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 11:45:00 -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 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.

neal