linux-kernel - Re: [PATCH] xen-netfront: Fix handling packets on compound pages with skb

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Date:	Sun, 3 Aug 2014 10:11:10 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	David Miller <davem@...emloft.net>
CC:	<wei.liu2@...rix.com>, <zoltan.kiss@...rix.com>,
	<konrad.wilk@...cle.com>, <boris.ostrovsky@...cle.com>,
	<david.vrabel@...rix.com>, <Ian.Campbell@...rix.com>,
	<paul.durrant@...rix.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH] xen-netfront: Fix handling packets on compound pages
 with skb_segment

On Sat, Aug 02, 2014 at 03:33:37PM -0700, David Miller wrote:
> From: Wei Liu <wei.liu2@...rix.com>
> Date: Fri, 1 Aug 2014 12:02:46 +0100
> 
> > On Thu, Jul 31, 2014 at 01:25:20PM -0700, David Miller wrote:
> >> If you were to have a 64-slot TX queue, you ought to be able to handle
> >> this theoretical 51 slot SKB.
> > 
> > There's two problems:
> > 1. IIRC a single page ring has 256 slots, allowing 64 slots packet
> >    yields 4 in-flight packets in worst case.
> > 2. Older netback could not handle this large number of slots and it's
> >    likely to deem the frontend malicious.
> > 
> > For #1, we don't actually care that much if guest screws itself by
> > generating 64 slot packets. #2 is more concerning.
> 
> How many slots can the older netback handle?

I listed those two problems in the context "if we were to lift this
limit in the latest net-next tree", so "older netback" actually refers
to netback from 3.10 to 3.16.

The current implementation allows the number of slots X:
 1. X <= 18, valid packet
 2. 18 < X < fatal_slot_count, dropped
 3. X >= fatal_slot_count, malicious frontend

fatal_slot_count has default value of 20.

Wei.

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