Re: [PATCH v8 2/3] mm, kasan: align free_meta_offset on sizeof(void*)

On 08/01/2016 05:56 PM, Alexander Potapenko wrote:

On Mon, Aug 1, 2016 at 4:55 PM, Andrey Ryabinin [off-list ref] wrote:

quoted

On 07/28/2016 06:31 PM, Alexander Potapenko wrote:

quoted

When free_meta_offset is not zero, it is usually aligned on 4 bytes,
because the size of preceding kasan_alloc_meta is aligned on 4 bytes.
As a result, accesses to kasan_free_meta fields may be misaligned.

Signed-off-by: Alexander Potapenko <glider@google.com>
---
 mm/kasan/kasan.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 6845f92..0379551 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c

@@ -390,7 +390,8 @@ void kasan_cache_create(struct kmem_cache *cache, size_t *size,
      /* Add free meta. */
      if (cache->flags & SLAB_DESTROY_BY_RCU || cache->ctor ||
          cache->object_size < sizeof(struct kasan_free_meta)) {
-             cache->kasan_info.free_meta_offset = *size;
+             cache->kasan_info.free_meta_offset =
+                     ALIGN(*size, sizeof(void *));

This cannot work.

Well, it does, at least on my tests.

JFYI. You aligned only meta offset, but didn't change the size, so after the '*size += sizeof(struct kasan_free_meta);'
*size may point into the middle of free_meta struct.
Plus, alignment wasn't taken into account in kasan_metadata_size().

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