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 <sperez...@gmail.com> > > > Signed-off-by: Sergio Perez Gonzalez <sperez...@gmail.com> > > > Acked-by: Vlastimil Babka <vba...@suse.cz> > > > Acked-by: David Rientjes <rient...@google.com> > > > Reviewed-by: Bagas Sanjaya <bagasdo...@gmail.com> > > > Signed-off-by: Kees Cook <k...@kernel.org> > > > --- > > > > Reviewed-by: Harry Yoo <harry....@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
