Re: [PATCH v2 02/14] kasan: sw_tags: Check kasan_flag_enabled at runtime

On 2025-02-20 at 00:30:09 +0100, Andrey Konovalov wrote:

On Tue, Feb 18, 2025 at 9:16 AM Maciej Wieczor-Retman
[off-list ref] wrote:

quoted

From: Samuel Holland <samuel.holland@sifive.com>

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.

Is this patch required on x86 as well? If so, I think it makes sense
to point it out here. And do the same in messages for other commits
that now mention RISC-V.

Not really necessary, I just thought all the general kasan patches from the
risc-v series could be added here at once. But you're right, I'll let Samuel
send these two (2nd and 3rd) patches since they relate to risc-v and not x86.

quoted

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

Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Signed-off-by: Maciej Wieczor-Retman <redacted>
---
 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 9a6927394b54..7f82af13b6a6 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.

@@ -259,9 +252,6 @@ 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(),
                str_on_off(kasan_vmalloc_enabled()),

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,
--

2.47.1

-- 
Kind regards
Maciej Wieczór-Retman