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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 5 May 2017 02:24:56 +0200
From:   Florian Westphal <fw@...len.de>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Florian Westphal <fw@...len.de>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net] tcp: randomize timestamps on syncookies

Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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, it is not used from
> a module.

I like the direction, but this is incomplete.

>  	if (want_cookie) {
>  		isn = cookie_init_sequence(af_ops, sk, skb, &req->mss);
> -		tcp_rsk(req)->ts_off = 0;

This breaks syncookies w. timestamps; cookie_timestamp_decode() lacks a tsoff
for readjustment.

We also need to pass the (recomputed) tsoff to tcp_get_cookie_sock().

Other than this, this patch looks good to me, thanks!