Re: [PATCH v6 7/8] ARM: mm: allow non-text sections to be non-executable

[Russell King - ARM Linux]

On Fri, Sep 26, 2014 at 09:43:12AM -0700, Kees Cook wrote:
> On Fri, Sep 26, 2014 at 4:42 AM, Russell King - ARM Linux
> <linux@xxxxxxxxxxxxxxxx> wrote:
> > On Thu, Sep 18, 2014 at 12:19:08PM -0700, Kees Cook wrote:
> >> @@ -1373,13 +1373,24 @@ static void __init map_lowmem(void)
> >>               if (start >= end)
> >>                       break;
> >>
> >> -             if (end < kernel_x_start || start >= kernel_x_end) {
> >> +             if (end < kernel_x_start) {
> >>                       map.pfn = __phys_to_pfn(start);
> >>                       map.virtual = __phys_to_virt(start);
> >>                       map.length = end - start;
> >>                       map.type = MT_MEMORY_RWX;
> >>
> >>                       create_mapping(&map);
> >> +             } else if (start >= kernel_x_end) {
> >> +                     map.pfn = __phys_to_pfn(start);
> >> +                     map.virtual = __phys_to_virt(start);
> >> +                     map.length = end - start;
> >> +#ifdef CONFIG_ARM_KERNMEM_PERMS
> >> +                     map.type = MT_MEMORY_RW;
> >> +#else
> >> +                     map.type = MT_MEMORY_RWX;
> >> +#endif
> >> +
> >> +                     create_mapping(&map);
> >
> >
> > I'm looking at this, and wondering two things.  Firstly, why isn't it
> > MT_MEMORY_RW in the first place.  Secondly, why do you need to make
> > this depend on CONFIG_ARM_KERNMEM_PERMS at all?
> >
> > I think the first is an oversight by me (and is a case which never came
> > up in my testing, because most of my platforms don't have segmented
> > memory layouts.)
> >
> > The second I think is not necessary - the memory region being considered
> > is not part of the kernel at all, and so should not be executable in any
> > shape or form.
> 
> I'm happy to change this. My original intent was to make layout for
> builds without CONFIG_ARM_KERNMEM_PERMS identical to what was there
> before, so I took this step to make sure.
> 
> I will drop the #ifdef and respin. Does the rest look ok? Shall I send
> a pull request?

I think all you need to is change that MT_MEMORY_RWX to be MT_MEMORY_RW.

However, I think I would also like to see the kernel text being read-only
as soon as possible - so maybe we should also have MT_MEMORY_RX, and
have map_lowmem() use this for the kernel text when we want this level
of protection?

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