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

Harry Yoo
2025-04-21T09:44:43Z

On Fri, Apr 18, 2025 at 01:13:20PM -0700, Kees Cook wrote:
> On Thu, Apr 17, 2025 at 11:15:11PM +0900, Harry Yoo wrote:
> > 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.
>
> Hmm, that's an excellent point. I don't see any mention of it in
> kernel-parameters.txt. Perhaps this?
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 4568572205ee..982e6511a225 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6483,6 +6483,10 @@
> Documentation/mm/slub.rst.
> (slub_debug legacy name also accepted for now)
>
> + Using this option implies the "no_hash_pointers"
> + option which can be undone by adding the
> + "hash_pointers=always" option.
> +

Looks good to me as it correctly states current behavior.

> slab_max_order= [MM]
> Determines the maximum allowed order for slabs.
> A high setting may cause OOMs due to memory
>
> >
> > > 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://urldefense.com/v3/__https://github.com/KSPP/linux/issues/368__;!!ACWV5N9M2RV99hQ!No7Nk2rrOwtdh-XVVxppszL38saNKUNOLWogxasxGNiGfiS7gTVkUmDrw8J_YVAsbujlEb3hfb30$ [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)
>
> The state of pointer hashing should not change after boot. (It is
> intentionally designed to use __ro_after_init.)

Okay. In security perspective that makes sense.

IIRC the only case that dynamically creates debug caches is rcutorture
so probably doesn't matter that much.

--
Cheers,
Harry / Hyeonggon