[PATCH v4 3/4] mm: kasan: Add free_meta size info in struct kasan_cache

[Feng Tang <feng.tang@intel.com>]

When kasan is enabled for slab/slub, it may save kasan' free_meta
data in the former part of slab object data area in slab object
free path, which works fine.

There is ongoing effort to extend slub's debug function which will
redzone the latter part of kmalloc object area, and when both of
the debug are enabled, there is possible conflict, especially when
the kmalloc object has small size, as caught by 0Day bot [1]

For better information for slab/slub, add free_meta's data size
info 'kasan_cache', so that its users can take right action to
avoid data conflict.

[1]. https://lore.kernel.org/lkml/YuYm3dWwpZwH58Hu@xsang-OptiPlex-9020/
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
---
 include/linux/kasan.h | 2 ++
 mm/kasan/common.c     | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index b092277bf48d..293bdaa0ba09 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -100,6 +100,8 @@ static inline bool kasan_has_integrated_init(void)
 struct kasan_cache {
 	int alloc_meta_offset;
 	int free_meta_offset;
+	/* size of free_meta data saved in object's data area */
+	int free_meta_size_in_object;
 	bool is_kmalloc;
 };
 
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 69f583855c8b..762ae7a7793e 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -201,6 +201,8 @@ void __kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
 			cache->kasan_info.free_meta_offset = KASAN_NO_FREE_META;
 			*size = ok_size;
 		}
+	} else {
+		cache->kasan_info.free_meta_size_in_object = sizeof(struct kasan_free_meta);
 	}
 
 	/* Calculate size with optimal redzone. */
-- 
2.34.1