Re: [RFC PATCH] make CONFIG_STRICT_DEVMEM a core non-debug feature

[Leif Lindholm <leif.lindholm@linaro.org>]

On Tue, Nov 04, 2014 at 10:43:00AM -0800, Kees Cook wrote:
> > diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
> > index efefd12..39f7817 100644
> > --- a/drivers/char/Kconfig
> > +++ b/drivers/char/Kconfig
> > @@ -6,6 +6,22 @@ menu "Character devices"
> >
> >  source "drivers/tty/Kconfig"
> >
> > +config STRICT_DEVMEM
> > +       bool "Reduced access to /dev/mem"
> > +       depends on HAVE_ARCH_RESTRICTED_DEVMEM
> > +       default y
> > +       help
> > +         If this option is disabled, you allow userspace (root) access to all
> > +         of memory, including kernel and userspace memory. Accidental
> > +         access to this is obviously disastrous, but specific access can
> > +         be used by people debugging the kernel.
> > +
> > +         If this option is switched on, the /dev/mem file restricts userspace
> > +         access to an architecture-specific subset of the physical address
> > +         space.
> 
> Great consolidation, thanks! I would probably expand this help text a
> bit to include some of details mentioned in the x86 portion of the
> option. For example:
> 
> 
> If this option is switched on, the /dev/mem file restricts userspace
> access to an architecture-specific subset of the physical address
> space. For example on x86, PCI space and BIOS code and data
> regions. This is sufficient for things like dosemu and non-KMS
> Xorg and all common users of /dev/mem.

I considered doing that, but didn't want to risk listing too many
details of one architecture, and too few of others.

One alternative would be to add a devmem.txt somewhere in
Documentation, listing the behaviours on different architectures (this
would also be a good place to describe restrictions on types of
mappings and suchlike). The help message could then contain a mention
of that file. Would that work for you?

I really don't have a strong opinion however, and would be happy to go
along with whatever the most people would like to see.

/
	Leif