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

[Dmitry Vyukov]

On Mon, Jul 1, 2019 at 11:56 AM Walter Wu <walter-zh.wu@xxxxxxxxxxxx> wrote:
> > > > > > > > This patch adds memory corruption identification at bug report for
> > > > > > > > software tag-based mode, the report show whether it is "use-after-free"
> > > > > > > > or "out-of-bound" error instead of "invalid-access" error.This will make
> > > > > > > > it easier for programmers to see the memory corruption problem.
> > > > > > > >
> > > > > > > > Now we extend the quarantine to support both generic and tag-based kasan.
> > > > > > > > For tag-based kasan, the quarantine stores only freed object information
> > > > > > > > to check if an object is freed recently. When tag-based kasan reports an
> > > > > > > > error, we can check if the tagged addr is in the quarantine and make a
> > > > > > > > good guess if the object is more like "use-after-free" or "out-of-bound".
> > > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > We already have all the information and don't need the quarantine to make such guess.
> > > > > > > Basically if shadow of the first byte of object has the same tag as tag in pointer than it's out-of-bounds,
> > > > > > > otherwise it's use-after-free.
> > > > > > >
> > > > > > > In pseudo-code it's something like this:
> > > > > > >
> > > > > > > u8 object_tag = *(u8 *)kasan_mem_to_shadow(nearest_object(cacche, page, access_addr));
> > > > > > >
> > > > > > > if (access_addr_tag == object_tag && object_tag != KASAN_TAG_INVALID)
> > > > > > >   // out-of-bounds
> > > > > > > else
> > > > > > >   // use-after-free
> > > > > >
> > > > > > Thanks your explanation.
> > > > > > I see, we can use it to decide corruption type.
> > > > > > But some use-after-free issues, it may not have accurate free-backtrace.
> > > > > > Unfortunately in that situation, free-backtrace is the most important.
> > > > > > please see below example
> > > > > >
> > > > > > In generic KASAN, it gets accurate free-backrace(ptr1).
> > > > > > In tag-based KASAN, it gets wrong free-backtrace(ptr2). It will make
> > > > > > programmer misjudge, so they may not believe tag-based KASAN.
> > > > > > So We provide this patch, we hope tag-based KASAN bug report is the same
> > > > > > accurate with generic KASAN.
> > > > > >
> > > > > > ---
> > > > > >     ptr1 = kmalloc(size, GFP_KERNEL);
> > > > > >     ptr1_free(ptr1);
> > > > > >
> > > > > >     ptr2 = kmalloc(size, GFP_KERNEL);
> > > > > >     ptr2_free(ptr2);
> > > > > >
> > > > > >     ptr1[size] = 'x';  //corruption here
> > > > > >
> > > > > >
> > > > > > static noinline void ptr1_free(char* ptr)
> > > > > > {
> > > > > >     kfree(ptr);
> > > > > > }
> > > > > > static noinline void ptr2_free(char* ptr)
> > > > > > {
> > > > > >     kfree(ptr);
> > > > > > }
> > > > > > ---
> > > > > >
> > > > > We think of another question about deciding by that shadow of the first
> > > > > byte.
> > > > > In tag-based KASAN, it is immediately released after calling kfree(), so
> > > > > the slub is easy to be used by another pointer, then it will change
> > > > > shadow memory to the tag of new pointer, it will not be the
> > > > > KASAN_TAG_INVALID, so there are many false negative cases, especially in
> > > > > small size allocation.
> > > > >
> > > > > Our patch is to solve those problems. so please consider it, thanks.
> > > > >
> > > > Hi, Andrey and Dmitry,
> > > >
> > > > I am sorry to bother you.
> > > > Would you tell me what you think about this patch?
> > > > We want to use tag-based KASAN, so we hope its bug report is clear and
> > > > correct as generic KASAN.
> > > >
> > > > Thanks your review.
> > > > Walter
> > >
> > > Hi Walter,
> > >
> > > I will probably be busy till the next week. Sorry for delays.
> >
> > It's ok. Thanks your kindly help.
> > I hope I can contribute to tag-based KASAN. It is a very important tool
> > for us.
>
> Hi, Dmitry,
>
> Would you have free time to discuss this patch together?
> Thanks.

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.
We also have a quarantine for hwasan in user-space. Though it works a
bit differently then the normal asan quarantine. We keep a per-thread
fixed-size ring-buffer of recent allocations:
https://github.com/llvm-mirror/compiler-rt/blob/master/lib/hwasan/hwasan_report.cpp#L274-L284
and scan these ring buffers during reports.