kasan: add memzero init for unaligned size at DEBUG

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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yee Lee <[email protected]>
Suggested-by: Andrey Konovalov <[email protected]>
Suggested-by: Marco Elver <[email protected]>
Reviewed-by: Marco Elver <[email protected]>
Reviewed-by: Andrey Konovalov <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Nicholas Tang <[email protected]>
Cc: Kuan-Ying Lee <[email protected]>
Cc: Chinwen Chang <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 12 insertions, 0 deletions

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 98e3059bfea4..d739cdd1621a 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() &&
+	    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);