Re: [PATCH v2] slab: Decouple slab_debug and no_hash_pointers

Rafael Aquini
2025-04-17T12:37:02Z

On Tue, Apr 15, 2025 at 10:02:33AM -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>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Rientjes <rientjes@google.com>
> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
> Signed-off-by: Kees Cook <kees@kernel.org>
> ---
> v2: use switch state to avoid missing states (pmladek)
> v1: https://lore.kernel.org/lkml/20250410174428.work.488-kees@kernel.org/
> Cc: Petr Mladek <pmladek@suse.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Jonathan Corbet <corbet@lwn.net>
> 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 | 61 +++++++++++++++++--
> mm/slub.c | 5 +-
> 4 files changed, 82 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.
> +
> hashdist= [KNL,NUMA] Large hashes allocated during boot
> are distributed across NUMA nodes. Defaults on
> for 64-bit NUMA, off otherwise.
> @@ -4120,18 +4141,7 @@
>
> no_hash_pointers
> [KNL,EARLY]
> - Force pointers printed to the console or buffers to be
> - unhashed. By default, when a pointer is printed via %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. However, if this command-line option is
> - specified, then all normal pointers will have their true
> - value printed. This option should only be specified when
> - debugging the kernel. Please do not use on production
> - kernels.
> + Alias for "hash_pointers=never".
>
> nohibernate [HIBERNATION] Disable hibernation and resume.
>
> diff --git a/include/linux/sprintf.h b/include/linux/sprintf.h
> index 51cab2def9ec..521bb2cd2648 100644
> --- a/include/linux/sprintf.h
> +++ b/include/linux/sprintf.h
> @@ -22,7 +22,7 @@ __scanf(2, 0) int vsscanf(const char *, const char *, va_list);
>
> /* These are for specific cases, do not use without real need */
> extern bool no_hash_pointers;
> -int no_hash_pointers_enable(char *str);
> +void hash_pointers_finalize(bool slub_debug);
>
> /* Used for Rust formatting ('%pA') */
> char *rust_fmt_argument(char *buf, char *end, const void *ptr);
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 01699852f30c..22cbd75266ef 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -60,6 +60,20 @@
> bool no_hash_pointers __ro_after_init;
> EXPORT_SYMBOL_GPL(no_hash_pointers);
>
> +/*
> + * Hashed pointers policy selected by "hash_pointers=..." boot param
> + *
> + * `auto` - Hashed pointers enabled unless disabled by slub_debug_enabled=true
> + * `always` - Hashed pointers enabled unconditionally
> + * `never` - Hashed pointers disabled unconditionally
> + */
> +enum hash_pointers_policy {
> + HASH_PTR_AUTO = 0,
> + HASH_PTR_ALWAYS,
> + HASH_PTR_NEVER
> +};
> +static enum hash_pointers_policy hash_pointers_mode __initdata;
> +
> noinline
> static unsigned long long simple_strntoull(const char *startp, char **endp, unsigned int base, size_t max_chars)
> {
> @@ -2271,12 +2285,23 @@ char *resource_or_range(const char *fmt, char *buf, char *end, void *ptr,
> return resource_string(buf, end, ptr, spec, fmt);
> }
>
> -int __init no_hash_pointers_enable(char *str)
> +void __init hash_pointers_finalize(bool slub_debug)
> {
> - if (no_hash_pointers)
> - return 0;
> + switch (hash_pointers_mode) {
> + case HASH_PTR_ALWAYS:
> + no_hash_pointers = false;
> + break;
> + case HASH_PTR_NEVER:
> + no_hash_pointers = true;
> + break;
> + case HASH_PTR_AUTO:
> + default:
> + no_hash_pointers = slub_debug;
> + break;
> + }
>
> - no_hash_pointers = true;
> + if (!no_hash_pointers)
> + return;
>
> pr_warn("**********************************************************\n");
> pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
> @@ -2289,11 +2314,39 @@ int __init no_hash_pointers_enable(char *str)
> pr_warn("** the kernel, report this immediately to your system **\n");
> pr_warn("** administrator! **\n");
> pr_warn("** **\n");
> + pr_warn("** Use hash_pointers=always to force this mode off **\n");
> + pr_warn("** **\n");
> pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
> pr_warn("**********************************************************\n");
> +}
> +
> +static int __init hash_pointers_mode_parse(char *str)
> +{
> + if (!str) {
> + pr_warn("Hash pointers mode empty; falling back to auto.\n");
> + hash_pointers_mode = HASH_PTR_AUTO;
> + } else if (strncmp(str, "auto", 4) == 0) {
> + pr_info("Hash pointers mode set to auto.\n");
> + hash_pointers_mode = HASH_PTR_AUTO;
> + } else if (strncmp(str, "never", 5) == 0) {
> + pr_info("Hash pointers mode set to never.\n");
> + hash_pointers_mode = HASH_PTR_NEVER;
> + } else if (strncmp(str, "always", 6) == 0) {
> + pr_info("Hash pointers mode set to always.\n");
> + hash_pointers_mode = HASH_PTR_ALWAYS;
> + } else {
> + pr_warn("Unknown hash_pointers mode '%s' specified; assuming auto.\n", str);
> + hash_pointers_mode = HASH_PTR_AUTO;
> + }
>
> return 0;
> }
> +early_param("hash_pointers", hash_pointers_mode_parse);
> +
> +static int __init no_hash_pointers_enable(char *str)
> +{
> + return hash_pointers_mode_parse("never");
> +}
> early_param("no_hash_pointers", no_hash_pointers_enable);
>
> /*
> diff --git a/mm/slub.c b/mm/slub.c
> index b46f87662e71..f3d61b330a76 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6314,9 +6314,8 @@ 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);
> + /* Inform pointer hashing choice about slub debugging state. */
> + hash_pointers_finalize(__slub_debug_enabled());
>
> kmem_cache_node = &boot_kmem_cache_node;
> kmem_cache = &boot_kmem_cache;
> --
> 2.34.1
>
>

Thank you, Kees.

Acked-by: Rafael Aquini <raquini@redhat.com>