[PATCH 07/12] arm64: mm: Place kImage at bottom of VA space

[Steve Capper]

Hi Ard,

On Mon, Dec 04, 2017 at 04:25:18PM +0000, Ard Biesheuvel wrote:
> On 4 December 2017 at 14:13, Steve Capper <steve.capper at arm.com> wrote:
> > Re-arrange the kernel memory map s.t. the kernel image resides in the
> > bottom 514MB of memory.
>
> I guess this breaks KASLR entirely, no? Given that it adds an offset
> in the range [0 ... sizeof(VMALLOC_SPACE) /4 ].

Yes, yes it does. Sorry about this. I had very carefully tested KASLR
with custom offsets... on my early page table code. I will have a think
about this.

>From a KASLR side, my (renewed) understanding is that a virtual address
as low as possible is desired for the kimage start as that affords the
most wiggle room?

>
> In any case, it makes sense to keep the kernel VA space adjacent to
> the VMALLOC space, rather than put stuff like PCI I/O and the fixmap
> in between.
>
> > With the modules, fixed map, PCI IO space placed
> > above it. At the very bottom of the memory map we set aside a 2MB guard
> > region to prevent ambiguity with PTR_ERR/ERR_PTR.
> >
>
> Interesting. In another thread, we discussed whether it is necessary
> to prevent the linear map randomization code from allocating at the
> very top [bottom in Steve-speak] of the kernel virtual address space,
> and this is a thing I did not consider.
>

I'll adjust my nomenclature to be less confusing.

> > Dynamically resizable objects such as KASAN shadow and sparsemem map
> > are placed above the fixed size objects.
> >
>
> The current placement of the sparsemem map was carefully chosen so
> that virt_to_page/page_to_virt translations are extremely cheap. Is
> that still the case?

I will double check the virt_to_page/page_to_virt. I had adjuested virt_to_phys
and phys_to_virt and I think this one escaped me.

Cheers,
--
Steve
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