On Tue, 8 Jun 2021 11:39:55 -0700 Kees Cook keescook@chromium.org wrote:
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.)
We already have https://lkml.kernel.org/r/6746FEEA-FD69-4792-8DDA-C78F5FE7DA02@psu.edu. Is this patch better?
--- 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.@@ -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
2.25.1