netdev - Re: [PATCH net-next 0/6] net: Call skb_get

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1425499383.5130.227.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 04 Mar 2015 12:03:03 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org, fw@...len.de
Subject: Re: [PATCH net-next 0/6] net: Call skb_get_hash in qdiscs

On Wed, 2015-03-04 at 10:39 -0800, Tom Herbert wrote:

> - The flow keys are passed in cb[] structure. This severely limits
>   our ability to increase the key space. For instance, the folding
>   of IPv6 addresses in flow_dissector to make a 32-bit value
>   represents a significant loss of information (it would be easy
>   to create millions of different 4-tuples that would always produce the
>   same hash value). 


Yes, but then your patch is all about reducing flow compares to a single
u32 comparison in qdiscs (and elsewhere)

choke for example explicitly wants to make sure we drop a companion
if incoming packet belongs to the same flow.

Relying on a 'strong hash' whatever it can be was not considered in
Choke paper. There is no mention of a stochastic match.

If we no longer can store the keys in skb->cb[], fine (although I claim
skb->cb[] size should be irrelevant, see our discussion on this topic
with Florian)
 -> Just recompute the keys, using a local variable, from packet
content. Yes, it will be more expensive, but hey, we get what we want.

Same for sfq : your skb_get_hash_perturb() doesn't address the point I
made earlier.

It is only giving a false sense of security.
I would rather not spread it.
(Note that there is no documentation or changelog to explain the
pro/cons)

I doubt OVS would condense their flow keys in a single u32...


--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html