On 6/8/21 8:39 PM, Kees Cook wrote:
The redzone area for SLUB exists between s->object_size and s->inuse (which is at least the word-aligned object_size). If a cache were created with an object_size smaller than sizeof(void *), the in-object stored freelist pointer would overwrite the redzone (e.g. with boot param "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____): f6 f4 a5 40 1d e8 ...@.. Redzone (____ptrval____): 1a aa .. Padding (____ptrval____): 00 00 00 00 00 00 00 00 ........
Store the freelist pointer out of line when object_size is smaller than sizeof(void *) and redzoning is enabled.
Additionally remove the "smaller than sizeof(void *)" check under CONFIG_DEBUG_VM in kmem_cache_sanity_check() as it is now redundant: SLAB and SLOB both handle small sizes.
(Note that no caches within this size range are known to exist in the kernel currently.)
Fixes: 81819f0fc828 ("SLUB core") Cc: stable@vger.kernel.org Signed-off-by: Kees Cook keescook@chromium.org
Acked-by: Vlastimil Babka vbabka@suse.cz
mm/slab_common.c | 3 +-- mm/slub.c | 8 +++++--- 2 files changed, 6 insertions(+), 5 deletions(-)
diff --git a/mm/slab_common.c b/mm/slab_common.c index a4a571428c51..7cab77655f11 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -97,8 +97,7 @@ EXPORT_SYMBOL(kmem_cache_size); #ifdef CONFIG_DEBUG_VM static int kmem_cache_sanity_check(const char *name, unsigned int size) {
- if (!name || in_interrupt() || size < sizeof(void *) ||
size > KMALLOC_MAX_SIZE) {
- if (!name || in_interrupt() || size > KMALLOC_MAX_SIZE) { pr_err("kmem_cache_create(%s) integrity check failed\n", name); return -EINVAL; }
diff --git a/mm/slub.c b/mm/slub.c index f91d9fe7d0d8..f58cfd456548 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -3734,15 +3734,17 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order) */ s->inuse = size;
- if (((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
s->ctor)) {
- if ((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
((flags & SLAB_RED_ZONE) && s->object_size < sizeof(void *)) ||s->ctor) {
- Relocate free pointer after the object if it is not
- permitted to overwrite the first word of the object on
- kmem_cache_free.
- This is the case if we do RCU, have a constructor or
* destructor or are poisoning the objects.
* destructor, are poisoning the objects, or are* redzoning an object smaller than sizeof(void *).
- The assumption that s->offset >= s->inuse means free
- pointer is outside of the object is used in the