[PATCH v3 1/1] kasan: Add memzero init for unaligned size under SLUB debug

[<yee.lee@mediatek.com>]

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.

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 | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 8f450bc28045..6f698f13dbe6 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -387,6 +387,16 @@ 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 (IS_ENABLED(CONFIG_SLUB_DEBUG) && 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