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

Ingo Molnar
2011-07-01T16:13:45Z

* 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