netdev - Re: [PATCH] tcp: should drop incoming frames without ACK flag set

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Date:	Wed, 26 Dec 2012 15:09:17 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, zhiyunq@...ch.edu, nanditad@...gle.com,
	ncardwell@...gle.com, john.dykstra1@...il.com
Subject: Re: [PATCH] tcp: should drop incoming frames without ACK flag set

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 26 Dec 2012 14:44:34 -0800

> [PATCH v2] tcp: should drop incoming frames without ACK flag set
> 
> In commit 96e0bf4b5193d (tcp: Discard segments that ack data not yet
> sent) John Dykstra enforced a check against ack sequences.
> 
> In commit 354e4aa391ed5 (tcp: RFC 5961 5.2 Blind Data Injection Attack
> Mitigation) I added more safety tests.
> 
> But we missed fact that these tests are not performed if ACK bit is
> not set.
> 
> RFC 793 3.9 mandates TCP should drop a frame without ACK flag set.
> 
> " fifth check the ACK field,
>       if the ACK bit is off drop the segment and return"
> 
> Not doing so permits an attacker to only guess an acceptable sequence
> number, evading stronger checks.
> 
> Many thanks to Zhiyun Qian for bringing this issue to our attention.
> 
> See :
> http://web.eecs.umich.edu/~zhiyunq/pub/ccs12_TCP_sequence_number_inference.pdf
> 
> Reported-by: Zhiyun Qian <zhiyunq@...ch.edu>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks Eric.
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