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: KSPP#368 [1]
Fixes: 7927029 ("slub: force on no_hash_pointers when slub_debug is enabled")
Co-developed-by: Sergio Perez Gonzalez <[email protected]>
Signed-off-by: Sergio Perez Gonzalez <[email protected]>
Signed-off-by: Kees Cook <[email protected]>

Author: kees

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.
 

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);

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 +2285,13 @@ 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;
+	if (hash_pointers_mode == HASH_PTR_AUTO && slub_debug)
+		no_hash_pointers = true;
 
-	no_hash_pointers = true;
+	if (!no_hash_pointers)
+		return;
 
 	pr_warn("**********************************************************\n");
 	pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
@@ -2289,11 +2304,39 @@ 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);
 
 /*

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;