mm/slub: fix redzoning for small allocations

The redzone area for SLUB exists between s->object_size and s->inuse
(which is at least the word-aligned object_size).  If a cache were
created with an object_size smaller than sizeof(void *), the in-object
stored freelist pointer would overwrite the redzone (e.g.  with boot
param "slub_debug=ZF"):

  BUG test (Tainted: G    B            ): Right Redzone overwritten

  INFO: 0xffff957ead1c05de-0xffff957ead1c05df @offset=1502. First byte 0x1a instead of 0xbb
  INFO: Slab 0xffffef3950b47000 objects=170 used=170 fp=0x0000000000000000 flags=0x8000000000000200
  INFO: Object 0xffff957ead1c05d8 @offset=1496 fp=0xffff957ead1c0620

  Redzone  (____ptrval____): bb bb bb bb bb bb bb bb    ........
  Object   (____ptrval____): f6 f4 a5 40 1d e8          ...@..
  Redzone  (____ptrval____): 1a aa                      ..
  Padding  (____ptrval____): 00 00 00 00 00 00 00 00    ........

Store the freelist pointer out of line when object_size is smaller than
sizeof(void *) and redzoning is enabled.

Additionally remove the "smaller than sizeof(void *)" check under
CONFIG_DEBUG_VM in kmem_cache_sanity_check() as it is now redundant:
SLAB and SLOB both handle small sizes.

(Note that no caches within this size range are known to exist in the
kernel currently.)

Fixes: 81819f0fc828 ("SLUB core")
Signed-off-by: Kees Cook <>
Acked-by: Vlastimil Babka <>
Cc: Christoph Lameter <>
Cc: David Rientjes <>
Cc: Joonsoo Kim <>
Cc: "Lin, Zhenpeng" <>
Cc: Marco Elver <>
Cc: Pekka Enberg <>
Cc: Roman Gushchin <>
Cc: <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
2 files changed