netdev - Re: [PATCH net-next] tcp: accept RST if SEQ matches right edge of SACK block

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Date:	Fri, 3 Jun 2016 15:49:17 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Pau Espin <pau.espin@...sares.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next] tcp: accept RST if SEQ matches right edge of
 SACK block

On Fri, Jun 3, 2016 at 3:44 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
> On Fri, Jun 3, 2016 at 12:24 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> On Fri, 2016-06-03 at 11:45 -0400, Neal Cardwell wrote:
>>> But I would also vote to tighten up the proposed logic slightly, and
>>> only check the seq of the incoming RST against the right edge of the
>>> *right-most* SACK block. That is, the code could loop through the
>>> tp->selective_acks to find the right-most of the right edges of the
>>> SACK blocks (the end_seq that has no other end_seq after() it). AFAICT
>>> it makes sense to expect that a legitimate incoming RST might match
>>> rcv_nxt, or might match the right-most edge of the right-most SACK.
>>> But allowing a RST to match a sequence of some SACK in the middle of
>>> the sequence range would seem to only increase the attack surface for
>>> RST attacks.
>>
>> Well, the most recent info would be in [0], no need to iterate, right ?
>>
>> So only look at the first sack block in the array, even if we have 3 or
>> 4 blocks there.
>
> Yes, good point. It should only need to check the first SACK block in
> the selective_acks  array.

Well, hmm. The most recent SACK block may not be the right-most one,
if there is reordering or packets are retransmitted. So AFAICT if we
wanted to try hard to just use the right-most SACK block we'd need to
check all the blocks.

But just checking the first SACK block seems like a reasonable
trade-off in terms of simplicity.

I don't feel strongly either way.

neal