linux-kernel - Re: [PATCH v2] slab: Decouple slab_debug and no_hash

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Message-ID: <aAEM73DrpbzdZF92@harry>
Date: Thu, 17 Apr 2025 23:15:11 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Kees Cook <kees@...nel.org>
Cc: Petr Mladek <pmladek@...e.com>,
        Sergio Perez Gonzalez <sperezglz@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>, David Rientjes <rientjes@...gle.com>,
        Bagas Sanjaya <bagasdotme@...il.com>, Jonathan Corbet <corbet@....net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Tamir Duberstein <tamird@...il.com>, Miguel Ojeda <ojeda@...nel.org>,
        Alice Ryhl <aliceryhl@...gle.com>, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, Thomas Huth <thuth@...hat.com>,
        "Borislav Petkov (AMD)" <bp@...en8.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andreas Hindborg <a.hindborg@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] slab: Decouple slab_debug and no_hash_pointers

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@...il.com>
> Signed-off-by: Sergio Perez Gonzalez <sperezglz@...il.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Acked-by: David Rientjes <rientjes@...gle.com>
> Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---

Reviewed-by: Harry Yoo <harry.yoo@...cle.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