linux-kernel - Re: extra large DMA buffer for PCI-E device under UIO

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Message-ID: <20111122185244.GD15508@local>
Date:	Tue, 22 Nov 2011 19:52:45 +0100
From:	"Hans J. Koch" <hjk@...sjkoch.de>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	"Hans J. Koch" <hjk@...sjkoch.de>,
	Jean-Francois Dagenais <jeff.dagenais@...il.com>,
	Greg KH <gregkh@...e.de>, tglx@...utronix.de,
	linux-pci@...r.kernel.org, open list <linux-kernel@...r.kernel.org>
Subject: Re: extra large DMA buffer for PCI-E device under UIO

On Tue, Nov 22, 2011 at 08:40:40PM +0200, Michael S. Tsirkin wrote:
> On Tue, Nov 22, 2011 at 06:54:02PM +0100, Hans J. Koch wrote:
> > On Tue, Nov 22, 2011 at 07:37:23PM +0200, Michael S. Tsirkin wrote:
> > [...]
> > > > Or am I better off with a UIO solution?
> > > 
> > > You should probably write a proper kernel driver, not a UIO one.
> > > your kernel driver would have to prevent the device fom DMA into memory
> > > outside the allocated range, even if userspace is malicious.
> > > That's why UIO is generally not recommended for PCI devices that do DMA.
> > 
> > When UIO was designed, the main goal was the ability to handle interrupts
> > from userspace. There was no requirement for DMA. In fact, in five years I
> > didn't get one real world device on my desk that needed it. That doesn't
> > mean there are no such devices. Adding DMA support to the UIO core was
> > discussed several times but noone ever did it. Ideas are still welcome...
> > 
> > If parts of the driver should be in userspace, you should really try
> > to extend the UIO core instead of re-implementing UIO functionality in
> > a "proper kernel driver".
> > 
> > Thanks,
> > Hans
> 
> Right, I really meant put all of the driver in the kernel.
> If parts are in userspace, and device can do DMA,
> you are faced with the problem as userspace suddenly
> can access arbitrary memory through the device.

That's nothing UIO specific. You have the same problem with /dev/mem
or graphic cards. If you're root, you can do lots of things that can
compromise security or crash your system.

Thanks,
Hans
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