Re: [PATCH 2/2] ramoops: remove module parameters

[Marco Stornelli]

Il 22/11/2011 19:14, Kees Cook ha scritto:
> On Tue, Nov 22, 2011 at 9:23 AM, Marco Stornelli
> <marco.stornelli@xxxxxxxxx>  wrote:
> > Il 21/11/2011 19:11, Kees Cook ha scritto:
> > > On Sat, Nov 19, 2011 at 1:25 AM, Marco Stornelli
> > > <marco.stornelli@xxxxxxxxx>    wrote:
> > > > Il 18/11/2011 20:31, Kees Cook ha scritto:
> > > > > The ramoops driver is intended to be used with platforms that define
> > > > > persistent memory regions. If memory regions were configurable with
> > > > > module parameters, it would be possible to read some RAM regions via
> > > > > the pstore interface without access to /dev/mem (which would result
> > > > > in a loss of kernel memory privacy when a system is built with
> > > > > STRICT_DEVMEM), so remove this ability completely.
> > > >
> > > > I don't like it very much. The loss of module parameters give us less
> > > > flexibility. The main goal of this driver is debug, so I think it should
> > > > be
> > > > fast to use. I mean it's not more possible reserve a memory region and
> > > > load
> > > > the module "on-the-fly", it needs a platform device, it's ok but I think
> > > > it's a little bit more complicated, (without talking about platforms
> > > > without
> > > > a device tree source).
> > > > I don't understand the problem of strict devmem. We shouldn't use kernel
> > > > memory region but only reserved ones and the driver doesn't use the
> > > > request_mem_region_exclusive, am I wrong?
> > >
> > > Hmmm, maybe I'm reading it backwards, but I think we want it to use
> > > ..._exclusive().
> > >
> > > int devmem_is_allowed(unsigned long pagenr)
> > > {
> > >          if (pagenr<= 256)
> > >                  return 1;
> > >          if (iomem_is_exclusive(pagenr<<    PAGE_SHIFT))
> > >                  return 0;
> > >          if (!page_is_ram(pagenr))
> > >                  return 1;
> > >          return 0;
> > > }
> > >
> > > If the region is exclusive, access is not allowed (return 0). ramoops
> > > currently uses request_mem_region() instead of
> > > request_mem_region_exclusive(). If we made that switch, I think I'd be
> > > happy. Would this create some problem I'm not seeing?
> >
> > I don't understand why we should use the exclusive version, to protect debug
> > data? You should provide a more valid reason to change, because the fact you
> > will be happier with this change is not enough for me :)
>
> I guess ..._exclusive() doesn't matter. My concern was that ramoops
> with the pstore interface and the module parameters could be used to
> bypass STRICT_DEVMEM if it were able to be loaded in some sensitive
> region of system memory. Perhaps the better approach would be to use a
> magic header so that uninitialized memory isn't visible? What do you
> think?
>
> -Kees

Sincerely, IMHO, if we consider the *debug* nature of this driver, it's 
sufficient a simple script (distributed with the kernel) to extract the 
all the information you need without touch the current implementation.

Marco
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