netdev - Re: [PATCH net-next 1/5] sctp: add the rhashtable apis for sctp global transport hashtable

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Date:	Mon, 11 Jan 2016 16:19:57 -0200
From:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	David Miller <davem@...emloft.net>, eric.dumazet@...il.com,
	lucien.xin@...il.com, netdev@...r.kernel.org,
	linux-sctp@...r.kernel.org, mleitner@...hat.com,
	vyasevic@...hat.com, daniel@...earbox.net
Subject: Re: [PATCH net-next 1/5] sctp: add the rhashtable apis for sctp
 global transport hashtable

On Mon, Jan 11, 2016 at 01:08:56PM -0500, Vlad Yasevich wrote:
> On 01/11/2016 11:33 AM, Marcelo Ricardo Leitner wrote:
> > On Mon, Jan 11, 2016 at 05:32:10PM +0800, Herbert Xu wrote:
> >> David Miller <davem@...emloft.net> wrote:
> >>> From: Eric Dumazet <eric.dumazet@...il.com>
> >>> Date: Wed, 30 Dec 2015 11:57:31 -0500
> >>>
> >>>> I am against using rhashtable in SCTP (or TCP) at this stage, given the
> >>>> number of bugs we have with it.
> >>>
> >>> Come on Eric, we've largely dealt with all of these problems.  I haven't
> >>> seen a serious report in a while.
> >>
> >> Well there is still the outstanding issue with softirq insertion
> >> potentially failing with ENOMEM if we fail to expand the hash
> >> table using just kmalloc.
> >>
> >> So if the target user does softirq insertions, I would wait until
> >> the fix for that is ready.
> > 
> > It does some, yes. If listening socket is not backlogged, there will be
> > N inserts at each new association, where N is the number of IP addresses
> > that the client is advertising.
> > 
> > This is done on the second stage of the SCTP handshake. Not easily
> > DoS-able as it requires receiving a packet from server and replying
> > based on it, plus N is limited by MTU.
> 
> How is N limited by MTU?  INIT and COOKIE chunks are allowed to exceed
> mtu and will be IP fragmented.  So it seems that N is limited by 65535-overhead,
> where overhead is all the headers plus the sctp cookie info.  That can
> be quite a lot of addresses.

Oups, you're right. One then can trigger quite some inserts with a
single new assoc attempt, yes.

  Marcelo