netdev - Re: [net-next PATCH 1/4] Revert "inet: limit length of fragment queue hash table bucket lists"

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Date:	Thu, 02 May 2013 08:16:41 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	netdev@...r.kernel.org
Subject: Re: [net-next PATCH 1/4] Revert "inet: limit length of fragment
 queue hash table bucket lists"

On Thu, 2013-05-02 at 09:59 +0200, Jesper Dangaard Brouer wrote:
> On Wed, 24 Apr 2013 17:00:30 -0700 Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > On Wed, 2013-04-24 at 17:48 +0200, Jesper Dangaard Brouer wrote:
> > > This reverts commit 5a3da1fe9561828d0ca7eca664b16ec2b9bf0055.
> > > 
> > > The problem with commit 5a3da1fe (inet: limit length of fragment
> > > queue hash table bucket lists) is that, once we hit the hash depth
> > > limit (of 128), the we *keep* the existing frag queues, not
> > > allowing new frag queues to be created.  Thus, an attacker can
> > > effectivly block handling of fragments for 30 sec (as each frag
> > > queue have a timeout of 30 sec)
> > > 
> > 
> > I do not think its good to revert this patch. It was a step in right
> > direction.
> 
> We need a revert, because we are too close to the merge window, and
> cannot complete the needed "steps" to make this patch safe, sorry.

Again, a limit of 128 is totally OK. Its in fact too big.

128 cache misses consume 5 us

Allowing a chain being non limited is a more severe bug.

Reverting will allow an attacker to consume all your cpu cycles.

We changed INETFRAGS_HASHSZ to 1024, so 128*1024 max frags is already a
very big limit.

No matter what we do, we need to limit both :

- Memory consumption
- Cpu consumption

For people willing to allow more memory to be used, the only way is to
resize hash table, or using a bigger INETFRAGS_HASHSZ

I do not think there is a hurry, current defrag code is already better
than what we had years ago.


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