netdev - Re: [net-next PATCH v2 1/1] net sched actions: skbedit add support for mod-ing skb pkt

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Date:	Mon, 13 Jun 2016 10:00:27 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Jamal Hadi Salim <jhs@...atatu.com>, davem@...emloft.net
CC:	netdev@...r.kernel.org, xiyou.wangcong@...il.com,
	Jamal Hadi Salim <hadi@...atatu.com>
Subject: Re: [net-next PATCH v2 1/1] net sched actions: skbedit add support
 for mod-ing skb pkt_type

Hi Jamal,

On 06/12/2016 11:24 PM, Jamal Hadi Salim wrote:
> From: Jamal Hadi Salim <hadi@...atatu.com>
>
> Extremely useful for setting packet type to host so i dont
> have to modify the dst mac address using pedit (which requires
> that i know the mac address)
>
> Signed-off-by: Jamal Hadi Salim <jhs@...atatu.com>

I'm wondering if this is a good idea, I was thinking about something
like this as well some time ago. So far pkt_type is just exposed as
read-only to user space right now and I'm a bit worried that when we
allow to set it arbitrarily, then this could lead to hard to debug
issues since skb->pkt_type is used in a lot of places with possibly
different assumptions and applications now need to mistrust the kernel
whether skb->pkt_type was actually what the kernel itself set in the
first place or skbedit with possibly some nonsense value (like rewriting
PACKET_OUTGOING into PACKET_LOOPBACK, etc). Did you audit that this
is safe?

Thanks,
Daniel