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

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Message-ID: <aPs6Na_GUhRzPW7v@hyeyoo>
Date: Fri, 24 Oct 2025 17:35:13 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Andrey Konovalov <andreyknvl@...il.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, David Rientjes <rientjes@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Feng Tang <feng.79.tang@...il.com>, Christoph Lameter <cl@...two.org>,
        Dmitry Vyukov <dvyukov@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        stable@...r.kernel.org
Subject: Re: [PATCH] mm/slab: ensure all metadata in slab object are
 word-aligned

On Fri, Oct 24, 2025 at 03:19:57AM +0200, Andrey Konovalov wrote:
> On Fri, Oct 24, 2025 at 2:41 AM Harry Yoo <harry.yoo@...cle.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.

This sounds reasonable, let me try this instead of unpoisoning
metadata. Thanks.

> 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.

Thanks for pointing this out!

Just to clarify, by calling kasan_reset_tag() we clear tag from the address
so that SW or HW tag based KASAN won't report access violation? (because
there is no valid tag in the address?)

-- 
Cheers,
Harry / Hyeonggon