Re: [PATCH v2 0/4] arm64: kasan: support CONFIG_KASAN_VMALLOC

Lecopzer Chen
2021-02-04T15:56:41Z

> On Sat, Jan 09, 2021 at 06:32:48PM +0800, Lecopzer Chen wrote:
> > Linux supports KAsan for VMALLOC since commit 3c5c3cfb9ef4da9
> > ("kasan: support backing vmalloc space with real shadow memory")
> >
> > Acroding to how x86 ported it [1], they early allocated p4d and pgd,
> > but in arm64 I just simulate how KAsan supports MODULES_VADDR in arm64
> > by not to populate the vmalloc area except for kimg address.
>
> The one thing I've failed to grok from your series is how you deal with
> vmalloc allocations where the shadow overlaps with the shadow which has
> already been allocated for the kernel image. Please can you explain?

The most key point is we don't map anything in the vmalloc shadow address.
So we don't care where the kernel image locate inside vmalloc area.

kasan_map_populate(kimg_shadow_start, kimg_shadow_end,...)

Kernel image was populated with real mapping in its shadow address.
I `bypass' the whole shadow of vmalloc area, the only place you can find
about vmalloc_shadow is
kasan_populate_early_shadow((void *)vmalloc_shadow_end,
(void *)KASAN_SHADOW_END);

----------- vmalloc_shadow_start
| |
| |
| | <= non-mapping
| |
| |
|-----------|
|///////////|<- kimage shadow with page table mapping.
|-----------|
| |
| | <= non-mapping
| |
------------- vmalloc_shadow_end
|00000000000|
|00000000000| <= Zero shadow
|00000000000|
------------- KASAN_SHADOW_END

vmalloc shadow will be mapped 'ondemend', see kasan_populate_vmalloc()
in mm/vmalloc.c in detail.
So the shadow of vmalloc will be allocated later if anyone use its va.

BRs,
Lecopzer