slub: avoid redzone when choosing freepointer location

Marco Elver reported system crashes when booting with "slub_debug=Z".

The freepointer location (s->offset) was not taking into account that
the "inuse" size that includes the redzone area should not be used by
the freelist pointer.  Change the calculation to save the area of the
object that an inline freepointer may be written into.

Fixes: 3202fa6 ("slub: relocate freelist pointer to middle of object")
Reported-by: Marco Elver <elver@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Marco Elver <elver@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Link: http://lkml.kernel.org/r/202004151054.BD695840@keescook
Link: https://lore.kernel.org/linux-mm/20200415164726.GA234932@google.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -3533,6 +3533,7 @@ 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;
 
 	/*
@@ -3541,6 +3542,13 @@ 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
 	/*
@@ -3582,13 +3590,13 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
 		 */
 		s->offset = size;
 		size += sizeof(void *);
-	} else if (size > sizeof(void *)) {
+	} else if (freepointer_area > sizeof(void *)) {
 		/*
 		 * 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(size / 2, sizeof(void *));
+		s->offset = ALIGN(freepointer_area / 2, sizeof(void *));
 	}
 
 #ifdef CONFIG_SLUB_DEBUG