netdev - RE: [net-next PATCH v1 1/3] net: sched: af_packet support for direct ring access

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Date:	Tue, 7 Oct 2014 15:59:35 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'David Miller' <davem@...emloft.net>
CC:	"willemb@...gle.com" <willemb@...gle.com>,
	"john.fastabend@...il.com" <john.fastabend@...il.com>,
	"dborkman@...hat.com" <dborkman@...hat.com>,
	"fw@...len.de" <fw@...len.de>,
	"gerlitz.or@...il.com" <gerlitz.or@...il.com>,
	"hannes@...essinduktion.org" <hannes@...essinduktion.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"john.ronciak@...el.com" <john.ronciak@...el.com>,
	"amirv@...lanox.com" <amirv@...lanox.com>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	"danny.zhou@...el.com" <danny.zhou@...el.com>
Subject: RE: [net-next PATCH v1 1/3] net: sched: af_packet support for
 direct ring access

From: David 
> From: David Laight <David.Laight@...LAB.COM>
> Date: Tue, 7 Oct 2014 09:27:03 +0000
> 
> > That is (probably) the only scheme that stops the application
> > accessing random parts of physical memory.
> 
> I don't know where this claim keeps coming from, it's false.
> 
> The application has to attach memory to the ring, and then the
> ring can only refer to that memory for the duration of the
> session.
> 
> There is no way that the user can program the address field of the
> descriptors to point at arbitrary physical memory locations.
> 
> There is protection and control.

I got the impression that the application was directly writing the ring
structure that the ethernet mac hardware uses to describe tx and rx buffers.
(ie they are mapped read-write into userspace).
Unless you have a system where you can limit the physical memory
ranges accessible to the mac hardware, I don't see how you can stop
the application putting rogue values into the ring.

Clearly I'm missing something in my quick read of the change.

	David



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