From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH net-next 1/2] net_sched: don't do precise pkt_len
 computation for untrusted packets
Date: Tue, 19 Mar 2013 05:10:40 -0700
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	 <20130317.121026.733329595817911776.davem@davemloft.net>
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Cc: David Miller <davem@davemloft.net>, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org, mst@redhat.com, edumazet@google.com,
	Daniel Borkmann <dborkman@redhat.com>
To: Jason Wang <jasowang@redhat.com>
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On Tue, 2013-03-19 at 17:25 +0800, Jason Wang wrote:

> I believe before doing header check for untrusted packets, the only
> thing we can trust is skb->len and that's we've used before
> 1def9238d4aa2. But after that, we're trying to use unchecked or
> meaningless value (e.g gso_segs were reset to zero in
> tun/macvtap/packet), and guest then can utilize this to result a very
> huge (-1U) pkt_len by filling evil value in the header. Can all kinds of
> packet scheduler survive this kinds of possible DOS?

I would use the flow dissector to fix the transport header from all
DODGY providers.

Daniel Borkmann is working on a patch serie adding nhoff into flow_keys,
and adding __skb_get_poff(const struct sk_buff *skb), for a BPF
extension we talked about in Copenhagen / Netfilter Workshop.

You could then set the transport header offset to the right value.

(and drop evil packets before they go further in the stack)

if (gso_packet(skb)) {
	u32 poff = __skb_get_poff(skb);

	if (!poff) {
		drop_evil_packet(skb);
	} else {
		skb_set_transport_header(skb, poff);
		...
	}
}