[{"id":3678953,"web_url":"http://patchwork.ozlabs.org/comment/3678953/","msgid":"","list_archive_url":null,"date":"2026-04-18T07:51:24","subject":"Re: [PATCH 4/4 nf] netfilter: xtables: fix L4 header parsing for\n non-first fragments","submitter":{"id":1315,"url":"http://patchwork.ozlabs.org/api/people/1315/","name":"Pablo Neira Ayuso","email":"pablo@netfilter.org"},"content":"On Fri, Apr 17, 2026 at 08:34:35PM +0200, Fernando Fernandez Mancera wrote:\n> The TPROXY target and osf match relies on L4 header to operate. For\n> fragmented packets, every fragment carries the transport protocol\n> identifier, but only the first fragment contains the L4 header.\n> \n> As the 'raw' table can be configured to run at priority -450 (before\n> defragmentation at -400), the target/match can be reached before\n> reassembly. In this case, non-first fragments have their payload\n> incorrectly parsed as a TCP/UDP header.\n\nI see, this refers to a misconfiguration scenario.\n\n> Add a fragment check to ensure TPROXY/osf only evaluates unfragmented\n> packets or the first fragment in the stream.\n\nLGTM this combo patch for osf and TPROXY in xtables.\n\nThanks.\n\n> Fixes: 902d6a4c2a4f (\"netfilter: nf_defrag: Skip defrag if NOTRACK is set\")\n> Signed-off-by: Fernando Fernandez Mancera \n> ---\n> net/netfilter/xt_TPROXY.c | 8 ++++++--\n> net/netfilter/xt_osf.c | 3 +++\n> 2 files changed, 9 insertions(+), 2 deletions(-)\n> \n> diff --git a/net/netfilter/xt_TPROXY.c b/net/netfilter/xt_TPROXY.c\n> index e4bea1d346cf..ac4b011ce48c 100644\n> --- a/net/netfilter/xt_TPROXY.c\n> +++ b/net/netfilter/xt_TPROXY.c\n> @@ -40,6 +40,9 @@ tproxy_tg4(struct net *net, struct sk_buff *skb, __be32 laddr, __be16 lport,\n> \tstruct udphdr _hdr, *hp;\n> \tstruct sock *sk;\n> \n> +\tif (ip_is_fragment(iph))\n> +\t\treturn NF_DROP;\n> +\n> \thp = skb_header_pointer(skb, ip_hdrlen(skb), sizeof(_hdr), &_hdr);\n> \tif (hp == NULL)\n> \t\treturn NF_DROP;\n> @@ -106,6 +109,7 @@ tproxy_tg6_v1(struct sk_buff *skb, const struct xt_action_param *par)\n> {\n> \tconst struct ipv6hdr *iph = ipv6_hdr(skb);\n> \tconst struct xt_tproxy_target_info_v1 *tgi = par->targinfo;\n> +\tunsigned short fragoff = 0;\n> \tstruct udphdr _hdr, *hp;\n> \tstruct sock *sk;\n> \tconst struct in6_addr *laddr;\n> @@ -113,8 +117,8 @@ tproxy_tg6_v1(struct sk_buff *skb, const struct xt_action_param *par)\n> \tint thoff = 0;\n> \tint tproto;\n> \n> -\ttproto = ipv6_find_hdr(skb, &thoff, -1, NULL, NULL);\n> -\tif (tproto < 0)\n> +\ttproto = ipv6_find_hdr(skb, &thoff, -1, &fragoff, NULL);\n> +\tif (tproto < 0 || fragoff)\n> \t\treturn NF_DROP;\n> \n> \thp = skb_header_pointer(skb, thoff, sizeof(_hdr), &_hdr);\n> diff --git a/net/netfilter/xt_osf.c b/net/netfilter/xt_osf.c\n> index dc9485854002..889dff4daff0 100644\n> --- a/net/netfilter/xt_osf.c\n> +++ b/net/netfilter/xt_osf.c\n> @@ -27,6 +27,9 @@\n> static bool\n> xt_osf_match_packet(const struct sk_buff *skb, struct xt_action_param *p)\n> {\n> +\tif (ip_is_fragment(ip_hdr(skb)))\n> +\t\treturn false;\n> +\n> \treturn nf_osf_match(skb, xt_family(p), xt_hooknum(p), xt_in(p),\n> \t\t\t xt_out(p), p->matchinfo, xt_net(p), nf_osf_fingers);\n> }\n> -- \n> 2.53.0\n>","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","netfilter-devel@vger.kernel.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=netfilter.org header.i=@netfilter.org\n header.a=rsa-sha256 header.s=2025 header.b=WRIFZ5PH;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=172.234.253.10; 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