* [PATCH v3 2/3] mm/slub: Fix redzoning for small allocations
[not found] <20201015033712.1491731-1-keescook@chromium.org>
@ 2020-10-15 3:37 ` Kees Cook
2020-10-15 3:37 ` [PATCH v3 3/3] mm/slub: Actually fix freelist pointer vs redzoning Kees Cook
1 sibling, 0 replies; 2+ messages in thread
From: Kees Cook @ 2020-10-15 3:37 UTC (permalink / raw)
To: Andrew Morton
Cc: Kees Cook, stable, Christoph Lameter, Vlastimil Babka,
Waiman Long, Marco Elver, Pekka Enberg, David Rientjes,
Joonsoo Kim, Roman Gushchin, linux-kernel, linux-doc, linux-mm
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
^ permalink raw reply related [flat|nested] 2+ messages in thread
* [PATCH v3 3/3] mm/slub: Actually fix freelist pointer vs redzoning
[not found] <20201015033712.1491731-1-keescook@chromium.org>
2020-10-15 3:37 ` [PATCH v3 2/3] mm/slub: Fix redzoning for small allocations Kees Cook
@ 2020-10-15 3:37 ` Kees Cook
1 sibling, 0 replies; 2+ messages in thread
From: Kees Cook @ 2020-10-15 3:37 UTC (permalink / raw)
To: Andrew Morton
Cc: Kees Cook, Marco Elver, stable, Vlastimil Babka,
Christoph Lameter, Waiman Long, Pekka Enberg, David Rientjes,
Joonsoo Kim, Roman Gushchin, linux-kernel, linux-doc, linux-mm
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 in this size range are known to exist in the kernel
currently.)
Reported-by: Marco Elver <elver@google.com>
Link: https://lore.kernel.org/linux-mm/20200807160627.GA1420741@elver.google.com/
Fixes: 89b83f282d8b (slub: avoid redzone when choosing freepointer location)
Cc: stable@vger.kernel.org
Tested-by: Marco Elver <elver@google.com>
Link: https://lore.kernel.org/lkml/CANpmjNOwZ5VpKQn+SYWovTkFB4VsT-RPwyENBmaK0dLcpqStkA@mail.gmail.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/lkml/0f7dd7b2-7496-5e2d-9488-2ec9f8e90441@suse.cz/
---
mm/slub.c | 14 +++-----------
1 file changed, 3 insertions(+), 11 deletions(-)
diff --git a/mm/slub.c b/mm/slub.c
index 752fad36522c..6f115e56c5d0 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3637,7 +3637,6 @@ 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;
/*
@@ -3646,13 +3645,6 @@ 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
/*
@@ -3678,7 +3670,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
/*
* 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;
@@ -3701,13 +3693,13 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
*/
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
--
2.25.1
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2020-10-15 3:37 ` [PATCH v3 2/3] mm/slub: Fix redzoning for small allocations Kees Cook
2020-10-15 3:37 ` [PATCH v3 3/3] mm/slub: Actually fix freelist pointer vs redzoning Kees Cook
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