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

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Date:	Fri, 1 Jul 2011 18:13:45 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Petr Tesarik <ptesarik@...e.cz>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.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


* H. Peter Anvin <hpa@...or.com> wrote:

> On 07/01/2011 08:36 AM, Ingo Molnar wrote:
> > 
> > So we could kill multiple birds with the same stone here:
> > 
> >  - remove various ugly uses of /dev/mem (including the rootkit usage),
> >    with or without strict-devmem
> > 
> >  - extending it to above-4G for inspection purposes
> > 
> >  - allowing to kill /dev/mem access runtime similar to the 
> >    disable_modules lock-down killswitch, for the so inclined.
> > 
> > Would you be interested in modifying your patch-set in such a 
> > fashion?
> > 
> 
> There is another use that I have looked at, as well: for testing 
> purposes, it would be extremely good to be able to dirty and/or 
> flush an arbitrary physical cache line for testing purposes.
> 
> This is very very similar to /dev/mem usage -- access to an 
> arbitrary chunk of memory -- and a fully enabled /dev/mem can of 
> course support this use (just mmap the page with the relevant cache 
> line).  However, it could also be a separate device which could 
> have looser permissions than /dev/mem; or a set of ioctls on 
> /dev/mem with a separate kill switch, because no data would ever be 
> have modified or returned to user space.
> 
> Either way, though, we found that it would share a lot of code with 
> the /dev/mem implementation, and as such fixing up the underlying 
> machinery is the sanest way to upstream this.

To me that cache flush thing sounds obscure (but still useful) enough 
to justify a new ioctl over /dev/mem.

Not sure it even needs a killswitch, unless there's some real 
security problem related to it.

Thanks,

	Ingo
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