linux-kernel - Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary physical addresses

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Message-ID: <4DFE9C27.3000305@gmail.com>
Date:	Mon, 20 Jun 2011 11:02:31 +1000
From:	Ryan Mallon <rmallon@...il.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	Ingo Molnar <mingo@...e.hu>, Petr Tesarik <ptesarik@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Luck <tony.luck@...el.com>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
	linux-sh@...r.kernel.org, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary
 physical addresses

On 20/06/11 10:52, Matthew Wilcox wrote:
> On Mon, Jun 20, 2011 at 10:46:08AM +1000, Ryan Mallon wrote:
>> On 20/06/11 10:42, Matthew Wilcox wrote:
>>> On Mon, Jun 20, 2011 at 09:02:17AM +1000, Ryan Mallon wrote:
>>>> There are drivers where this makes sense. For example an FPGA device
>>>> with a proprietary register layout on the memory bus can be done this
>>>> way. The FPGA can simply be mapped in user-space via /dev/mem and
>>>> handled there. If the device requires no access other than memory bus
>>>> reads and writes then writing a custom char device driver just to get an
>>>> mmap function seems a bit overkill.
>>> Calling a 30 line device driver "overkill" might in itself be overkill?
>>>
>> I mean overkill in the sense of having to write the driver at all. Why
>> write a 30 line driver just to re-implement some functionality of
>> /dev/mem?
> Because it pushes the tradeoff in the right direction.  Somebody wants
> to do something weird is a little inconvenienced vs protecting the vast
> majority of users from some security escalation problems.
How does it protect against security escalation? A process mapping a 
region either from /dev/mem or from some custom char device can't escape 
that region right? In either case you need root privileges to make the 
mapping in the first place.
> Besides, if you have a real bus with discoverable regions
> (like PCI BARs), the bus should have sysfs entries like
> /sys/bus/pci/devices/0000\:06\:06.0/resource0 that can be mmaped.
> Then there's no need for a device driver at all, *and* the privilege
> escalation isn't achievable.
>
> Of course, most embedded architectures have crap discoverability.
Which is also where devices like FPGAs tend to exist :-).

~Ryan

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