netdev - Re: [PATCH net] ip: make IP identifiers less predictable

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Message-ID: <53D2BDF3.5060909@cs.unm.edu>
Date:	Fri, 25 Jul 2014 14:28:35 -0600
From:	Jeffrey Knockel <jeffk@...unm.edu>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	"Jedidiah R. Crandall" <crandall@...unm.edu>,
	Willy Tarreau <w@....eu>,
	"security@...nel.org" <security@...nel.org>
Subject: Re: [PATCH net] ip: make IP identifiers less predictable

On 07/25/2014 12:09 PM, Eric Dumazet wrote:
> What do you mean by "an attacker who controls a large number of
> addresses" ?

In general, I mean any attacker who can read the packets sent to a large
number of different Internet addresses.  Even an attacker who has one
address but can cycle through different assignments may be a problem.

> The hash(daddr) -> slot function is not known, as we use a Jenkin hash
> with a secret ( ip_idents_hashrnd & ip6_idents_hashrnd )

That's true, but the secret never changes, right?  I may not be able to
identify the slot number that any address is hashed to, but I can
identify when some victim address hashes to the same slot as one of my
addresses whose packets I can read.  For instance, if I in short succession

1. Probe value of the IP id counter for each of my addresses
2. Spoof a large number of (e.g.) echo requests from victim address (or
something else to the distribution that I can measure)
3. Again probe value of the IP id counter for each of my addresses

Then I can tell which of my addresses hash to the same slot as the
victim address by whose value of the IP id counter has jumped as a
result of the linux machine sending echo replies to the victim.

Jeff
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