Re: [PATCH v1 00/11] mm/kasan: support per-page shadow memory to reduce memory consumption

[Vladimir Murzin]

On 30/05/17 09:15, Dmitry Vyukov wrote:
> On Tue, May 30, 2017 at 9:58 AM, Vladimir Murzin
> <vladimir.murzin@xxxxxxx> wrote:
>> On 29/05/17 16:29, Dmitry Vyukov wrote:
>>> I have an alternative proposal. It should be conceptually simpler and
>>> also less arch-dependent. But I don't know if I miss something
>>> important that will render it non working.
>>> Namely, we add a pointer to shadow to the page struct. Then, create a
>>> slab allocator for 512B shadow blocks. Then, attach/detach these
>>> shadow blocks to page structs as necessary. It should lead to even
>>> smaller memory consumption because we won't need a whole shadow page
>>> when only 1 out of 8 corresponding kernel pages are used (we will need
>>> just a single 512B block). I guess with some fragmentation we need
>>> lots of excessive shadow with the current proposed patch.
>>> This does not depend on TLB in any way and does not require hooking
>>> into buddy allocator.
>>> The main downside is that we will need to be careful to not assume
>>> that shadow is continuous. In particular this means that this mode
>>> will work only with outline instrumentation and will need some ifdefs.
>>> Also it will be slower due to the additional indirection when
>>> accessing shadow, but that's meant as "small but slow" mode as far as
>>> I understand.
>>>
>>> But the main win as I see it is that that's basically complete support
>>> for 32-bit arches. People do ask about arm32 support:
>>> https://groups.google.com/d/msg/kasan-dev/Sk6BsSPMRRc/Gqh4oD_wAAAJ
>>> https://groups.google.com/d/msg/kasan-dev/B22vOFp-QWg/EVJPbrsgAgAJ
>>> and probably mips32 is relevant as well.
>>> Such mode does not require a huge continuous address space range, has
>>> minimal memory consumption and requires minimal arch-dependent code.
>>> Works only with outline instrumentation, but I think that's a
>>> reasonable compromise.
>>
>> .. or you can just keep shadow in page extension. It was suggested back in
>> 2015 [1], but seems that lack of stack instrumentation was "no-way"...
>>
>> [1] https://lkml.org/lkml/2015/8/24/573
> 
> Right. It describes basically the same idea.
> 
> How is page_ext better than adding data page struct?

page_ext is already here along with some other debug options ;)

> It seems that memory for all page_ext is preallocated along with page
> structs; but just the lookup is slower.
> 

Yup. Lookup would look like (based on v4.0):

...
page_ext = lookup_page_ext_begin(virt_to_page(start));

do {
	page_ext->shadow[idx++] = value;
} while (idx < bound);

lookup_page_ext_end((void *)page_ext);

...

Cheers
Vladimir