[PATCH 1/9] kasan: unify static kasan_flag_enabled across modes

[Sabyrzhan Tasbolatov <snovitoll@gmail.com>]

Historically the fast-path static key `kasan_flag_enabled` existed
only for `CONFIG_KASAN_HW_TAGS`. Generic and SW_TAGS either relied on
`kasan_arch_is_ready()` or evaluated KASAN checks unconditionally.
As a result every architecture had to toggle a private flag
in its `kasan_init()`.

This patch turns the flag into a single global runtime predicate that
is built for every `CONFIG_KASAN` mode and adds a helper that flips
the key once KASAN is ready.

Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218315
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
---
 include/linux/kasan-enabled.h | 22 ++++++++++++++++------
 include/linux/kasan.h         |  6 ++++++
 mm/kasan/common.c             |  7 +++++++
 mm/kasan/generic.c            | 11 +++++++++++
 mm/kasan/hw_tags.c            |  7 -------
 mm/kasan/sw_tags.c            |  2 ++
 6 files changed, 42 insertions(+), 13 deletions(-)

diff --git a/include/linux/kasan-enabled.h b/include/linux/kasan-enabled.h
index 6f612d69ea0c..2436eb45cfee 100644
--- a/include/linux/kasan-enabled.h
+++ b/include/linux/kasan-enabled.h
@@ -4,8 +4,12 @@
 
 #include <linux/static_key.h>
 
-#ifdef CONFIG_KASAN_HW_TAGS
+#ifdef CONFIG_KASAN
 
+/*
+ * Global runtime flag. Starts ‘false’; switched to ‘true’ by
+ * the appropriate kasan_init_*() once KASAN is fully initialized.
+ */
 DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
 
 static __always_inline bool kasan_enabled(void)
@@ -13,18 +17,24 @@ static __always_inline bool kasan_enabled(void)
 	return static_branch_likely(&kasan_flag_enabled);
 }
 
-static inline bool kasan_hw_tags_enabled(void)
+#else /* !CONFIG_KASAN */
+
+static __always_inline bool kasan_enabled(void)
 {
-	return kasan_enabled();
+	return false;
 }
 
-#else /* CONFIG_KASAN_HW_TAGS */
+#endif /* CONFIG_KASAN */
 
-static inline bool kasan_enabled(void)
+#ifdef CONFIG_KASAN_HW_TAGS
+
+static inline bool kasan_hw_tags_enabled(void)
 {
-	return IS_ENABLED(CONFIG_KASAN);
+	return kasan_enabled();
 }
 
+#else /* !CONFIG_KASAN_HW_TAGS */
+
 static inline bool kasan_hw_tags_enabled(void)
 {
 	return false;
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 890011071f2b..51a8293d1af6 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -543,6 +543,12 @@ void kasan_report_async(void);
 
 #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
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index ed4873e18c75..525194da25fa 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -32,6 +32,13 @@
 #include "kasan.h"
 #include "../slab.h"
 
+/*
+ * Definition of the unified static key declared in kasan-enabled.h.
+ * This provides consistent runtime enable/disable across all KASAN modes.
+ */
+DEFINE_STATIC_KEY_FALSE(kasan_flag_enabled);
+EXPORT_SYMBOL(kasan_flag_enabled);
+
 struct slab *kasan_addr_to_slab(const void *addr)
 {
 	if (virt_addr_valid(addr))
diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index d54e89f8c3e7..32c432df24aa 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -36,6 +36,17 @@
 #include "kasan.h"
 #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)
+{
+	static_branch_enable(&kasan_flag_enabled);
+
+	pr_info("KernelAddressSanitizer initialized (generic)\n");
+}
+
 /*
  * All functions below always inlined so compiler could
  * perform better optimizations in each of __asan_loadX/__assn_storeX
diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index 9a6927394b54..8e819fc4a260 100644
--- a/mm/kasan/hw_tags.c
+++ b/mm/kasan/hw_tags.c
@@ -45,13 +45,6 @@ static enum kasan_arg kasan_arg __ro_after_init;
 static enum kasan_arg_mode kasan_arg_mode __ro_after_init;
 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.
diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
index b9382b5b6a37..525bc91e2fcd 100644
--- a/mm/kasan/sw_tags.c
+++ b/mm/kasan/sw_tags.c
@@ -45,6 +45,8 @@ void __init kasan_init_sw_tags(void)
 
 	kasan_init_tags();
 
+	static_branch_enable(&kasan_flag_enabled);
+
 	pr_info("KernelAddressSanitizer initialized (sw-tags, stacktrace=%s)\n",
 		str_on_off(kasan_stack_collection_enabled()));
 }
-- 
2.34.1