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

Kees Cook
2025-04-18T20:13:22Z

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.
+
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://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)

The state of pointer hashing should not change after boot. (It is
intentionally designed to use __ro_after_init.) Honestly, I'd prefer
that slab_debug was not tied to no_hash_pointers at all...

-Kees

--
Kees Cook