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Date: Fri, 1 Jul 2011 18:13:45 +0200
From: Ingo Molnar <mingo@elte.hu>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Petr Tesarik <ptesarik@suse.cz>, Christoph Hellwig <hch@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Fenghua Yu <fenghua.yu@intel.com>, Ingo Molnar <mingo@redhat.com>,
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        Dave Jones <davej@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary
 physical addresses
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* H. Peter Anvin <hpa@zytor.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