Re: [PATCH] mm/slab: ensure all metadata in slab object are word-aligned

[Andrey Konovalov <andreyknvl@gmail.com>]

On Fri, Oct 24, 2025 at 3:19 AM Andrey Konovalov <andreyknvl@gmail.com> wrote:
>
> On Fri, Oct 24, 2025 at 2:41 AM Harry Yoo <harry.yoo@oracle.com> wrote:
> >
> > Adding more details on how I discovered this and why I care:
> >
> > I was developing a feature that uses unused bytes in s->size as the
> > slabobj_ext metadata. Unlike other metadata where slab disables KASAN
> > when accessing it, this should be unpoisoned to avoid adding complexity
> > and overhead when accessing it.
>
> Generally, unpoisoining parts of slabs that should not be accessed by
> non-slab code is undesirable - this would prevent KASAN from detecting
> OOB accesses into that memory.
>
> An alternative to unpoisoning or disabling KASAN could be to add
> helper functions annotated with __no_sanitize_address that do the
> required accesses. And make them inlined when KASAN is disabled to
> avoid the performance hit.
>
> On a side note, you might also need to check whether SW_TAGS KASAN and
> KMSAN would be unhappy with your changes:
>
> - When we do kasan_disable_current() or metadata_access_enable(), we
> also do kasan_reset_tag();
> - In metadata_access_enable(), we disable KMSAN as well.
>
> > This warning is from kasan_unpoison():
> >         if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
> >                 return;
> >
> > on x86_64, the address passed to kasan_{poison,unpoison}() should be at
> > least aligned with 8 bytes.
> >
> > After manual investigation it turns out when the SLAB_STORE_USER flag is
> > specified, any metadata after the original kmalloc request size is
> > misaligned.
> >
> > Questions:
> > - Could it cause any issues other than the one described above?
> > - Does KASAN even support architectures that have issues with unaligned
> >   accesses?
>
> Unaligned accesses are handled just fine. It's just that the start of
> any unpoisoned/accessible memory region must be aligned to 8 (or 16
> for SW_TAGS) bytes due to how KASAN encodes shadow memory values.

Misread your question: my response was about whether unaligned
accesses are instrumented/checked correctly on architectures that do
support them.

For architectures that do not: there might indeed be an issue. Though
there's KASAN support for xtensa and I suppose it works (does xtensa
support unaligned accesses?).

>
> > - How come we haven't seen any issues regarding this so far? :/
>
> As you pointed out, we don't unpoison the memory that stores KASAN
> metadata and instead just disable KASAN error reporting. This is done
> deliberately to allow KASAN catching accesses into that memory that
> happen outside of the slab/KASAN code.