Re: [PATCH 00/10] Enhance /dev/mem to allow read/write of arbitrary

Ingo Molnar
2011-07-01T14:37:35Z

* Christoph Hellwig <hch@infradead.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