[PATCH] slub: choose the right freelist pointer location when creating small caches

["Lin, Zhenpeng" <zplin@psu.edu>]

When enabling CONFIG_SLUB_DEBUG and booting with "slub_debug=Z", the
kernel crashes at creating caches if the object size is smaller
than 2*sizeof(void*). The problem is due to the wrong calculation
of freepointer_area. The freelist pointer can be stored in the
middle of object only if the object size is not smaller than
2*sizeof(void*). Otherwise, the freelist pointer will be corrupted by
SLUB_RED_ZONE.

Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
Fixes: 89b83f282d8b ("slub: avoid redzone when choosing freepointer location")
Signed-off-by: Zhenpeng Lin <zplin@psu.edu>
---
mm/slub.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 3f96e099817a..cb23233ee683 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3704,7 +3704,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
	 * can't use that portion for writing the freepointer, so
	 * s->offset must be limited within this for the general case.
	 */
-	freepointer_area = size;
+	freepointer_area = s->object_size;

#ifdef CONFIG_SLUB_DEBUG
	/*
@@ -3751,7 +3751,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
		 */
		s->offset = size;
		size += sizeof(void *);
-	} else if (freepointer_area > sizeof(void *)) {
+	} else if (freepointer_area >= 2 * sizeof(void *)) {
		/*
		 * Store freelist pointer near middle of object to keep
		 * it away from the edges of the object to avoid small
--
2.17.1