netdev - Re: [PATCH v2 net-next] tcp: avoid tx starvation by SYNACK packets

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Message-ID: <20120627195032.GI1269@breakpoint.cc>
Date:	Wed, 27 Jun 2012 21:50:32 +0200
From:	Florian Westphal <fw@...len.de>
To:	David Miller <davem@...emloft.net>
Cc:	brouer@...hat.com, eric.dumazet@...il.com,
	hans.schillstrom@...csson.com, subramanian.vijay@...il.com,
	dave.taht@...il.com, netdev@...r.kernel.org, ncardwell@...gle.com,
	therbert@...gle.com, mph@...h.dk
Subject: Re: [PATCH v2 net-next] tcp: avoid tx starvation by SYNACK packets

David Miller <davem@...emloft.net> wrote:
> From: Jesper Dangaard Brouer <brouer@...hat.com>
> Date: Wed, 27 Jun 2012 08:32:13 +0200
> 
> > Using it as default, might be "dangerous" and open an attack vector
> > on SYN cookies in Linux.
> 
> If it's dangerous for syncookies then it's just as dangerous for
> the routing hash and the socket hashes where we use it already.
> Therefore, this sounds like a baseless claim to me.

I doubt using jhash is safe for syncookies.

There a several differences to other uses in kernel:
- all hash input except u32 cookie_secret[2] is known
- we transmit hash result (i.e, its visible to 3rd party)
- we do not re-seed the secret, ever

it should be quite easy to recompute cookie_secret[] from known syncookie
values?
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