netdev - Re: [PATCH v3 net] tcp: randomize timestamps on syncookies

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Message-Id: <20170505.120043.1050625612733133272.davem@davemloft.net>
Date:   Fri, 05 May 2017 12:00:43 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     fw@...len.de, netdev@...r.kernel.org, ycheng@...gle.com
Subject: Re: [PATCH v3 net] tcp: randomize timestamps on syncookies

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 05 May 2017 06:56:54 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Whole point of randomization was to hide server uptime, but an attacker
> can simply start a syn flood and TCP generates 'old style' timestamps,
> directly revealing server jiffies value.
> 
> Also, TSval sent by the server to a particular remote address vary
> depending on syncookies being sent or not, potentially triggering PAWS
> drops for innocent clients.
> 
> Lets implement proper randomization, including for SYNcookies.
> 
> Also we do not need to export sysctl_tcp_timestamps, since it is not
> used from a module.
> 
> In v2, I added Florian feedback and contribution, adding tsoff to
> tcp_get_cookie_sock().
> 
> v3 removed one unused variable in tcp_v4_connect() as Florian spotted.
> 
> Fixes: 95a22caee396c ("tcp: randomize tcp timestamp offsets for each connection")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reviewed-by: Florian Westphal <fw@...len.de>
> Tested-by: Florian Westphal <fw@...len.de>

Applied and queued up for -stable, thanks Eric.