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

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Date:	Fri, 03 Jun 2016 09:24:54 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neal Cardwell <ncardwell@...gle.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, 2016-06-03 at 11:45 -0400, Neal Cardwell wrote:
> On Fri, Jun 3, 2016 at 11:13 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > I have no strict opinion on this.
> >
> > It seems to me that checking at most 4 right edges (at least in current
> > linux implementation) is not adding a huge risk, and allows for better
> > interoperability.
> >
> > I vote for no extra sysctl.
> 
> I vote for no extra sysctl as well.
> 
> 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.