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

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Message-ID: <20110701143735.GA21367@elte.hu>
Date:	Fri, 1 Jul 2011 16:37:35 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	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


* Christoph Hellwig <hch@...radead.org> wrote:

> On Fri, Jul 01, 2011 at 02:58:02PM +0200, Ingo Molnar wrote:
> > So what was not mentioned in your series, what is *your* motivation 
> > and your usecase? Enabling closed-source userspace drivers? Enabling 
> > the crash utility?
> 
> He stated it pretty clearly in the thread, it's the crash utility.

True. I only re-read the first patch and forgot about the resulting 
discussion. Sorry Petr!

> > If the former then shame on you, if the latter then how do you 
> > explain that distros appear to disable the RAM aspect of 
> > /dev/mem:
> > 
> >  $ grep DEVMEM $(rpm -ql kernel-2.6.38-0.rc7.git2.3.fc16.x86_64 | grep config-2.6 )
> >  CONFIG_STRICT_DEVMEM=y
> > 
> > So the crash utility use-case does not work on unpatched, default 
> > kernels, right?
> 
> Not if you have highmem.  That's why Redhat or Fedora to quote your
> example patch in the /dev/crash driver, which totally defeats the
> CONFIG_STRICT_DEVMEM setting.  But apparently it's good enough that no
> one either noticed or at least doesn't care.

After initial modules have loaded i essentially disable crash.ko via 
/proc/sys/kernel/modules_disabled so rootkits have to work a bit 
harder than that.

But yeah, crash.ko is a rootkit-and-other-badness-enabler as it 
stands today.

Thanks,

	Ingo
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