Date:   Mon, 30 Oct 2023 19:32:46 +0800
From:   Juntong Deng <juntong.deng@...look.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrey Konovalov <andreyknvl@...il.com>, ryabinin.a.a@...il.com,
        glider@...gle.com, vincenzo.frascino@....com,
        akpm@...ux-foundation.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kernel-mentees@...ts.linuxfoundation.org" 
        <linux-kernel-mentees@...ts.linuxfoundation.org>
Subject: Re: [RFC] mm/kasan: Add Allocation, Free, Error timestamps to KASAN
 report

On 2023/10/30 18:10, Dmitry Vyukov wrote:
> On Mon, 30 Oct 2023 at 10:28, Juntong Deng <juntong.deng@...look.com> wrote:
>>
>> On 2023/10/30 14:29, Dmitry Vyukov wrote:
>>> On Sun, 29 Oct 2023 at 10:05, Juntong Deng <juntong.deng@...look.com> wrote:
>>>>
>>>> On 2023/10/26 3:22, Andrey Konovalov wrote:
>>>>> On Tue, Oct 17, 2023 at 9:40 PM Juntong Deng <juntong.deng@...look.com> wrote:
>>>>>>
>>>>>> The idea came from the bug I was fixing recently,
>>>>>> 'KASAN: slab-use-after-free Read in tls_encrypt_done'.
>>>>>>
>>>>>> This bug is caused by subtle race condition, where the data structure
>>>>>> is freed early on another CPU, resulting in use-after-free.
>>>>>>
>>>>>> Like this bug, some of the use-after-free bugs are caused by race
>>>>>> condition, but it is not easy to quickly conclude that the cause of the
>>>>>> use-after-free is race condition if only looking at the stack trace.
>>>>>>
>>>>>> I did not think this use-after-free was caused by race condition at the
>>>>>> beginning, it took me some time to read the source code carefully and
>>>>>> think about it to determine that it was caused by race condition.
>>>>>>
>>>>>> By adding timestamps for Allocation, Free, and Error to the KASAN
>>>>>> report, it will be much easier to determine if use-after-free is
>>>>>> caused by race condition.
>>>>>
>>>>> An alternative would be to add the CPU number to the alloc/free stack
>>>>> traces. Something like:
>>>>>
>>>>> Allocated by task 42 on CPU 2:
>>>>> (stack trace)
>>>>>
>>>>> The bad access stack trace already prints the CPU number.
>>>>
>>>> Yes, that is a great idea and the CPU number would help a lot.
>>>>
>>>> But I think the CPU number cannot completely replace the free timestamp,
>>>> because some freeing really should be done at another CPU.
>>>>
>>>> We need the free timestamp to help us distinguish whether it was freed
>>>> a long time ago or whether it was caused to be freed during the
>>>> current operation.
>>>>
>>>> I think both the CPU number and the timestamp should be displayed, more
>>>> information would help us find the real cause of the error faster.
>>>>
>>>> Should I implement these features?
>>>
>>> Hi Juntong,
>>>
>>> There is also an aspect of memory consumption. KASAN headers increase
>>> the size of every heap object. So we tried to keep them as compact as
>>> possible. At some point CPU numbers and timestamps (IIRC) were already
>>> part of the header, but we removed them to shrink the header to 16
>>> bytes.
>>> PID gives a good approximation of potential races. I usually look at
>>> PIDs to understand if it's a "plain old single-threaded
>>> use-after-free", or free and access happened in different threads.
>>> Re timestamps, I see you referenced a syzbot report. With syzkaller
>>> most timestamps will be very close even for non-racing case.
>>> So if this is added, this should be added at least under a separate config.
>>>
>>> If you are looking for potential KASAN improvements, here is a good list:
>>> https://bugzilla.kernel.org/buglist.cgi?bug_status=__open__&component=Sanitizers&list_id=1134168&product=Memory%20Management
>>
>> Hi Dmitry,
>>
>> I think PID cannot completely replace timestamp for reason similar to
>> CPU number, some frees really should be done in another thread, but it
>> is difficult for us to distinguish if it is a free that was done some
>> time ago, or under subtle race conditions.
> 
> I agree it's not a complete replacement, it just does not consume
> additional memory.
> 
>> As to whether most of the timestamps will be very close even for
>> non-racing case, this I am not sure, because I do not have
>> enough samples.
>>
>> I agree that these features should be in a separate config and
>> the user should be free to choose whether to enable them or not.
>>
>> We can divide KASAN into normal mode and depth mode. Normal mode
>> records only minimal critical information, while depth mode records
>> more potentially useful information.
>>
>> Also, honestly, I think a small amount of extra memory consumption
>> should not stop us from recording more information.
>>
>> Because if someone enables KASAN for debugging, then memory consumption
>> and performance are no longer his main concern.
> 
> There are a number of debugging tools created with the "performance
> does not matter" attitude. They tend to be barely usable, not usable
> in wide scale testing, not usable in canaries, etc.
> All of sanitizers were created with lots of attention to performance,
> attention on the level of the most performance critical production
> code (sanitizer code is hotter than any production piece of code).
> That's what made them so widely used. Think of interactive uses,
> smaller devices, etc. Please let's keep this attitude.

Yes, I agree that debugging tools used at a wide scale need to have
more rigorous performance considerations.

Do you think it is worth using the extra bytes to record more
information? If this is a user-configurable feature.

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