Re: [PATCH 1/2] kasan, mm: reset tag when access metadata

[Andrey Konovalov <andreyknvl@gmail.com>]

On Tue, Jul 27, 2021 at 9:22 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Tue, Jul 27, 2021 at 04:32:02PM +0800, Kuan-Ying Lee wrote:
> > On Tue, 2021-07-27 at 09:10 +0200, Marco Elver wrote:
> > > +Cc Catalin
> > >
> > > On Tue, 27 Jul 2021 at 06:00, Kuan-Ying Lee <
> > > Kuan-Ying.Lee@mediatek.com> wrote:
> > > >
> > > > Hardware tag-based KASAN doesn't use compiler instrumentation, we
> > > > can not use kasan_disable_current() to ignore tag check.
> > > >
> > > > Thus, we need to reset tags when accessing metadata.
> > > >
> > > > Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
> > >
> > > This looks reasonable, but the patch title is not saying this is
> > > kmemleak, nor does the description say what the problem is. What
> > > problem did you encounter? Was it a false positive?
> >
> > kmemleak would scan kernel memory to check memory leak.
> > When it scans on the invalid slab and dereference, the issue
> > will occur like below.
> >
> > So I think we should reset the tag before scanning.
> >
> > # echo scan > /sys/kernel/debug/kmemleak
> > [  151.905804]
> > ==================================================================
> > [  151.907120] BUG: KASAN: out-of-bounds in scan_block+0x58/0x170
> > [  151.908773] Read at addr f7ff0000c0074eb0 by task kmemleak/138
> > [  151.909656] Pointer tag: [f7], memory tag: [fe]
>
> It would be interesting to find out why the tag doesn't match. Kmemleak
> should in principle only scan valid objects that have been allocated and
> the pointer can be safely dereferenced. 0xfe is KASAN_TAG_INVALID, so it
> either goes past the size of the object (into the red zone) or it still
> accesses the object after it was marked as freed but before being
> released from kmemleak.
>
> With slab, looking at __cache_free(), it calls kasan_slab_free() before
> ___cache_free() -> kmemleak_free_recursive(), so the second scenario is
> possible. With slub, however, slab_free_hook() first releases the object
> from kmemleak before poisoning it. Based on the stack dump, you are
> using slub, so it may be that kmemleak goes into the object red zones.
>
> I'd like this clarified before blindly resetting the tag.

AFAIK, kmemleak scans the whole object including the leftover redzone
for kmalloc-allocated objects.

Looking at the report, there are 11 0xf7 granules, which amounts to
176 bytes, and the object is allocated from the kmalloc-256 cache. So
when kmemleak accesses the last 256-176 bytes, it causes faults, as
those are marked with KASAN_KMALLOC_REDZONE == KASAN_TAG_INVALID ==
0xfe.

Generally, resetting tags in kasan_disable/enable_current() section
should be fine to suppress MTE faults, provided those sections had
been added correctly in the first place.