atomic_t variables are currently used to implement reference
counters with the following properties:
- counter is initialized to 1 using atomic_set()
- a resource is freed upon counter reaching zero
- once counter reaches zero, its further
increments aren't allowed
- counter schema uses basic atomic operations
(set, inc, inc_not_zero, dec_and_test, etc.)
Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.
The variable xen_blkif.refcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.
Suggested-by: Kees Cook <keesc...@chromium.org>
Reviewed-by: David Windsor <dwind...@gmail.com>
Reviewed-by: Hans Liljestrand <ishkam...@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshet...@intel.com>
---
drivers/block/xen-blkback/common.h | 7 ++++---
drivers/block/xen-blkback/xenbus.c | 2 +-
2 files changed, 5 insertions(+), 4 deletions(-)
diff --git a/drivers/block/xen-blkback/common.h
b/drivers/block/xen-blkback/common.h
index ecb35fe..0c3320d 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -35,6 +35,7 @@
#include <linux/wait.h>
#include <linux/io.h>
#include <linux/rbtree.h>
+#include <linux/refcount.h>
#include <asm/setup.h>
#include <asm/pgalloc.h>
#include <asm/hypervisor.h>
@@ -319,7 +320,7 @@ struct xen_blkif {
struct xen_vbd vbd;
/* Back pointer to the backend_info. */
struct backend_info *be;
- atomic_t refcnt;
+ refcount_t refcnt;
/* for barrier (drain) requests */
struct completion drain_complete;
atomic_t drain;
@@ -372,10 +373,10 @@ struct pending_req {
(_v)->bdev->bd_part->nr_sects : \
get_capacity((_v)->bdev->bd_disk))
-#define xen_blkif_get(_b) (atomic_inc(&(_b)->refcnt))
+#define xen_blkif_get(_b) (refcount_inc(&(_b)->refcnt))
#define xen_blkif_put(_b) \
do { \
- if (atomic_dec_and_test(&(_b)->refcnt)) \
+ if (refcount_dec_and_test(&(_b)->refcnt)) \
schedule_work(&(_b)->free_work);\
} while (0)
diff --git a/drivers/block/xen-blkback/xenbus.c
b/drivers/block/xen-blkback/xenbus.c
index 21c1be1..5955b61 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -176,7 +176,7 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
return ERR_PTR(-ENOMEM);
blkif->domid = domid;
- atomic_set(&blkif->refcnt, 1);
+ refcount_set(&blkif->refcnt, 1);
init_completion(&blkif->drain_complete);
INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
--
2.7.4
