Re: [PATCH] slab: Decouple slab_debug and no_hash_pointers

Bagas Sanjaya
2025-04-13T23:57:12Z

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On Thu, Apr 10, 2025 at 10:44:31AM -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]
>
> 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>
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Roman Gushchin <roman.gushchin@linux.dev>
> Cc: Harry Yoo <harry.yoo@oracle.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Tamir Duberstein <tamird@gmail.com>
> Cc: Miguel Ojeda <ojeda@kernel.org>
> Cc: Alice Ryhl <aliceryhl@google.com>
> Cc: linux-doc@vger.kernel.org
> Cc: linux-mm@kvack.org
> ---
> .../admin-guide/kernel-parameters.txt | 34 ++++++++-----
> include/linux/sprintf.h | 2 +-
> lib/vsprintf.c | 51 +++++++++++++++++--
> mm/slub.c | 5 +-
> 4 files changed, 72 insertions(+), 20 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 76e538c77e31..d0fd9c745db9 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1798,6 +1798,27 @@
> backtraces on all cpus.
> Format: 0 | 1
>
> + hash_pointers=
> + [KNL,EARLY]
> + By default, when pointers are printed to the console
> + or buffers via the %p format string, that pointer is
> + "hashed", i.e. obscured by hashing the pointer value.
> + This is a security feature that hides actual kernel
> + addresses from unprivileged users, but it also makes
> + debugging the kernel more difficult since unequal
> + pointers can no longer be compared. The choices are:
> + Format: { auto | always | never }
> + Default: auto
> +
> + auto - Hash pointers unless slab_debug is enabled.
> + always - Always hash pointers (even if slab_debug is
> + enabled).
> + never - Never hash pointers. This option should only
> + be specified when debugging the kernel. Do
> + not use on production kernels. The boot
> + param "no_hash_pointers" is an alias for
> + this mode.

So on production one would want hash pointers (i.e. auto/always), right?

Anyway, the wording LGTM.

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>

PS: I'm confused on unspoken convention that anything but otherwise specified
is suitable for production, as I'm expecting explicit version deployment
(e.g. use <insert something> version instead of don't use <otherwise>).

--
An old man doll... just what I always wanted! - Clara

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