[PATCH] Decouple slub_debug= from no_hash_pointers again

[Peter Gerber]

While, as mentioned in 792702911f58, no_hash_pointers is what
one wants for debugging, this option is also used for hardening.

Various places recommend or use slub_debug for hardening:

a) The Kernel Self Protection Project lists slub_debug as
   a recommended setting. [1]
b) Debian offers package hardening-runtime [2] which enables
   slub_debug for hardening.
c) Security- and privacy-oriented Tails enables slub_debug
   by default [3].

I understand that encountering hashed pointers during debugging
is most unwanted. Thus, I updated the documentation to make
it as clear as possible that no_hash_pointers is what one
wants when using slub_debug for debugging. I also added a
mentioned of the hardening use case in order to discourage
any other, well-meant, tries to disable hashing with slub_debug.

[1]: https://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project/Recommended_Settings
[2]: https://packages.debian.org/bullseye/hardening-runtime
[3]: https://tails.boum.org/contribute/design/kernel_hardening/

Fixes: 792702911f58 ("slub: force on no_hash_pointers when slub_debug is enabled")
Cc: Stephen Boyd <swboyd@xxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/kernel-hardening/202204121715.11B2CA80@keescook/T/#t
---
 Documentation/admin-guide/kernel-parameters.txt |  5 +++--
 Documentation/vm/slub.rst                       | 10 ++++++++++
 include/linux/kernel.h                          |  2 --
 mm/slub.c                                       |  4 ----
 4 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3f1cc5e317ed..8987c07e206c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5344,8 +5344,9 @@
 			culprit if slab objects become corrupted. Enabling
 			slub_debug can create guard zones around objects and
 			may poison objects when not in use. Also tracks the
-			last alloc / free. For more information see
-			Documentation/vm/slub.rst.
+			last alloc / free. If used for debugging, rather
+			than hardening, also set no_hash_pointers.
+			For more information see Documentation/vm/slub.rst.
 
 	slub_max_order= [MM, SLUB]
 			Determines the maximum allowed order for slabs.
diff --git a/Documentation/vm/slub.rst b/Documentation/vm/slub.rst
index d3028554b1e9..1ae6c27d0ff0 100644
--- a/Documentation/vm/slub.rst
+++ b/Documentation/vm/slub.rst
@@ -41,6 +41,16 @@ slub_debug=<Debug-Options>,<slab name1>,<slab name2>,...
 	Enable options only for select slabs (no spaces
 	after a comma)
 
+.. hint::
+
+   **Also enable no_hash_pointers** for debugging. Otherwise, only hashed
+   pointers are printed.
+
+   Hashing is disabled by default because this option, despite having
+   debug in its name, is also in useed to provide `additional hardening`_.
+
+.. _additional hardening: https://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project/Recommended_Settings
+
 Multiple blocks of options for all slabs or selected slabs can be given, with
 blocks of options delimited by ';'. The last of "all slabs" blocks is applied
 to all slabs except those that match one of the "select slabs" block. Options
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index fe6efb24d151..e3d9d3879495 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -229,8 +229,6 @@ int sscanf(const char *, const char *, ...);
 extern __scanf(2, 0)
 int vsscanf(const char *, const char *, va_list);
 
-extern int no_hash_pointers_enable(char *str);
-
 extern int get_option(char **str, int *pint);
 extern char *get_options(const char *str, int nints, int *ints);
 extern unsigned long long memparse(const char *ptr, char **retptr);
diff --git a/mm/slub.c b/mm/slub.c
index ed5c2c03a47a..b78ccfde9214 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4800,10 +4800,6 @@ void __init kmem_cache_init(void)
 	if (debug_guardpage_minorder())
 		slub_max_order = 0;
 
-	/* Print slub debugging pointers without hashing */
-	if (__slub_debug_enabled())
-		no_hash_pointers_enable(NULL);
-
 	kmem_cache_node = &boot_kmem_cache_node;
 	kmem_cache = &boot_kmem_cache;
 
-- 
2.35.1