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

Lecopzer Chen
2021-02-04T18:46:44Z

> On Thu, Feb 04, 2021 at 11:53:46PM +0800, Lecopzer Chen wrote:
> > > 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.
>
> Indeed, but the question I'm asking is what happens when an on-demand shadow
> allocation from vmalloc overlaps with the shadow that we allocated early for
> the kernel image?
>
> Sounds like I have to go and read the code...
oh, sorry I misunderstood your question.

FWIW,
I think this won't happend because this mean vmalloc() provides va
which already allocated by kimg, as I know, vmalloc_init() will insert
early allocated vma into its vmalloc rb tree

, and this early allocated vma will include kernel image.

After quick review of mm init code,
this early allocated for vma is at map_kernel() in arch/arm64/mm/mmu.c

BRs
Lecopzer