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

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Message-ID: <202506090823.33ED63C@keescook>
Date: Mon, 9 Jun 2025 08:24:47 -0700
From: Kees Cook <kees@...nel.org>
To: Petr Mladek <pmladek@...e.com>
Cc: 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>,
	Harry Yoo <harry.yoo@...cle.com>,
	"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 Mon, Jun 09, 2025 at 04:39:25PM +0200, Petr Mladek wrote:
> On Thu 2025-06-05 13:15:32, Kees Cook wrote:
> > On Wed, Apr 16, 2025 at 02:06:21PM +0200, Petr Mladek wrote:
> > > On Tue 2025-04-15 10:02:33, 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@...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>
> > > 
> > > Tested-by: Petr Mladek <pmladek@...e.com>
> > > Reviewed-by: Petr Mladek <pmladek@...e.com>
> > > 
> > > I am going to wait few more days for a potential feedback.
> > > I'll queue it for 6.16 unless anyone complains.
> > 
> > Hi,
> > 
> > Is this still planned for 6.16? I just noticed it wasn't in -next, and
> > we're almost to the end of the merge window...
> 
> I am terribly sorry. I have forgot this patch...
> 
> I have just pushed it into printk/linux.git, branch
> for-6.17-hash_pointers.
> 
> Also I updated the documentation, aka applied the diff
> from https://lore.kernel.org/r/202504181307.254F81843@keescook
> See
> https://web.git.kernel.org/pub/scm/linux/kernel/git/printk/linux.git/commit/?h=for-6.17-hash_pointers&id=de1c831a7898f164c1c2703c6b2b9e4fb4bebefc
> 
> As the branch name suggests, I am going to push it for 6.17.
> If it is too late for you then please let me know.

As long as it's in -next and scheduled to land, I'm happy. I'd always
like it earlier, but the less workflow disruption the better! :)

Thanks!

-Kees

-- 
Kees Cook