[RFC PATCH 2/7] kasan: sw_tags: Check kasan_flag_enabled at runtime

[Samuel Holland]

On RISC-V, the ISA extension required to dereference tagged pointers is
optional, and the interface to enable pointer masking requires firmware
support. Therefore, we must detect at runtime if sw_tags is usable on a
given machine. Reuse the logic from hw_tags to dynamically enable KASAN.

This commit makes no functional change to the KASAN_HW_TAGS code path.

Signed-off-by: Samuel Holland <samuel.holland at sifive.com>
---

 include/linux/kasan-enabled.h | 15 +++++----------
 mm/kasan/hw_tags.c            | 10 ----------
 mm/kasan/tags.c               | 10 ++++++++++
 3 files changed, 15 insertions(+), 20 deletions(-)

diff --git a/include/linux/kasan-enabled.h b/include/linux/kasan-enabled.h
index 6f612d69ea0c..648bda9495b7 100644
--- a/include/linux/kasan-enabled.h
+++ b/include/linux/kasan-enabled.h
@@ -4,7 +4,7 @@
 
 #include <linux/static_key.h>
 
-#ifdef CONFIG_KASAN_HW_TAGS
+#if defined(CONFIG_KASAN_SW_TAGS) || defined(CONFIG_KASAN_HW_TAGS)
 
 DECLARE_STATIC_KEY_FALSE(kasan_flag_enabled);
 
@@ -13,23 +13,18 @@ static __always_inline bool kasan_enabled(void)
 	return static_branch_likely(&kasan_flag_enabled);
 }
 
-static inline bool kasan_hw_tags_enabled(void)
-{
-	return kasan_enabled();
-}
-
-#else /* CONFIG_KASAN_HW_TAGS */
+#else /* CONFIG_KASAN_SW_TAGS || CONFIG_KASAN_HW_TAGS */
 
 static inline bool kasan_enabled(void)
 {
 	return IS_ENABLED(CONFIG_KASAN);
 }
 
+#endif /* CONFIG_KASAN_SW_TAGS || CONFIG_KASAN_HW_TAGS */
+
 static inline bool kasan_hw_tags_enabled(void)
 {
-	return false;
+	return IS_ENABLED(CONFIG_KASAN_HW_TAGS) && kasan_enabled();
 }
 
-#endif /* CONFIG_KASAN_HW_TAGS */
-
 #endif /* LINUX_KASAN_ENABLED_H */
diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index 9958ebc15d38..c3beeb94efa5 100644
--- a/mm/kasan/hw_tags.c
+++ b/mm/kasan/hw_tags.c
@@ -43,13 +43,6 @@ 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.
@@ -257,9 +250,6 @@ void __init kasan_init_hw_tags(void)
 
 	kasan_init_tags();
 
-	/* KASAN is now initialized, enable it. */
-	static_branch_enable(&kasan_flag_enabled);
-
 	pr_info("KernelAddressSanitizer initialized (hw-tags, mode=%s, vmalloc=%s, stacktrace=%s)\n",
 		kasan_mode_info(),
 		kasan_vmalloc_enabled() ? "on" : "off",
diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
index d65d48b85f90..c111d98961ed 100644
--- a/mm/kasan/tags.c
+++ b/mm/kasan/tags.c
@@ -32,6 +32,13 @@ enum kasan_arg_stacktrace {
 
 static enum kasan_arg_stacktrace kasan_arg_stacktrace __initdata;
 
+/*
+ * Whether KASAN is enabled at all.
+ * The value remains false until KASAN is initialized by kasan_init_tags().
+ */
+DEFINE_STATIC_KEY_FALSE(kasan_flag_enabled);
+EXPORT_SYMBOL(kasan_flag_enabled);
+
 /* Whether to collect alloc/free stack traces. */
 DEFINE_STATIC_KEY_TRUE(kasan_flag_stacktrace);
 
@@ -92,6 +99,9 @@ void __init kasan_init_tags(void)
 		if (WARN_ON(!stack_ring.entries))
 			static_branch_disable(&kasan_flag_stacktrace);
 	}
+
+	/* KASAN is now initialized, enable it. */
+	static_branch_enable(&kasan_flag_enabled);
 }
 
 static void save_stack_info(struct kmem_cache *cache, void *object,
-- 
2.45.1