netdev - Re: IP oversized ip oacket from - header size should be skipped?

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Message-ID: <CAA85sZvo54saR-wrVhQ=LLz7P28tzA-sO3Bg=YuBANZTcY0PpQ@mail.gmail.com>
Date: Fri, 28 Jun 2024 12:55:58 +0200
From: Ian Kumlien <ian.kumlien@...il.com>
To: Florian Westphal <fw@...len.de>
Cc: Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: IP oversized ip oacket from - header size should be skipped?

On Fri, Jun 28, 2024 at 12:53 PM Florian Westphal <fw@...len.de> wrote:
>
> Ian Kumlien <ian.kumlien@...il.com> wrote:
> > Hi,
> >
> > In net/ipv4/ip_fragment.c line 412:
> > static int ip_frag_reasm(struct ipq *qp, struct sk_buff *skb,
> >                          struct sk_buff *prev_tail, struct net_device *dev)
> > {
> > ...
> >         len = ip_hdrlen(skb) + qp->q.len;
> >         err = -E2BIG;
> >         if (len > 65535)
> >                 goto out_oversize;
> > ....
> >
> > We can expand the expression to:
> > len = (ip_hdr(skb)->ihl * 4) + qp->q.len;
> >
> > But it's still weird since the definition of q->len is: "total length
> > of the original datagram"
>
> AFAICS datagram == l4 payload, so adding ihl is correct.

But then it should be added and multiplied by the count of fragments?
which doesn't make sense to me...

I have a security scanner that generates big packets (looking for
overflows using nmap nasl) that causes this to happen on send....