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

Greg Hackmann
2017-07-13T22:40:03Z

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.

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);