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

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, May 30, 2017 at 10:40 AM, Vladimir Murzin
<vladimir.murzin@arm.com> wrote:
> On 30/05/17 09:31, Vladimir Murzin wrote:
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>>
>> On 30/05/17 09:15, Dmitry Vyukov wrote:
>>> On Tue, May 30, 2017 at 9:58 AM, Vladimir Murzin
>>> <vladimir.murzin@arm.com> 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 ;)


But page struct is also here. What am I missing?


>>> 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);
>>
>> ...
>
> Correction: please, ignore that *_{begin,end} stuff - mainline only
> lookup_page_ext() is only used.


Note that this added code will be executed during handling of each and
every memory access in kernel. Every instruction matters on that path.
The additional indirection via page struct will also slow down it, but
that's the cost for lower memory consumption and potentially 32-bit
support. For page_ext it looks like even more overhead for no gain.