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. (This object_size is not actually considered valid, as tested by kmem_cache_sanity_check() under CONFIG_DEBUG_VM. This is being added for extra robustness, since it IS possible to build kernels where this is allowed -- why keep foot-guns around?)
(Note that no caches in 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 --- mm/slub.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c index f4f1d63f0ab9..752fad36522c 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -3682,15 +3682,17 @@ 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