Re: [PATCH] slab: Decouple slab_debug and no_hash_pointers

[Bagas Sanjaya]

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@xxxxxxxxx>
> Signed-off-by: Sergio Perez Gonzalez <sperezglz@xxxxxxxxx>
> Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
> ---
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Petr Mladek <pmladek@xxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> Cc: Harry Yoo <harry.yoo@xxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Cc: Tamir Duberstein <tamird@xxxxxxxxx>
> Cc: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Cc: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> ---
>  .../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@xxxxxxxxx>

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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