netdev - Re: [PATCH] fragment: Improved handling of incorrect IP fragments

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Message-ID: <20200102112731.299b5fe4@hermes.lan>
Date:   Thu, 2 Jan 2020 11:27:31 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Ttttabcd <ttttabcd@...tonmail.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        "kuznet@....inr.ac.ru" <kuznet@....inr.ac.ru>,
        "yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH] fragment: Improved handling of incorrect IP fragments

On Tue, 31 Dec 2019 01:19:27 +0000
Ttttabcd <ttttabcd@...tonmail.com> wrote:

> @@ -267,8 +278,6 @@ static int ip6_frag_reasm(struct frag_queue *fq, struct sk_buff *skb,
>  	payload_len = ((skb->data - skb_network_header(skb)) -
>  		       sizeof(struct ipv6hdr) + fq->q.len -
>  		       sizeof(struct frag_hdr));
> -	if (payload_len > IPV6_MAXPLEN)
> -		goto out_oversize;

You can not safely drop this check.
With recursive fragmentation it is possible that the initial payload ends
up exceeding the maximum packet length.