mm/slub: actually fix freelist pointer vs redzoning

It turns out that SLUB redzoning ("slub_debug=Z") checks from
s->object_size rather than from s->inuse (which is normally bumped to
make room for the freelist pointer), so a cache created with an object
size less than 24 would have the freelist pointer written beyond
s->object_size, causing the redzone to be corrupted by the freelist
pointer.  This was very visible with "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____): 00 00 00 00 00 f6 f4 a5               ........
  Redzone  (____ptrval____): 40 1d e8 1a aa                        @....
  Padding  (____ptrval____): 00 00 00 00 00 00 00 00               ........

Adjust the offset to stay within s->object_size.

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

Link: 89b83f282d8b (slub: avoid redzone when choosing freepointer location)
Signed-off-by: Kees Cook <>
Reported-by: Marco Elver <>
Reported-by: "Lin, Zhenpeng" <>
Tested-by: Marco Elver <>
Acked-by: Vlastimil Babka <>
Cc: Christoph Lameter <>
Cc: David Rientjes <>
Cc: Joonsoo Kim <>
Cc: Pekka Enberg <>
Cc: Roman Gushchin <>
Cc: <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
1 file changed