Re: [RFC PATCH v2 13/15] khwasan: add hooks implementation

From: Andrey Konovalov <andreyknvl@google.com>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
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Subject: Re: [RFC PATCH v2 13/15] khwasan: add hooks implementation
Date: Thu, 12 Apr 2018 18:45:35 +0200	[thread overview]
Message-ID: <CAAeHK+wzwXnJh1bbhUN6bm788q52BA2EfC+Q3dMS=peP7Px4Rg@mail.gmail.com> (raw)
In-Reply-To: <bfc3da50-66df-c6ed-ad6a-a285efe617ec@virtuozzo.com>

On Tue, Apr 10, 2018 at 6:31 PM, Andrey Ryabinin
<aryabinin@virtuozzo.com> wrote:
>
>
> On 04/10/2018 07:07 PM, Andrey Konovalov wrote:
>> On Fri, Apr 6, 2018 at 2:27 PM, Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>>> On 04/06/2018 03:14 PM, Andrey Konovalov wrote:
>>>> On Thu, Apr 5, 2018 at 3:02 PM, Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>>>>> Nevertheless, this doesn't mean that we should ignore *all* accesses to !slab memory.
>>>>
>>>> So you mean we need to find a way to ignore accesses via pointers
>>>> returned by page_address(), but still check accesses through all other
>>>> pointers tagged with 0xFF? I don't see an obvious way to do this. I'm
>>>> open to suggestions though.
>>>>
>>>
>>> I'm saying that we need to ignore accesses to slab objects if pointer
>>> to slab object obtained via page_address() + offset_in_page() trick, but don't ignore
>>> anything else.
>>>
>>> So, save tag somewhere in page struct and poison shadow with that tag. Make page_address() to
>>> return tagged address for all !PageSlab() pages. For PageSlab() pages page_address() should return
>>> 0xff tagged address, so we could ignore such accesses.
>>
>> Which pages do you mean by !PageSlab()?
>
> Literally the "PageSlab(page) == false" pages.
>
>> The ones that are allocated and freed by pagealloc, but mot managed by the slab allocator?
>
> Yes.
>
>> Perhaps we should then add tagging to the pagealloc hook instead?
>>
>
> Of course the tagging would be in kasan_alloc_pages(), where else that could be? And instead of what?

I think I misunderstood your suggestion twice already :)

To make it clear, you're suggesting:

1. Tag memory with a random tag in kasan_alloc_pages() and returned a
tagged pointer from pagealloc.

2. Restore the tag for the pointers returned from page_address for
!PageSlab() pages.

3. Set the tag to 0xff for the pointers returned from page_address for
PageSlab() pages.

Is this correct?

In 2 instead of storing the tag in page_struct, we can just recover it
from the shadow memory that corresponds to that page. What do you
think about this?