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

[yee.lee at mediatek.com]

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