Re: [PATCH 1/4] kasan: support alloca() poisoning

[Dmitry Vyukov]

On Fri, Jul 14, 2017 at 12:40 AM, Greg Hackmann <ghackmann@xxxxxxxxxx> wrote:
> Hi,
>
> Thanks for taking a look at this patchstack.  I apologize for the delay in
> responding.
>
> On 07/10/2017 01:44 AM, Dmitry Vyukov wrote:
>>>
>>> +
>>> +       const void *left_redzone = (const void *)(addr -
>>> +                       KASAN_ALLOCA_REDZONE_SIZE);
>>> +       const void *right_redzone = (const void *)(addr +
>>> rounded_up_size);
>>
>>
>> Please check that size is rounded to KASAN_ALLOCA_REDZONE_SIZE. That's
>> the expectation, right? That can change is clang silently.
>>
>>> +       kasan_poison_shadow(left_redzone, KASAN_ALLOCA_REDZONE_SIZE,
>>> +                       KASAN_ALLOCA_LEFT);
>>> +       kasan_poison_shadow(right_redzone,
>>> +                       padding_size + KASAN_ALLOCA_REDZONE_SIZE,
>>> +                       KASAN_ALLOCA_RIGHT);
>>
>>
>> We also need to poison the unaligned part at the end of the object
>> from size to rounded_up_size. You can see how we do it for heap
>> objects.
>
>
> The expectation is that `size' is the exact size of the alloca()ed object.
> `rounded_up_size' then adds the 0-7 bytes needed to adjust the size to the
> ASAN shadow scale.  So `addr + rounded_up_size' should be the correct place
> to start poisoning.


We need to start poisoning at addr+size exactly.
Asan shadow scheme supports this. It's not possible to poison
beginning of an aligned 8-byte block, but leave tail unpoisoned. But
it is possible to poison tail of an aligned 8-byte block and leave
beginning unpoisoned. Look at what we do for kmalloc.


> In retrospect this part of the code was pretty confusing.  How about this?
> I think its intent is clearer, plus it's a closer match for the description
> in my commit message:
>
>         unsigned long left_redzone_start;
>         unsigned long object_end;
>         unsigned long right_redzone_start, right_redzone_end;
>
>         left_redzone_start = addr - KASAN_ALLOCA_REDZONE_SIZE;
>         kasan_poison_shadow((const void *)left_redzone_start,
>                         KASAN_ALLOCA_REDZONE_SIZE,
>                         KASAN_ALLOCA_LEFT);
>
>         object_end = round_up(addr + size, KASAN_SHADOW_SCALE_SIZE);
>         right_redzone_start = round_up(object_end,
> KASAN_ALLOCA_REDZONE_SIZE);
>         right_redzone_end = right_redzone_start + KASAN_ALLOCA_REDZONE_SIZE;
>         kasan_poison_shadow((const void *)object_end,
>                         right_redzone_end - object_end,
>                         KASAN_ALLOCA_RIGHT);