netdev - Re: [PATCH net-next 6/6] sched: Eliminate use of flow_keys in sch

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Date:	Mon, 02 Mar 2015 11:24:38 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Florian Westphal <fw@...len.de>
Subject: Re: [PATCH net-next 6/6] sched: Eliminate use of flow_keys in
 sch_sfq

On Mon, 2015-03-02 at 07:31 -0800, Tom Herbert wrote:

> As I already mentioned, the probability of skb_get_hash returning the
> same value for two flows is 1/2^32. I suspect it's greater probability
> to get a hash collision between two IPv6 flows with subtlety different
> addresses or simply match 4-tuples in different address spaces (like
> VLAN or with VNID). If hash collision is really a problem, we can
> periodically rekey skb->hash (either SW or HW) or maybe move to 64 bit
> hashes somehow. Anyway, we potentially have the same problem in RSS
> and other uses of the hash, if hashes collide then two flows share the
> same bucket forever unless there is an action to break that.

Main historical use of SFQ is for routers.

We do not control skb->hash for forwarding workload, unless you
reprogram RSS keys on the NIC.

If you remember, we used to have a problem on some NIC using a well
known RSS key. It is fortunate SFQ/fq_codel did not rely on skb->hash
too much.

If to target one flow, attackers need to attack your big boxes to flood
one particular RX queue at NIC level, this has a huge cost for them.

While targeting one hash bucket on a qdisc might be much easier, if they
know even a hash perturbation has no effect against their attack.

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