------- 1 file changed, 32 insertions(+), 7 deletions(-) diff --git a/net/netfilter/nft_payload.c b/net/netfilter/nft_payload.c index e315d35f73d4..f732e2d61f8c 100644 --- a/net/netfilter/nft_payload.c +++ b/net/netfilter/nft_payload.c @@ -20,6 +20,7 @@ #include #include #include +#include #include #include #include @@ -1008,11 +1009,13 @@ 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 +1049,27 @@ 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); + case IPPROTO_ICMP: + return priv->csum_offset == offsetof(struct icmphdr, checksum); + } + + return false; +} + static bool nft_payload_csum_write_ok(const struct nft_pktinfo *pkt, const struct nft_payload_set *priv) { @@ -1055,9 +1079,10 @@ 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: -- 2.55.0