Re: [PATCH v4 1/2] kasan: detect negative size in memory operation function

Andrey Ryabinin
2019-11-21T22:20:48Z

On 11/21/19 10:58 PM, Dmitry Vyukov wrote:
> On Thu, Nov 21, 2019 at 1:27 PM Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
>>> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
>>> index 6814d6d6a023..4bfce0af881f 100644
>>> --- a/mm/kasan/common.c
>>> +++ b/mm/kasan/common.c
>>> @@ -102,7 +102,8 @@ EXPORT_SYMBOL(__kasan_check_write);
>>> #undef memset
>>> void *memset(void *addr, int c, size_t len)
>>> {
>>> - check_memory_region((unsigned long)addr, len, true, _RET_IP_);
>>> + if (!check_memory_region((unsigned long)addr, len, true, _RET_IP_))
>>> + return NULL;
>>>
>>> return __memset(addr, c, len);
>>> }
>>> @@ -110,8 +111,9 @@ void *memset(void *addr, int c, size_t len)
>>> #undef memmove
>>> void *memmove(void *dest, const void *src, size_t len)
>>> {
>>> - check_memory_region((unsigned long)src, len, false, _RET_IP_);
>>> - check_memory_region((unsigned long)dest, len, true, _RET_IP_);
>>> + if (!check_memory_region((unsigned long)src, len, false, _RET_IP_) ||
>>> + !check_memory_region((unsigned long)dest, len, true, _RET_IP_))
>>> + return NULL;
>>>
>>> return __memmove(dest, src, len);
>>> }
>>> @@ -119,8 +121,9 @@ void *memmove(void *dest, const void *src, size_t len)
>>> #undef memcpy
>>> void *memcpy(void *dest, const void *src, size_t len)
>>> {
>>> - check_memory_region((unsigned long)src, len, false, _RET_IP_);
>>> - check_memory_region((unsigned long)dest, len, true, _RET_IP_);
>>> + if (!check_memory_region((unsigned long)src, len, false, _RET_IP_) ||
>>> + !check_memory_region((unsigned long)dest, len, true, _RET_IP_))
>>> + return NULL;
>>>
>>
>> I realized that we are going a wrong direction here. Entirely skipping mem*() operation on any
>> poisoned shadow value might only make things worse. Some bugs just don't have any serious consequences,
>> but skipping the mem*() ops entirely might introduce such consequences, which wouldn't happen otherwise.
>>
>> So let's keep this code as this, no need to check the result of check_memory_region().
>
> I suggested it.
>
> For our production runs it won't matter, we always panic on first report.
> If one does not panic, there is no right answer. You say: _some_ bugs
> don't have any serious consequences, but skipping the mem*() ops
> entirely might introduce such consequences. The opposite is true as
> well, right? :) And it's not hard to come up with a scenario where
> overwriting memory after free or out of bounds badly corrupts memory.
> I don't think we can somehow magically avoid bad consequences in all
> cases.
>

Absolutely right. My point was that if it's bad consequences either way,
than there is no point in complicating this code, it doesn't buy us anything.

> What I was thinking about is tests. We need tests for this. And we
> tried to construct tests specifically so that they don't badly corrupt
> memory (e.g. OOB/UAF reads, or writes to unused redzones, etc), so
> that it's possible to run all of them to completion reliably. Skipping
> the actual memory options allows to write such tests for all possible
> scenarios. That's was my motivation.

But I see you point now. No objections to the patch in that case.