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Date:   Wed, 22 Feb 2023 00:24:13 +0100
From:   Pablo Neira Ayuso <pablo@netfilter.org>
To:     Xin Long <lucien.xin@gmail.com>
Cc:     netfilter-devel@vger.kernel.org,
        network dev <netdev@vger.kernel.org>,
        Jozsef Kadlecsik <kadlec@netfilter.org>,
        Florian Westphal <fw@strlen.de>, davem@davemloft.net,
        kuba@kernel.org, Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>
Subject: Re: [PATCH nf] netfilter: use skb len to match in length_mt6
Message-ID: <Y/VSnV8FFRWU4TzC@salvia>
References: <361acd69270a8c2746da5774644dda9147b407a1.1676676177.git.lucien.xin@gmail.com>
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On Fri, Feb 17, 2023 at 06:22:57PM -0500, Xin Long wrote:
> For IPv6 Jumbo packets, the ipv6_hdr(skb)->payload_len is always 0,
> and its real payload_len ( > 65535) is saved in hbh exthdr. With 0
> length for the jumbo packets, it may mismatch.
> 
> To fix this, we can just use skb->len instead of parsing exthdrs, as
> the hbh exthdr parsing has been done before coming to length_mt6 in
> ip6_rcv_core() and br_validate_ipv6() and also the packet has been
> trimmed according to the correct IPv6 (ext)hdr length there, and skb
> len is trustable in length_mt6().
> 
> Note that this patch is especially needed after the IPv6 BIG TCP was
> supported in kernel, which is using IPv6 Jumbo packets. Besides, to
> match the packets greater than 65535 more properly, a v1 revision of
> xt_length may be needed to extend "min, max" to u32 in the future,
> and for now the IPv6 Jumbo packets can be matched by:
> 
>   # ip6tables -m length ! --length 0:65535

Applied, thanks