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Subject: Re: [PATCH v3 1/2] kasan: detect negative size in memory operation
 function
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Walter Wu <walter-zh.wu@mediatek.com>,
        Alexander Potapenko <glider@google.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        wsd_upstream <wsd_upstream@mediatek.com>
References: <20191104020519.27988-1-walter-zh.wu@mediatek.com>
 <34bf9c08-d2f2-a6c6-1dbe-29b1456d8284@virtuozzo.com>
 <CACT4Y+bfGrJemwyMVqd2Kt19mF2i=3GwXRKHP0qGJaT_5OhSCA@mail.gmail.com>
From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <20df03c5-e733-98b0-84e9-8d52ddce5c98@virtuozzo.com>
Date:   Mon, 11 Nov 2019 11:24:36 +0300
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On 11/11/19 10:57 AM, Dmitry Vyukov wrote:
> On Fri, Nov 8, 2019 at 11:32 PM Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:

>>> diff --git a/mm/kasan/generic_report.c b/mm/kasan/generic_report.c
>>> index 36c645939bc9..52a92c7db697 100644
>>> --- a/mm/kasan/generic_report.c
>>> +++ b/mm/kasan/generic_report.c
>>> @@ -107,6 +107,24 @@ static const char *get_wild_bug_type(struct kasan_access_info *info)
>>>
>>>  const char *get_bug_type(struct kasan_access_info *info)
>>>  {
>>> +     /*
>>> +      * If access_size is negative numbers, then it has three reasons
>>> +      * to be defined as heap-out-of-bounds bug type.
>>> +      * 1) Casting negative numbers to size_t would indeed turn up as
>>> +      *    a large size_t and its value will be larger than ULONG_MAX/2,
>>> +      *    so that this can qualify as out-of-bounds.
>>> +      * 2) If KASAN has new bug type and user-space passes negative size,
>>> +      *    then there are duplicate reports. So don't produce new bug type
>>> +      *    in order to prevent duplicate reports by some systems
>>> +      *    (e.g. syzbot) to report the same bug twice.
>>> +      * 3) When size is negative numbers, it may be passed from user-space.
>>> +      *    So we always print heap-out-of-bounds in order to prevent that
>>> +      *    kernel-space and user-space have the same bug but have duplicate
>>> +      *    reports.
>>> +      */
>>
>> Completely fail to understand 2) and 3). 2) talks something about *NOT* producing new bug
>> type, but at the same time you code actually does that.
>> 3) says something about user-space which have nothing to do with kasan.
> 
> The idea was to use one of the existing bug titles so that syzbot does
> not produce 2 versions for OOBs where size is user-controlled. We
> don't know if it's overflow from heap, global or stack, but heap is
> the most common bug, so saying heap overflow will reduce chances of
> producing duplicates the most.
> But for all of this to work we do need to use one of the existing bug titles.

The "heap-out-of-bounds" is not one of the existing bug titles.