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

[Greg Hackmann <ghackmann@google.com>]

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);

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