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Subject[PATCH 3/6] block: convert blkcg_gq.refcnt from atomic_t to refcount_t
Date
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 blkcg_gq.refcnt is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
---
block/blk-cgroup.c | 2 +-
include/linux/blk-cgroup.h | 11 ++++++-----
2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index d3f56ba..1e7cedc 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -107,7 +107,7 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
blkg->q = q;
INIT_LIST_HEAD(&blkg->q_node);
blkg->blkcg = blkcg;
- atomic_set(&blkg->refcnt, 1);
+ refcount_set(&blkg->refcnt, 1);

/* root blkg uses @q->root_rl, init rl only for !root blkgs */
if (blkcg != &blkcg_root) {
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 9d92153..c95d29d 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -19,6 +19,7 @@
#include <linux/radix-tree.h>
#include <linux/blkdev.h>
#include <linux/atomic.h>
+#include <linux/refcount.h>

/* percpu_counter batch for blkg_[rw]stats, per-cpu drift doesn't matter */
#define BLKG_STAT_CPU_BATCH (INT_MAX / 2)
@@ -122,7 +123,7 @@ struct blkcg_gq {
struct request_list rl;

/* reference count */
- atomic_t refcnt;
+ refcount_t refcnt;

/* is this blkg online? protected by both blkcg and q locks */
bool online;
@@ -354,8 +355,8 @@ static inline int blkg_path(struct blkcg_gq *blkg, char *buf, int buflen)
*/
static inline void blkg_get(struct blkcg_gq *blkg)
{
- WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
- atomic_inc(&blkg->refcnt);
+ WARN_ON_ONCE(refcount_read(&blkg->refcnt) == 0);
+ refcount_inc(&blkg->refcnt);
}

void __blkg_release_rcu(struct rcu_head *rcu);
@@ -366,8 +367,8 @@ void __blkg_release_rcu(struct rcu_head *rcu);
*/
static inline void blkg_put(struct blkcg_gq *blkg)
{
- WARN_ON_ONCE(atomic_read(&blkg->refcnt) <= 0);
- if (atomic_dec_and_test(&blkg->refcnt))
+ WARN_ON_ONCE(refcount_read(&blkg->refcnt) == 0);
+ if (refcount_dec_and_test(&blkg->refcnt))
call_rcu(&blkg->rcu_head, __blkg_release_rcu);
}

--
2.7.4
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