kasan: introduce ARCH_DEFER_KASAN and unify static key across modes

Patch series "kasan: unify kasan_enabled() and remove arch-specific
implementations", v6.

This patch series addresses the fragmentation in KASAN initialization
across architectures by introducing a unified approach that eliminates
duplicate static keys and arch-specific kasan_arch_is_ready()
implementations.

The core issue is that different architectures have inconsistent approaches
to KASAN readiness tracking:
- PowerPC, LoongArch, and UML arch, each implement own kasan_arch_is_ready()
- Only HW_TAGS mode had a unified static key (kasan_flag_enabled)
- Generic and SW_TAGS modes relied on arch-specific solutions
or always-on behavior


This patch (of 2):

Introduce CONFIG_ARCH_DEFER_KASAN to identify architectures [1] that need
to defer KASAN initialization until shadow memory is properly set up, and
unify the static key infrastructure across all KASAN modes.

[1] PowerPC, UML, LoongArch selects ARCH_DEFER_KASAN.

The core issue is that different architectures haveinconsistent approaches
to KASAN readiness tracking:
- PowerPC, LoongArch, and UML arch, each implement own
kasan_arch_is_ready()
- Only HW_TAGS mode had a unified static key (kasan_flag_enabled)
- Generic and SW_TAGS modes relied on arch-specific solutions or always-on
behavior

This patch addresses the fragmentation in KASAN initialization across
architectures by introducing a unified approach that eliminates duplicate
static keys and arch-specific kasan_arch_is_ready() implementations.

Let's replace kasan_arch_is_ready() with existing kasan_enabled() check,
which examines the static key being enabled if arch selects
ARCH_DEFER_KASAN or has HW_TAGS mode support. For other arch,
kasan_enabled() checks the enablement during compile time.

Now KASAN users can use a single kasan_enabled() check everywhere.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217049
Signed-off-by: Sabyrzhan Tasbolatov <[e-mail]>
Reviewed-by: Christophe Leroy <[e-mail]>
Reviewed-by: Ritesh Harjani (IBM) <[e-mail]> #powerpc
Cc: Alexander Gordeev <[e-mail]>
Cc: Alexander Potapenko <[e-mail]>
Cc: Alexandre Ghiti <[e-mail]>
Cc: Alexandre Ghiti <[e-mail]>
Cc: Andrey Konovalov <[e-mail]>
Cc: Andrey Ryabinin <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: David Gow <[e-mail]>
Cc: Dmitriy Vyukov <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Huacai Chen <[e-mail]>
Cc: Marco Elver <[e-mail]>
Cc: Qing Zhang <[e-mail]>
Cc: Sabyrzhan Tasbolatov <[e-mail]>
Cc: Vincenzo Frascino <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

arch/loongarch/Kconfig

···
 	select ACPI_PPTT if ACPI
 	select ACPI_SYSTEM_POWER_STATES_SUPPORT	if ACPI
 	select ARCH_BINFMT_ELF_STATE
+	select ARCH_NEEDS_DEFER_KASAN
 	select ARCH_DISABLE_KASAN_INLINE
 	select ARCH_ENABLE_MEMORY_HOTPLUG
 	select ARCH_ENABLE_MEMORY_HOTREMOVE

arch/loongarch/include/asm/kasan.h

···
 #define XKPRANGE_WC_SHADOW_OFFSET	(KASAN_SHADOW_START + XKPRANGE_WC_KASAN_OFFSET)
 #define XKVRANGE_VC_SHADOW_OFFSET	(KASAN_SHADOW_START + XKVRANGE_VC_KASAN_OFFSET)
 
-extern bool kasan_early_stage;
 extern unsigned char kasan_early_shadow_page[PAGE_SIZE];
 
 #define kasan_mem_to_shadow kasan_mem_to_shadow
···
 
 #define kasan_shadow_to_mem kasan_shadow_to_mem
 const void *kasan_shadow_to_mem(const void *shadow_addr);
-
-#define kasan_arch_is_ready kasan_arch_is_ready
-static __always_inline bool kasan_arch_is_ready(void)
-{
-	return !kasan_early_stage;
-}
 
 #define addr_has_metadata addr_has_metadata
 static __always_inline bool addr_has_metadata(const void *addr)

arch/loongarch/mm/kasan_init.c

···
 #define __pte_none(early, pte) (early ? pte_none(pte) : \
 ((pte_val(pte) & _PFN_MASK) == (unsigned long)__pa(kasan_early_shadow_page)))
 
-bool kasan_early_stage = true;
-
 void *kasan_mem_to_shadow(const void *addr)
 {
-	if (!kasan_arch_is_ready()) {
+	if (!kasan_enabled()) {
 		return (void *)(kasan_early_shadow_page);
 	} else {
 		unsigned long maddr = (unsigned long)addr;
···
 	kasan_populate_early_shadow(kasan_mem_to_shadow((void *)VMALLOC_START),
 					kasan_mem_to_shadow((void *)KFENCE_AREA_END));
 
-	kasan_early_stage = false;
+	/* Enable KASAN here before kasan_mem_to_shadow(). */
+	kasan_init_generic();
 
 	/* Populate the linear mapping */
 	for_each_mem_range(i, &pa_start, &pa_end) {
···
 
 	/* At this point kasan is fully initialized. Enable error messages */
 	init_task.kasan_depth = 0;
-	pr_info("KernelAddressSanitizer initialized.\n");
 }

arch/powerpc/Kconfig

···
 	# Please keep this list sorted alphabetically.
 	#
 	select ARCH_32BIT_OFF_T if PPC32
+	select ARCH_NEEDS_DEFER_KASAN		if PPC_RADIX_MMU
 	select ARCH_DISABLE_KASAN_INLINE	if PPC_RADIX_MMU
 	select ARCH_DMA_DEFAULT_COHERENT	if !NOT_COHERENT_CACHE
 	select ARCH_ENABLE_MEMORY_HOTPLUG

arch/powerpc/include/asm/kasan.h

···
 #endif
 
 #ifdef CONFIG_KASAN
-#ifdef CONFIG_PPC_BOOK3S_64
-DECLARE_STATIC_KEY_FALSE(powerpc_kasan_enabled_key);
-
-static __always_inline bool kasan_arch_is_ready(void)
-{
-	if (static_branch_likely(&powerpc_kasan_enabled_key))
-		return true;
-	return false;
-}
-
-#define kasan_arch_is_ready kasan_arch_is_ready
-#endif
 
 void kasan_early_init(void);
 void kasan_mmu_init(void);

arch/powerpc/mm/kasan/init_32.c

···
 
 	/* At this point kasan is fully initialized. Enable error messages */
 	init_task.kasan_depth = 0;
-	pr_info("KASAN init done\n");
+	kasan_init_generic();
 }
 
 void __init kasan_late_init(void)

arch/powerpc/mm/kasan/init_book3e_64.c

···
 
 	/* Enable error messages */
 	init_task.kasan_depth = 0;
-	pr_info("KASAN init done\n");
+	kasan_init_generic();
 }
 
 void __init kasan_late_init(void) { }

arch/powerpc/mm/kasan/init_book3s_64.c

···
 #include <linux/memblock.h>
 #include <asm/pgalloc.h>
 
-DEFINE_STATIC_KEY_FALSE(powerpc_kasan_enabled_key);
-
 static void __init kasan_init_phys_region(void *start, void *end)
 {
 	unsigned long k_start, k_end, k_cur;
···
 	 */
 	memset(kasan_early_shadow_page, 0, PAGE_SIZE);
 
-	static_branch_inc(&powerpc_kasan_enabled_key);
-
 	/* Enable error messages */
 	init_task.kasan_depth = 0;
-	pr_info("KASAN init done\n");
+	kasan_init_generic();
 }
 
 void __init kasan_early_init(void) { }

arch/um/Kconfig

···
 config UML
 	bool
 	default y
+	select ARCH_NEEDS_DEFER_KASAN if STATIC_LINK
 	select ARCH_WANTS_DYNAMIC_TASK_STRUCT
 	select ARCH_HAS_CACHE_LINE_SIZE
 	select ARCH_HAS_CPU_FINALIZE_INIT

arch/um/include/asm/kasan.h

···
 
 #ifdef CONFIG_KASAN
 void kasan_init(void);
-extern int kasan_um_is_ready;
 
-#ifdef CONFIG_STATIC_LINK
-#define kasan_arch_is_ready() (kasan_um_is_ready)
+#if defined(CONFIG_STATIC_LINK) && defined(CONFIG_KASAN_INLINE)
+#error UML does not work in KASAN_INLINE mode with STATIC_LINK enabled!
 #endif
 #else
 static inline void kasan_init(void) { }

arch/um/kernel/mem.c

···
 #include <os.h>
 #include <um_malloc.h>
 #include <linux/sched/task.h>
+#include <linux/kasan.h>
 
 #ifdef CONFIG_KASAN
-int kasan_um_is_ready;
-void kasan_init(void)
+void __init kasan_init(void)
 {
 	/*
 	 * kasan_map_memory will map all of the required address space and
···
 	 */
 	kasan_map_memory((void *)KASAN_SHADOW_START, KASAN_SHADOW_SIZE);
 	init_task.kasan_depth = 0;
-	kasan_um_is_ready = true;
+	/*
+	 * Since kasan_init() is called before main(),
+	 * KASAN is initialized but the enablement is deferred after
+	 * jump_label_init(). See arch_mm_preinit().
+	 */
 }
 
 static void (*kasan_init_ptr)(void)
···
 
 void __init arch_mm_preinit(void)
 {
+	/* Safe to call after jump_label_init(). Enables KASAN. */
+	kasan_init_generic();
+
 	/* clear the zero-page */
 	memset(empty_zero_page, 0, PAGE_SIZE);
 

include/linux/kasan-enabled.h

···
 
 #include <linux/static_key.h>
 
-#ifdef CONFIG_KASAN_HW_TAGS
-
+#if defined(CONFIG_ARCH_DEFER_KASAN) || defined(CONFIG_KASAN_HW_TAGS)
+/*
+ * Global runtime flag for KASAN modes that need runtime control.
+ * Used by ARCH_DEFER_KASAN architectures and HW_TAGS mode.
+ */
 DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
 
+/*
+ * Runtime control for shadow memory initialization or HW_TAGS mode.
+ * Uses static key for architectures that need deferred KASAN or HW_TAGS.
+ */
 static __always_inline bool kasan_enabled(void)
 {
 	return static_branch_likely(&kasan_flag_enabled);
 }
 
-static inline bool kasan_hw_tags_enabled(void)
+static inline void kasan_enable(void)
 {
-	return kasan_enabled();
+	static_branch_enable(&kasan_flag_enabled);
 }
-
-#else /* CONFIG_KASAN_HW_TAGS */
-
-static inline bool kasan_enabled(void)
+#else
+/* For architectures that can enable KASAN early, use compile-time check. */
+static __always_inline bool kasan_enabled(void)
 {
 	return IS_ENABLED(CONFIG_KASAN);
 }
 
+static inline void kasan_enable(void) {}
+#endif /* CONFIG_ARCH_DEFER_KASAN || CONFIG_KASAN_HW_TAGS */
+
+#ifdef CONFIG_KASAN_HW_TAGS
+static inline bool kasan_hw_tags_enabled(void)
+{
+	return kasan_enabled();
+}
+#else
 static inline bool kasan_hw_tags_enabled(void)
 {
 	return false;
 }
-
 #endif /* CONFIG_KASAN_HW_TAGS */
 
 #endif /* LINUX_KASAN_ENABLED_H */

include/linux/kasan.h

···
 
 #endif /* CONFIG_KASAN_HW_TAGS */
 
+#ifdef CONFIG_KASAN_GENERIC
+void __init kasan_init_generic(void);
+#else
+static inline void kasan_init_generic(void) { }
+#endif
+
 #ifdef CONFIG_KASAN_SW_TAGS
 void __init kasan_init_sw_tags(void);
 #else

lib/Kconfig.kasan

···
 	  Disables both inline and stack instrumentation. Selected by
 	  architectures that do not support these instrumentation types.
 
+config ARCH_NEEDS_DEFER_KASAN
+	bool
+
+config ARCH_DEFER_KASAN
+	def_bool y
+	depends on KASAN && ARCH_NEEDS_DEFER_KASAN
+	help
+	  Architectures should select this if they need to defer KASAN
+	  initialization until shadow memory is properly set up. This
+	  enables runtime control via static keys. Otherwise, KASAN uses
+	  compile-time constants for better performance.
+
 config CC_HAS_KASAN_GENERIC
 	def_bool $(cc-option, -fsanitize=kernel-address)
 

mm/kasan/common.c

···
 #include "kasan.h"
 #include "../slab.h"
 
+#if defined(CONFIG_ARCH_DEFER_KASAN) || defined(CONFIG_KASAN_HW_TAGS)
+/*
+ * Definition of the unified static key declared in kasan-enabled.h.
+ * This provides consistent runtime enable/disable across KASAN modes.
+ */
+DEFINE_STATIC_KEY_FALSE(kasan_flag_enabled);
+EXPORT_SYMBOL_GPL(kasan_flag_enabled);
+#endif
+
 struct slab *kasan_addr_to_slab(const void *addr)
 {
 	if (virt_addr_valid(addr))
···
 bool __kasan_slab_pre_free(struct kmem_cache *cache, void *object,
 				unsigned long ip)
 {
-	if (!kasan_arch_is_ready() || is_kfence_address(object))
+	if (is_kfence_address(object))
 		return false;
 	return check_slab_allocation(cache, object, ip);
 }
···
 bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
 		       bool still_accessible)
 {
-	if (!kasan_arch_is_ready() || is_kfence_address(object))
+	if (is_kfence_address(object))
 		return false;
 
 	/*
···
 
 static inline bool check_page_allocation(void *ptr, unsigned long ip)
 {
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return false;
 
 	if (ptr != page_address(virt_to_head_page(ptr))) {
···
 		return true;
 	}
 
-	if (is_kfence_address(ptr) || !kasan_arch_is_ready())
+	if (is_kfence_address(ptr))
 		return true;
 
 	slab = folio_slab(folio);

mm/kasan/generic.c

···
 #include "../slab.h"
 
 /*
+ * Initialize Generic KASAN and enable runtime checks.
+ * This should be called from arch kasan_init() once shadow memory is ready.
+ */
+void __init kasan_init_generic(void)
+{
+	kasan_enable();
+
+	pr_info("KernelAddressSanitizer initialized (generic)\n");
+}
+
+/*
  * All functions below always inlined so compiler could
  * perform better optimizations in each of __asan_loadX/__assn_storeX
  * depending on memory access size X.
···
 						size_t size, bool write,
 						unsigned long ret_ip)
 {
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return true;
 
 	if (unlikely(size == 0))
···
 {
 	s8 shadow_byte;
 
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return true;
 
 	shadow_byte = READ_ONCE(*(s8 *)kasan_mem_to_shadow(addr));
···
 
 static void release_free_meta(const void *object, struct kasan_free_meta *meta)
 {
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return;
 
 	/* Check if free meta is valid. */
···
 	kasan_save_track(&alloc_meta->alloc_track, flags);
 }
 
-void kasan_save_free_info(struct kmem_cache *cache, void *object)
+void __kasan_save_free_info(struct kmem_cache *cache, void *object)
 {
 	struct kasan_free_meta *free_meta;
 

mm/kasan/hw_tags.c

···
 static enum kasan_arg_vmalloc kasan_arg_vmalloc __initdata;
 
 /*
- * Whether KASAN is enabled at all.
- * The value remains false until KASAN is initialized by kasan_init_hw_tags().
- */
-DEFINE_STATIC_KEY_FALSE(kasan_flag_enabled);
-EXPORT_SYMBOL(kasan_flag_enabled);
-
-/*
  * Whether the selected mode is synchronous, asynchronous, or asymmetric.
  * Defaults to KASAN_MODE_SYNC.
  */
···
 	kasan_init_tags();
 
 	/* KASAN is now initialized, enable it. */
-	static_branch_enable(&kasan_flag_enabled);
+	kasan_enable();
 
 	pr_info("KernelAddressSanitizer initialized (hw-tags, mode=%s, vmalloc=%s, stacktrace=%s)\n",
 		kasan_mode_info(),

mm/kasan/kasan.h

···
 void kasan_set_track(struct kasan_track *track, depot_stack_handle_t stack);
 void kasan_save_track(struct kasan_track *track, gfp_t flags);
 void kasan_save_alloc_info(struct kmem_cache *cache, void *object, gfp_t flags);
-void kasan_save_free_info(struct kmem_cache *cache, void *object);
+
+void __kasan_save_free_info(struct kmem_cache *cache, void *object);
+static inline void kasan_save_free_info(struct kmem_cache *cache, void *object)
+{
+	if (kasan_enabled())
+		__kasan_save_free_info(cache, object);
+}
 
 #ifdef CONFIG_KASAN_GENERIC
 bool kasan_quarantine_put(struct kmem_cache *cache, void *object);

mm/kasan/shadow.c

···
 {
 	void *shadow_start, *shadow_end;
 
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return;
 
 	/*
···
 #ifdef CONFIG_KASAN_GENERIC
 void kasan_poison_last_granule(const void *addr, size_t size)
 {
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return;
 
 	if (size & KASAN_GRANULE_MASK) {
···
 	unsigned long shadow_start, shadow_end;
 	int ret;
 
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return 0;
 
 	if (!is_vmalloc_or_module_addr((void *)addr))
···
 	unsigned long region_start, region_end;
 	unsigned long size;
 
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return;
 
 	region_start = ALIGN(start, KASAN_MEMORY_PER_SHADOW_PAGE);
···
 	 * with setting memory tags, so the KASAN_VMALLOC_INIT flag is ignored.
 	 */
 
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return (void *)start;
 
 	if (!is_vmalloc_or_module_addr(start))
···
  */
 void __kasan_poison_vmalloc(const void *start, unsigned long size)
 {
-	if (!kasan_arch_is_ready())
+	if (!kasan_enabled())
 		return;
 
 	if (!is_vmalloc_or_module_addr(start))

mm/kasan/sw_tags.c

···
 		per_cpu(prng_state, cpu) = (u32)get_cycles();
 
 	kasan_init_tags();
+	kasan_enable();
 
 	pr_info("KernelAddressSanitizer initialized (sw-tags, stacktrace=%s)\n",
 		str_on_off(kasan_stack_collection_enabled()));

mm/kasan/tags.c

···
 	save_stack_info(cache, object, flags, false);
 }
 
-void kasan_save_free_info(struct kmem_cache *cache, void *object)
+void __kasan_save_free_info(struct kmem_cache *cache, void *object)
 {
 	save_stack_info(cache, object, 0, true);
 }