Re: [PATCH v2 -next] netfilter: don't use per-destination incrementing ports in nat random mode

[Daniel Borkmann <dborkman@redhat.com>]

On 01/03/2014 11:52 PM, Pablo Neira Ayuso wrote:
> On Fri, Dec 20, 2013 at 10:40:29PM +0100, Hannes Frederic Sowa wrote:
>> From: Daniel Borkmann <dborkman@redhat.com>
>>
>> We currently use prandom_u32() for allocation of ports in tcp bind(0)
>> and udp code. In case of plain SNAT we try to keep the ports as is
>> or increment on collision.
>>
>> SNAT --random mode does use per-destination incrementing port
>> allocation. As a recent paper pointed out in [1] that this mode of
>> port allocation makes it possible to an attacker to find the randomly
>> allocated ports through a timing side-channel in a socket overloading
>> attack conducted through an off-path attacker.
>>
>> So, NF_NAT_RANGE_PROTO_RANDOM actually weakens the port randomization
>> in regard to the attack described in this paper. As we need to keep
>> compatibility, add another flag called NF_NAT_RANGE_PROTO_RANDOM_FULLY
>> that would replace the NF_NAT_RANGE_PROTO_RANDOM hash-based port
>> selection algorithm with a simple prandom_u32() in order to mitigate
>> this attack vector. Note that the lfsr113's internal state is
>> periodically reseeded by the kernel through a local secure entropy
>> source.
>>
>> More details can be found in [1], the basic idea is to send bursts
>> of packets to a socket to overflow its receive queue and measure
>> the latency to detect a possible retransmit when the port is found.
>> Because of increasing ports to given destination and port, further
>> allocations can be predicted. This information could then be used by
>> an attacker for e.g. for cache-poisoning, NS pinning, and degradation
>> of service attacks against DNS servers [1]:
>>
>>    The best defense against the poisoning attacks is to properly
>>    deploy and validate DNSSEC; DNSSEC provides security not only
>>    against off-path attacker but even against MitM attacker. We hope
>>    that our results will help motivate administrators to adopt DNSSEC.
>>    However, full DNSSEC deployment make take significant time, and
>>    until that happens, we recommend short-term, non-cryptographic
>>    defenses. We recommend to support full port randomisation,
>>    according to practices recommended in [2], and to avoid
>>    per-destination sequential port allocation, which we show may be
>>    vulnerable to derandomisation attacks.
>>
>> Joint work between Hannes Frederic Sowa and Daniel Borkmann.
>
> Applied, thanks.
>
> I have renamed the title of this patch to: "add full port
> randomization support" which I though a bit more descriptive with the
> final patch that has settled down, just in case you look for it in the
> nf-next tree.

That's fine, thanks a lot Pablo!