Subject: Re: [PATCH 4/4 nf] netfilter: xtables: fix L4 header parsing for non-first fragments

From: Pablo Neira Ayuso <[email-masked]>
Date: Sat, 18 Apr 2026 09:51:24 +0200

On Fri, Apr 17, 2026 at 08:34:35PM +0200, Fernando Fernandez Mancera wrote:
> The TPROXY target and osf match relies on L4 header to operate. For
> fragmented packets, every fragment carries the transport protocol
> identifier, but only the first fragment contains the L4 header.
> 
> As the 'raw' table can be configured to run at priority -450 (before
> defragmentation at -400), the target/match can be reached before
> reassembly. In this case, non-first fragments have their payload
> incorrectly parsed as a TCP/UDP header.

I see, this refers to a misconfiguration scenario.

> Add a fragment check to ensure TPROXY/osf only evaluates unfragmented
> packets or the first fragment in the stream.

LGTM this combo patch for osf and TPROXY in xtables.

Thanks.

> Fixes: 902d6a4c2a4f ("netfilter: nf_defrag: Skip defrag if NOTRACK is set")
> Signed-off-by: Fernando Fernandez Mancera <[email-masked]>
> ---
>  net/netfilter/xt_TPROXY.c | 8 ++++++--
>  net/netfilter/xt_osf.c    | 3 +++
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/net/netfilter/xt_TPROXY.c b/net/netfilter/xt_TPROXY.c
> index e4bea1d346cf..ac4b011ce48c 100644
> --- a/net/netfilter/xt_TPROXY.c
> +++ b/net/netfilter/xt_TPROXY.c
> @@ -40,6 +40,9 @@ tproxy_tg4(struct net *net, struct sk_buff *skb, __be32 laddr, __be16 lport,
>  	struct udphdr _hdr, *hp;
>  	struct sock *sk;
>  
> +	if (ip_is_fragment(iph))
> +		return NF_DROP;
> +
>  	hp = skb_header_pointer(skb, ip_hdrlen(skb), sizeof(_hdr), &_hdr);
>  	if (hp == NULL)
>  		return NF_DROP;
> @@ -106,6 +109,7 @@ tproxy_tg6_v1(struct sk_buff *skb, const struct xt_action_param *par)
>  {
>  	const struct ipv6hdr *iph = ipv6_hdr(skb);
>  	const struct xt_tproxy_target_info_v1 *tgi = par->targinfo;
> +	unsigned short fragoff = 0;
>  	struct udphdr _hdr, *hp;
>  	struct sock *sk;
>  	const struct in6_addr *laddr;
> @@ -113,8 +117,8 @@ tproxy_tg6_v1(struct sk_buff *skb, const struct xt_action_param *par)
>  	int thoff = 0;
>  	int tproto;
>  
> -	tproto = ipv6_find_hdr(skb, &thoff, -1, NULL, NULL);
> -	if (tproto < 0)
> +	tproto = ipv6_find_hdr(skb, &thoff, -1, &fragoff, NULL);
> +	if (tproto < 0 || fragoff)
>  		return NF_DROP;
>  
>  	hp = skb_header_pointer(skb, thoff, sizeof(_hdr), &_hdr);
> diff --git a/net/netfilter/xt_osf.c b/net/netfilter/xt_osf.c
> index dc9485854002..889dff4daff0 100644
> --- a/net/netfilter/xt_osf.c
> +++ b/net/netfilter/xt_osf.c
> @@ -27,6 +27,9 @@
>  static bool
>  xt_osf_match_packet(const struct sk_buff *skb, struct xt_action_param *p)
>  {
> +	if (ip_is_fragment(ip_hdr(skb)))
> +		return false;
> +
>  	return nf_osf_match(skb, xt_family(p), xt_hooknum(p), xt_in(p),
>  			    xt_out(p), p->matchinfo, xt_net(p), nf_osf_fingers);
>  }
> -- 
> 2.53.0
>