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

[Walter Wu <walter-zh.wu@mediatek.com>]

On Fri, 2019-07-26 at 15:00 +0300, Andrey Ryabinin wrote:
> 
> On 7/22/19 12:52 PM, Walter Wu wrote:
> > On Thu, 2019-07-18 at 19:11 +0300, Andrey Ryabinin wrote:
> >>
> >> 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.
> > 
> > 
> > I remember that there are already the lists which you concern. Maybe we
> > can try to solve those problems one by one.
> > 
> > 1. deadlock issue? cause by kmalloc() after kfree()?
> 
> smp_call_on_cpu()

> > 2. decrease allocation fail, to modify GFP_NOWAIT flag to GFP_KERNEL?
> 
> No, this is not gonna work. Ideally we shouldn't have any allocations there.
> It's not reliable and it hurts performance.
> 
I dont know this meaning, we need create a qobject and put into
quarantine, so may need to call kmem_cache_alloc(), would you agree this
action?

> 
> > 3. check whether slim 48 bytes (sizeof (qlist_object) +
> > sizeof(kasan_alloc_meta)) and additional unique stacktrace in
> > stackdepot?
> > 4. duplicate struct 'kasan_track' information in two different places
> > 
> 
> Yup.
> 
> > Would you have any other concern? or?
> > 
> 
> It would be nice to see some performance numbers. Something that uses slab allocations a lot, e.g. netperf STREAM_STREAM test.
> 
ok, we will do it.