[patch 05/18] mm/slub: actually fix freelist pointer vs redzoning

From: Kees Cook <redacted>
Subject: mm/slub: actually fix freelist pointer vs redzoning

It turns out that SLUB redzoning ("slub_debug=Z") checks from
s->object_size rather than from s->inuse (which is normally bumped to make
room for the freelist pointer), so a cache created with an object size
less than 24 would have the freelist pointer written beyond
s->object_size, causing the redzone to be corrupted by the freelist
pointer.  This was very visible with "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____): 00 00 00 00 00 f6 f4 a5               ........
Redzone  (____ptrval____): 40 1d e8 1a aa                        @....
Padding  (____ptrval____): 00 00 00 00 00 00 00 00               ........

Adjust the offset to stay within s->object_size.

(Note that no caches of in this size range are known to exist in the
kernel currently.)

Link: https://lkml.kernel.org/r/20210608183955.280836-4-keescook@chromium.org
Link: https://lore.kernel.org/linux-mm/20200807160627.GA1420741@elver.google.com/ (local)
Link: https://lore.kernel.org/lkml/0f7dd7b2-7496-5e2d-9488-2ec9f8e90441@suse.cz/Fixes (local): 89b83f282d8b (slub: avoid redzone when choosing freepointer location)
Link: https://lore.kernel.org/lkml/CANpmjNOwZ5VpKQn+SYWovTkFB4VsT-RPwyENBmaK0dLcpqStkA@mail.gmail.com (local)
Signed-off-by: Kees Cook <redacted>
Reported-by: Marco Elver <elver@google.com>
Reported-by: "Lin, Zhenpeng" <redacted>
Tested-by: Marco Elver <elver@google.com>
Acked-by: Vlastimil Babka <redacted>
Cc: Christoph Lameter <redacted>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <redacted>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <redacted>
Cc: <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/slub.c |   14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

--- a/mm/slub.c~mm-slub-actually-fix-freelist-pointer-vs-redzoning
+++ a/mm/slub.c

@@ -3689,7 +3689,6 @@ static int calculate_sizes(struct kmem_c
 {
 	slab_flags_t flags = s->flags;
 	unsigned int size = s->object_size;
-	unsigned int freepointer_area;
 	unsigned int order;
 
 	/*

@@ -3698,13 +3697,6 @@ static int calculate_sizes(struct kmem_c
 	 * 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
 	/*

@@ -3730,7 +3722,7 @@ static int calculate_sizes(struct kmem_c
 
 	/*
 	 * With that we have determined the number of bytes in actual use
-	 * by the object. This is the potential offset to the free pointer.
+	 * by the object and redzoning.
 	 */
 	s->inuse = size;
 

@@ -3753,13 +3745,13 @@ static int calculate_sizes(struct kmem_c
 		 */
 		s->offset = size;
 		size += sizeof(void *);
-	} else if (freepointer_area > sizeof(void *)) {
+	} else {
 		/*
 		 * 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(freepointer_area / 2, sizeof(void *));
+		s->offset = ALIGN_DOWN(s->object_size / 2, sizeof(void *));
 	}
 
 #ifdef CONFIG_SLUB_DEBUG

_