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@xxxxxxxxx> > > Signed-off-by: Sergio Perez Gonzalez <sperezglz@xxxxxxxxx> > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx> > > Acked-by: David Rientjes <rientjes@xxxxxxxxxx> > > Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> > > Signed-off-by: Kees Cook <kees@xxxxxxxxxx> > > --- > > Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx> > > 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
