netdev - Re: [PATCH net-next 1/7] tcp: apply Kern's check on RTTs used for congestion control

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: Windows password security audit tool. GUI, reports in PDF.

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

Date:	Tue, 2 Feb 2016 20:30:27 +0100
From:	Kenneth Klette Jonassen <kennetkl@....uio.no>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 1/7] tcp: apply Kern's check on RTTs used for
 congestion control

On Sat, Oct 17, 2015 at 6:57 AM, Yuchung Cheng <ycheng@...gle.com> wrote:
> Currently ca_seq_rtt_us does not use Kern's check. Fix that by
> checking if any packet acked is a retransmit, for both RTT used
> for RTT estimation and congestion control.
>
*snip*

This patch (commit 9e45a3e) puzzles me, because Karn's check was
already in effect:
http://lxr.free-electrons.com/source/net/ipv4/tcp_input.c?v=4.3#L3117

Since first_ackt/last_ackt is only set when non-retransmitted packets
are ACKed (sequentially), we know them to be unambiguous samples for
RTTM. Even if a (sequential) ACK covers retransmitted packets, we can
still make a valid RTTM if that ACK also covers non-retransmitted
packets. But this patch seems to prevent that?