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Subject[PATCH 1/4] bcache: convert cached_dev.count 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 cached_dev.count 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>
---
drivers/md/bcache/bcache.h | 7 ++++---
drivers/md/bcache/super.c | 6 +++---
drivers/md/bcache/writeback.h | 2 +-
3 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 2ed9bd2..8ed8ad6 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -184,6 +184,7 @@
#include <linux/mutex.h>
#include <linux/rbtree.h>
#include <linux/rwsem.h>
+#include <linux/refcount.h>
#include <linux/types.h>
#include <linux/workqueue.h>

@@ -299,7 +300,7 @@ struct cached_dev {
struct semaphore sb_write_mutex;

/* Refcount on the cache set. Always nonzero when we're caching. */
- atomic_t count;
+ refcount_t count;
struct work_struct detach;

/*
@@ -806,13 +807,13 @@ do { \

static inline void cached_dev_put(struct cached_dev *dc)
{
- if (atomic_dec_and_test(&dc->count))
+ if (refcount_dec_and_test(&dc->count))
schedule_work(&dc->detach);
}

static inline bool cached_dev_get(struct cached_dev *dc)
{
- if (!atomic_inc_not_zero(&dc->count))
+ if (!refcount_inc_not_zero(&dc->count))
return false;

/* Paired with the mb in cached_dev_attach */
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index fc0a31b..676e6f4 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -889,7 +889,7 @@ static void cached_dev_detach_finish(struct work_struct *w)
closure_init_stack(&cl);

BUG_ON(!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags));
- BUG_ON(atomic_read(&dc->count));
+ BUG_ON(refcount_read(&dc->count));

mutex_lock(&bch_register_lock);

@@ -1016,7 +1016,7 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
* dc->c must be set before dc->count != 0 - paired with the mb in
* cached_dev_get()
*/
- atomic_set(&dc->count, 1);
+ refcount_set(&dc->count, 1);

/* Block writeback thread, but spawn it */
down_write(&dc->writeback_lock);
@@ -1028,7 +1028,7 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
if (BDEV_STATE(&dc->sb) == BDEV_STATE_DIRTY) {
bch_sectors_dirty_init(&dc->disk);
atomic_set(&dc->has_dirty, 1);
- atomic_inc(&dc->count);
+ refcount_inc(&dc->count);
bch_writeback_queue(dc);
}

diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h
index e35421d..5937e0d 100644
--- a/drivers/md/bcache/writeback.h
+++ b/drivers/md/bcache/writeback.h
@@ -89,7 +89,7 @@ static inline void bch_writeback_add(struct cached_dev *dc)
{
if (!atomic_read(&dc->has_dirty) &&
!atomic_xchg(&dc->has_dirty, 1)) {
- atomic_inc(&dc->count);
+ refcount_inc(&dc->count);

if (BDEV_STATE(&dc->sb) != BDEV_STATE_DIRTY) {
SET_BDEV_STATE(&dc->sb, BDEV_STATE_DIRTY);
--
2.7.4
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