netdev - Re: [PATCH] ipv4: mitigate an integer underflow when comparing tcp timestamps

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Date:	Sun, 14 Nov 2010 09:52:25 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Zhang Le <r0bertz@...too.org>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	"Pekka Savola (ipv6)" <pekkas@...core.fi>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH] ipv4: mitigate an integer underflow when comparing tcp
 timestamps

Le dimanche 14 novembre 2010 à 15:35 +0800, Zhang Le a écrit :
> Behind a loadbalancer which does NAT, peer->tcp_ts could be much smaller than
> req->ts_recent. In this case, theoretically the req should not be ignored.
> 
> But in fact, it could be ignored, if peer->tcp_ts is so small that the
> difference between this two number is larger than 2 to the power of 31.
> 
> I understand that under this situation, timestamp does not make sense any more,
> because it actually comes from difference machines. However, if anyone
> ever need to do the same investigation which I have done, this will
> save some time for him.
> 
> Signed-off-by: Zhang Le <r0bertz@...too.org>
> ---
>  net/ipv4/tcp_ipv4.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index 8f8527d..1eb4974 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -1352,8 +1352,8 @@ int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
>  		    peer->v4daddr == saddr) {
>  			inet_peer_refcheck(peer);
>  			if ((u32)get_seconds() - peer->tcp_ts_stamp < TCP_PAWS_MSL &&
> -			    (s32)(peer->tcp_ts - req->ts_recent) >
> -							TCP_PAWS_WINDOW) {
> +			    ((s32)(peer->tcp_ts - req->ts_recent) > TCP_PAWS_WINDOW &&
> +			     peer->tcp_ts > req->ts_recent)) {
>  				NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSPASSIVEREJECTED);
>  				goto drop_and_release;
>  			}

This seems very wrong to me.

Adding a : if (peer->tcp_ts > req->ts_recent) condition is _not_ going
to help. And it might break some working setups, because of wrap around.

Really, if you have multiple clients behind a common NAT, you cannot use
this code at all, since NAT doesnt usually change TCP timestamps.

What about following patch instead ?

[PATCH] doc: extend tcp_tw_recycle documentation

tcp_tw_recycle should not be used on a server if there is a chance
clients are behind a same NAT. Document this fact before too many users
discover this too late.

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
 Documentation/networking/ip-sysctl.txt |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index c7165f4..406f0d5 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -446,7 +446,12 @@ tcp_tso_win_divisor - INTEGER
 tcp_tw_recycle - BOOLEAN
 	Enable fast recycling TIME-WAIT sockets. Default value is 0.
 	It should not be changed without advice/request of technical
-	experts.
+	experts. If you set it to 1, make sure you dont miss connections
+	attempts (check LINUX_MIB_PAWSPASSIVEREJECTED netstat counter).
+	In particular, this might break if several clients are behind
+	a common NAT device, since their TCP timestamp wont be changed
+	by the NAT. tcp_tw_recycle should be used with care, most
+	probably in private networks.
 
 tcp_tw_reuse - BOOLEAN
 	Allow to reuse TIME-WAIT sockets for new connections when it is


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