Re: [PATCH v2] slab: Decouple slab_debug and no_hash_pointers

Harry Yoo
2025-04-17T14:16:16Z

On Tue, Apr 15, 2025 at 10:02:33AM -0700, Kees Cook wrote:
> Some system owners use slab_debug=FPZ (or similar) as a hardening option,
> but do not want to be forced into having kernel addresses exposed due
> to the implicit "no_hash_pointers" boot param setting.[1]

Is this behavior documented somewhere or it's only in the code?
I couldn't find anything other than the code.

> Introduce the "hash_pointers" boot param, which defaults to "auto"
> (the current behavior), but also includes "always" (forcing on hashing
> even when "slab_debug=..." is defined), and "never". The existing
> "no_hash_pointers" boot param becomes an alias for "hash_pointers=never".
>
> This makes it possible to boot with "slab_debug=FPZ hash_pointers=always".
>
> Link: https://github.com/KSPP/linux/issues/368 [1]
> Fixes: 792702911f58 ("slub: force on no_hash_pointers when slub_debug is enabled")
> Co-developed-by: Sergio Perez Gonzalez <sperezglz@gmail.com>
> Signed-off-by: Sergio Perez Gonzalez <sperezglz@gmail.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Rientjes <rientjes@google.com>
> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---

Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

By the way, while this patch does not change existing behavior of
slub_debug implying no_hash_pointers, kmem_cache_init() is not the only
place that enables slub_debug_enabled static key.

Maybe we should update __kmem_cache_create_args() too?
(in a separate patch)

--
Cheers,
Harry / Hyeonggon