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

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Message-ID: <CA+fCnZeMwOE1EwuP-3Xrs2e0qp_A5eo2aiXB_q243GiLFZV-=Q@mail.gmail.com>
Date: Fri, 24 Oct 2025 16:17:41 +0200
From: Andrey Konovalov <andreyknvl@...il.com>
To: Harry Yoo <harry.yoo@...cle.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 10:35 AM Harry Yoo <harry.yoo@...cle.com> wrote:
>
> > 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.

But note that you still need kasan_reset_tag() for HW_TAGS KASAN: this
mode is not based on compiler instrumentation and thus
__no_sanitize_address has no effect on it.

(There's been some discussion on making __no_sanitize_address work for
HW_TAGS: https://bugzilla.kernel.org/show_bug.cgi?id=212513#c2, but
this was never attempted.)

> > 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?)

Yeah, kind of: kasan_reset_tag() sets the pointer tag (the top byte)
to 0xFF. With SW_TAGS KASAN, the compiler knows not to embed validity
checks for accesses through pointers with 0xFF in the top byte. With
HW_TAGS KASAN, the CPU is instructed to behave the same.

(This is slightly different than kasan_disable_current(): with
kasan_reset_tag(), validity checks do not happen at all. With
kasan_disable_current(), the checks happen but the bug reports are
ignored.)

Thank you!