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/20210608183955.280836-4-keescook@chromium.org
Link: https://lore.kernel.org/linux-mm/20200807160627.GA1420741@elver.google.com/
Link: https://lore.kernel.org/lkml/0f7dd7b2-7496-5e2d-9488-2ec9f8e90441@suse.cz/Fixes:
Link: https://lore.kernel.org/lkml/CANpmjNOwZ5VpKQn+SYWovTkFB4VsT-RPwyENBmaK0dLcpqStkA@mail.gmail.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Marco Elver <elver@google.com>
Reported-by: "Lin, Zhenpeng" <zplin@psu.edu>
Tested-by: Marco Elver <elver@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <guro@fb.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/slub.c b/mm/slub.c
index f58cfd4..fe30df4 100644 (file)
--- 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