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
