[PATCH v3 2/3] mm/slub: Fix redzoning for small allocations

[Kees Cook <keescook@chromium.org>]

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
-- 
2.25.1