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 in this size range are known to exist in the kernel currently.) Reported-by: Marco Elver <el...@google.com> Link: https://lore.kernel.org/linux-mm/20200807160627.ga1420...@elver.google.com/ Fixes: 89b83f282d8b (slub: avoid redzone when choosing freepointer location) Cc: sta...@vger.kernel.org Tested-by: Marco Elver <el...@google.com> Link: https://lore.kernel.org/lkml/canpmjnowz5vpkqn+sywovtkfb4vst-rpwyenbmak0dlcpqs...@mail.gmail.com Signed-off-by: Kees Cook <keesc...@chromium.org> Acked-by: Vlastimil Babka <vba...@suse.cz> Link: https://lore.kernel.org/lkml/0f7dd7b2-7496-5e2d-9488-2ec9f8e90...@suse.cz/ --- mm/slub.c | 14 +++----------- 1 file changed, 3 insertions(+), 11 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index 752fad36522c..6f115e56c5d0 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -3637,7 +3637,6 @@ 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; /* @@ -3646,13 +3645,6 @@ 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 /* @@ -3678,7 +3670,7 @@ 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; @@ -3701,13 +3693,13 @@ 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
