From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from Chamillionaire.breakpoint.cc (Chamillionaire.breakpoint.cc [91.216.245.30]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 96D04377006 for ; Mon, 24 Aug 2026 19:28:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.216.245.30 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787599733; cv=none; b=XD5RS2b5W2u5A5aa04W0WiNW2CG75poF5gUqXE9wBnOz09A0cB2eDhH+wSwPV/SNVEBcZHf3Hhco8QG+z8nr8gExrA9TMkDPDSmHdmd3okGi3INZ25f2swZezOvgvnN50OfOv3xjiAQTmKJwKIcNpmWajcYh8Llgvg8bV1doEoU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787599733; c=relaxed/simple; bh=DDwf6BISWyWN0neFvwIljoSQqVqz0425nHhIrVbuNkg=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=VPgmoS4Uskg7BXLb5Xj1pc7EzDjxZQGhLpIq9qqVq4UvllAqRdvxSbvHjB8w15RvROSZt7tSQkH3BprdIvNhYgF0y0PspZxzGfKbq/M3MfsEMwamhU/TT3Nvf2yxNd8NZoRuqn2/NUeHBWHXoScojsuSCexySyeEu3V5LTfMIpA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=strlen.de; spf=pass smtp.mailfrom=Chamillionaire.breakpoint.cc; arc=none smtp.client-ip=91.216.245.30 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=strlen.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=Chamillionaire.breakpoint.cc Received: by Chamillionaire.breakpoint.cc (Postfix, from userid 1003) id 520DF602A3; Mon, 24 Aug 2026 21:28:49 +0200 (CEST) From: Florian Westphal To: Cc: Florian Westphal Subject: [PATCH nf] netfilter: nft_payload: restrict checksum offsets to known values Date: Mon, 24 Aug 2026 21:28:43 +0200 Message-ID: <20260824192843.28439-1-fw@strlen.de> X-Mailer: git-send-email 2.54.0 Precedence: bulk X-Mailing-List: netfilter-devel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 --- 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: -- 2.54.0