Re: [PATCH v3] kasan: add memory corruption identification for software tag-based mode

Andrey Ryabinin
2019-07-18T16:11:48Z

On 7/15/19 6:06 AM, Walter Wu wrote:
> On Fri, 2019-07-12 at 13:52 +0300, Andrey Ryabinin wrote:
>>
>> On 7/11/19 1:06 PM, Walter Wu wrote:
>>> On Wed, 2019-07-10 at 21:24 +0300, Andrey Ryabinin wrote:
>>>>
>>>> On 7/9/19 5:53 AM, Walter Wu wrote:
>>>>> On Mon, 2019-07-08 at 19:33 +0300, Andrey Ryabinin wrote:
>>>>>>
>>>>>> On 7/5/19 4:34 PM, Dmitry Vyukov wrote:
>>>>>>> On Mon, Jul 1, 2019 at 11:56 AM Walter Wu <walter-zh.wu@mediatek.com> wrote:
>>>>
>>>>>>>
>>>>>>> Sorry for delays. I am overwhelm by some urgent work. I afraid to
>>>>>>> promise any dates because the next week I am on a conference, then
>>>>>>> again a backlog and an intern starting...
>>>>>>>
>>>>>>> Andrey, do you still have concerns re this patch? This change allows
>>>>>>> to print the free stack.
>>>>>>
>>>>>> I 'm not sure that quarantine is a best way to do that. Quarantine is made to delay freeing, but we don't that here.
>>>>>> If we want to remember more free stacks wouldn't be easier simply to remember more stacks in object itself?
>>>>>> Same for previously used tags for better use-after-free identification.
>>>>>>
>>>>>
>>>>> Hi Andrey,
>>>>>
>>>>> We ever tried to use object itself to determine use-after-free
>>>>> identification, but tag-based KASAN immediately released the pointer
>>>>> after call kfree(), the original object will be used by another
>>>>> pointer, if we use object itself to determine use-after-free issue, then
>>>>> it has many false negative cases. so we create a lite quarantine(ring
>>>>> buffers) to record recent free stacks in order to avoid those false
>>>>> negative situations.
>>>>
>>>> I'm telling that *more* than one free stack and also tags per object can be stored.
>>>> If object reused we would still have information about n-last usages of the object.
>>>> It seems like much easier and more efficient solution than patch you proposing.
>>>>
>>> To make the object reused, we must ensure that no other pointers uses it
>>> after kfree() release the pointer.
>>> Scenario:
>>> 1). The object reused information is valid when no another pointer uses
>>> it.
>>> 2). The object reused information is invalid when another pointer uses
>>> it.
>>> Do you mean that the object reused is scenario 1) ?
>>> If yes, maybe we can change the calling quarantine_put() location. It
>>> will be fully use that quarantine, but at scenario 2) it looks like to
>>> need this patch.
>>> If no, maybe i miss your meaning, would you tell me how to use invalid
>>> object information? or?
>>>
>>
>>
>> KASAN keeps information about object with the object, right after payload in the kasan_alloc_meta struct.
>> This information is always valid as long as slab page allocated. Currently it keeps only one last free stacktrace.
>> It could be extended to record more free stacktraces and also record previously used tags which will allow you
>> to identify use-after-free and extract right free stacktrace.
>
> Thanks for your explanation.
>
> For extend slub object, if one record is 9B (sizeof(u8)+ sizeof(struct
> kasan_track)) and add five records into slub object, every slub object
> may add 45B usage after the system runs longer.
> Slub object number is easy more than 1,000,000(maybe it may be more
> bigger), then the extending object memory usage should be 45MB, and
> unfortunately it is no limit. The memory usage is more bigger than our
> patch.

No, it's not necessarily more.
And there are other aspects to consider such as performance, how simple reliable the code is.

>
> We hope tag-based KASAN advantage is smaller memory usage. If it’s
> possible, we should spend less memory in order to identify
> use-after-free. Would you accept our patch after fine tune it?

Sure, if you manage to fix issues and demonstrate that performance penalty of your
patch is close to zero.