linux-kernel - Re: [PATCH net-next] hv_netvsc: don't make assumptions on struct flow

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Date:	Thu, 14 Jan 2016 14:08:06 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Haiyang Zhang <haiyangz@...rosoft.com>
Cc:	Tom Herbert <tom@...bertland.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	David Miller <davem@...emloft.net>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	KY Srinivasan <kys@...rosoft.com>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] hv_netvsc: don't make assumptions on struct
 flow_keys layout

On Thu, 2016-01-14 at 20:23 +0000, Haiyang Zhang wrote:
> 

> For non-random inputs, I used the port selection of iperf that increases 
> the port number by 2 for each connection. Only send-port numbers are 
> different, other values are the same. I also tested some other fixed 
> increment, Toeplitz spreads the connections evenly. For real applications, 
> if the load came from local area, then the IP/port combinations are 
> likely to have some non-random patterns.

We are not putting code in core networking stack favoring non secure
behavior.

The +2 behavior for connections from A to B:<fixed port> is something
that we will eventually remove in the future. It used to be +1 not a
long time ago...

Say if we implement the following,

https://tools.ietf.org/html/rfc6056#section-3.3.4

The fact that Toeplitz hash has this linear property should not be a
valid reason to help hackers to exploit vulnerabilities.

In my tests I was using netperf, which randomizes both source &
destination ports.

This is why I could not reproduce your results based on iperf, which
generates 5-tuple in a totally predictable way.

This reminds me some drivers had a well known Toeplitz RSS key, allowing
attackers to direct their attack on a single queue.

I guess we could replace sk_txhash generator by a simple linear
allocator and boom, your driver will be pleased.

But this is only for a very specific workload.

diff --git a/include/net/sock.h b/include/net/sock.h
index e830c1006935..949527413cfb 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1689,7 +1689,8 @@ unsigned long sock_i_ino(struct sock *sk);

 static inline u32 net_tx_rndhash(void)
 {
-       u32 v = prandom_u32();
+       static u32 last_hash;
+       u32 v = ++last_hash; // do not care about SMP races.

        return v ?: 1;
 }