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Message-Id: <20250421165508.make.689-kees@kernel.org>
Date: Mon, 21 Apr 2025 09:55:12 -0700
From: Kees Cook <kees@...nel.org>
To: Petr Mladek <pmladek@...e.com>
Cc: Kees Cook <kees@...nel.org>,
Sergio Perez Gonzalez <sperezglz@...il.com>,
Vlastimil Babka <vbabka@...e.cz>,
David Rientjes <rientjes@...gle.com>,
Bagas Sanjaya <bagasdotme@...il.com>,
Harry Yoo <harry.yoo@...cle.com>,
Rafael Aquini <raquini@...hat.com>,
Jonathan Corbet <corbet@....net>,
Andrew Morton <akpm@...ux-foundation.org>,
Steven Rostedt <rostedt@...dmis.org>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Sergey Senozhatsky <senozhatsky@...omium.org>,
Christoph Lameter <cl@...ux.com>,
Pekka Enberg <penberg@...nel.org>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Roman Gushchin <roman.gushchin@...ux.dev>,
"Paul E. McKenney" <paulmck@...nel.org>,
Thomas Huth <thuth@...hat.com>,
"Borislav Petkov (AMD)" <bp@...en8.de>,
Ard Biesheuvel <ardb@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Tamir Duberstein <tamird@...il.com>,
Alice Ryhl <aliceryhl@...gle.com>,
Stephen Boyd <swboyd@...omium.org>,
linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org,
linux-mm@...ck.org,
linux-hardening@...r.kernel.org
Subject: [PATCH v3] slab: Decouple slab_debug and no_hash_pointers
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>
Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
Acked-by: Rafael Aquini <raquini@...hat.com>
Tested-by: Petr Mladek <pmladek@...e.com>
Reviewed-by: Petr Mladek <pmladek@...e.com>
Signed-off-by: Kees Cook <kees@...nel.org>
---
v3: add docs about implicit "no_hash_pointers" when using "slab_debug" (harry.yoo)
v2: https://lore.kernel.org/all/20250415170232.it.467-kees@kernel.org/
v1: https://lore.kernel.org/lkml/20250410174428.work.488-kees@kernel.org/
---
.../admin-guide/kernel-parameters.txt | 38 ++++++++----
include/linux/sprintf.h | 2 +-
lib/vsprintf.c | 61 +++++++++++++++++--
mm/slub.c | 5 +-
4 files changed, 86 insertions(+), 20 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d9fd26b95b34..cb12531b2100 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1795,6 +1795,27 @@
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.
@@ -4117,18 +4138,7 @@
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.
@@ -6478,6 +6488,10 @@
Documentation/mm/slub.rst.
(slub_debug legacy name also accepted for now)
+ Using this option implies the "no_hash_pointers"
+ option which can be undone by adding the
+ "hash_pointers=always" option.
+
slab_max_order= [MM]
Determines the maximum allowed order for slabs.
A high setting may cause OOMs due to memory
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
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