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

Dmitry Vyukov
2019-06-13T13:05:27Z

On Thu, Jun 13, 2019 at 2:27 PM Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> On 6/13/19 11:13 AM, Walter Wu 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

But we don't have redzones in tag mode (intentionally), so unless I am
missing something we don't have the necessary info. Both cases look
the same -- we hit a different tag.
There may only be a small trailer for kmalloc-allocated objects that
is painted with a different tag. I don't remember if we actually use a
different tag for the trailer. Since tag mode granularity is 16 bytes,
for smaller objects the trailer is impossible at all.