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: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/linux-mm/[email protected]/
Link: https://lore.kernel.org/lkml/[email protected]/Fixes: 89b83f282d8b (slub: avoid redzone when choosing freepointer location)
Link: https://lore.kernel.org/lkml/CANpmjNOwZ5VpKQn+SYWovTkFB4VsT-RPwyENBmaK0dLcpqStkA@mail.gmail.com
Signed-off-by: Kees Cook <[email protected]>
Reported-by: Marco Elver <[email protected]>
Reported-by: "Lin, Zhenpeng" <[email protected]>
Tested-by: Marco Elver <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 3 insertions, 11 deletions

diff --git a/mm/slub.c b/mm/slub.c
index f58cfd456548..fe30df460fad 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3689,7 +3689,6 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
 {
 	slab_flags_t flags = s->flags;
 	unsigned int size = s->object_size;
-	unsigned int freepointer_area;
 	unsigned int order;
 
 	/*
@@ -3698,13 +3697,6 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
 	 * the possible location of the free pointer.
 	 */
 	size = ALIGN(size, sizeof(void *));
-	/*
-	 * This is the area of the object where a freepointer can be
-	 * safely written. If redzoning adds more to the inuse size, we
-	 * can't use that portion for writing the freepointer, so
-	 * s->offset must be limited within this for the general case.
-	 */
-	freepointer_area = size;
 
 #ifdef CONFIG_SLUB_DEBUG
 	/*
@@ -3730,7 +3722,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
 
 	/*
 	 * With that we have determined the number of bytes in actual use
-	 * by the object. This is the potential offset to the free pointer.
+	 * by the object and redzoning.
 	 */
 	s->inuse = size;
 
@@ -3753,13 +3745,13 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
 		 */
 		s->offset = size;
 		size += sizeof(void *);
-	} else if (freepointer_area > sizeof(void *)) {
+	} else {
 		/*
 		 * Store freelist pointer near middle of object to keep
 		 * it away from the edges of the object to avoid small
 		 * sized over/underflows from neighboring allocations.
 		 */
-		s->offset = ALIGN(freepointer_area / 2, sizeof(void *));
+		s->offset = ALIGN_DOWN(s->object_size / 2, sizeof(void *));
 	}
 
 #ifdef CONFIG_SLUB_DEBUG