From: Kees Cook keescook@chromium.org Subject: 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/Fi...: 89b83f282d8b (slub: avoid redzone when choosing freepointer location) Link: https://lore.kernel.org/lkml/CANpmjNOwZ5VpKQn+SYWovTkFB4VsT-RPwyENBmaK0dLcpq... 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 ---
mm/slub.c | 14 +++----------- 1 file changed, 3 insertions(+), 11 deletions(-)
--- a/mm/slub.c~mm-slub-actually-fix-freelist-pointer-vs-redzoning +++ a/mm/slub.c @@ -3689,7 +3689,6 @@ static int calculate_sizes(struct kmem_c { 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_c * 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_c
/* * 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_c */ 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 _