[PATCH v4 2/2] kasan: Add memzero int for unaligned size at DEBUG

[PATCH v4 2/2] kasan: Add memzero int for unaligned size at DEBUG

[yee.lee]

From: Yee Lee <yee.lee@mediatek.com>

Issue: when SLUB debug is on, hwtag kasan_unpoison() would overwrite
the redzone of object with unaligned size.

An additional memzero_explicit() path is added to replacing init by
hwtag instruction for those unaligned size at SLUB debug mode.

The penalty is acceptable since they are only enabled in debug mode,
not production builds. A block of comment is added for explanation.

---
 v4:
 - Add "slab.h" header
 - Use slub_debug_enabled_unlikely() to replace IS_ENABLED
 - Refine the comment block

---
Signed-off-by: Yee Lee <yee.lee@mediatek.com>
Suggested-by: Marco Elver <elver@google.com>
Suggested-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>

---
 mm/kasan/kasan.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 98e3059bfea4..a9d837197302 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -9,6 +9,7 @@
 #ifdef CONFIG_KASAN_HW_TAGS
 
 #include <linux/static_key.h>
+#include "../slab.h"
 
 DECLARE_STATIC_KEY_FALSE(kasan_flag_stacktrace);
 extern bool kasan_flag_async __ro_after_init;
@@ -387,6 +388,17 @@ static inline void kasan_unpoison(const void *addr, size_t size, bool init)
 
 	if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
 		return;
+	/*
+	 * Explicitly initialize the memory with the precise object size to
+	 * avoid overwriting the SLAB redzone. This disables initialization in
+	 * the arch code and may thus lead to performance penalty. The penalty
+	 * is accepted since SLAB redzones aren't enabled in production builds.
+	 */
+	if (slub_debug_enabled_unlikely() &&
+	    init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
+		init = false;
+		memzero_explicit((void *)addr, size);
+	}
 	size = round_up(size, KASAN_GRANULE_SIZE);
 
 	hw_set_mem_tag_range((void *)addr, size, tag, init);
-- 
2.18.0

Re: [PATCH v4 2/2] kasan: Add memzero int for unaligned size at DEBUG

[Marco Elver]

On Fri, Jul 02, 2021 at 04:54PM +0800, yee.lee@mediatek.com wrote:
> From: Yee Lee <yee.lee@mediatek.com>
> 
> Issue: when SLUB debug is on, hwtag kasan_unpoison() would overwrite
> the redzone of object with unaligned size.
> 
> An additional memzero_explicit() path is added to replacing init by
> hwtag instruction for those unaligned size at SLUB debug mode.
> 
> The penalty is acceptable since they are only enabled in debug mode,
> not production builds. A block of comment is added for explanation.
> 
> ---
>  v4:
>  - Add "slab.h" header
>  - Use slub_debug_enabled_unlikely() to replace IS_ENABLED
>  - Refine the comment block

^^ this changelog ...
 
> ---

^^  this '---' is wrong unfortunately.

> Signed-off-by: Yee Lee <yee.lee@mediatek.com>
> Suggested-by: Marco Elver <elver@google.com>
> Suggested-by: Andrey Konovalov <andreyknvl@gmail.com>
> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>


... should come after the tags.


git am removes anything between the first '---' and the actual patch
from the commit message.

The typical convention is to place the changelog after a '---' _after_
the tags, so that it is removed from the final commit message.

I think the code looks fine now, so please go ahead and send v5.

Thanks,
-- Marco