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Subject[PATCH 3/4] dm: convert dm_dev_internal.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 dm_dev_internal.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/dm-table.c | 6 +++---
drivers/md/dm.h | 3 ++-
2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index ef7b8f2..fc7d240 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -451,15 +451,15 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
return r;
}

- atomic_set(&dd->count, 0);
+ refcount_set(&dd->count, 1);
list_add(&dd->list, &t->devices);

} else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
r = upgrade_mode(dd, mode, t->md);
if (r)
return r;
+ refcount_inc(&dd->count);
}
- atomic_inc(&dd->count);

*result = dd->dm_dev;
return 0;
@@ -515,7 +515,7 @@ void dm_put_device(struct dm_target *ti, struct dm_dev *d)
dm_device_name(ti->table->md), d->name);
return;
}
- if (atomic_dec_and_test(&dd->count)) {
+ if (refcount_dec_and_test(&dd->count)) {
dm_put_table_device(ti->table->md, d);
list_del(&dd->list);
kfree(dd);
diff --git a/drivers/md/dm.h b/drivers/md/dm.h
index 38c84c0..36399bb8 100644
--- a/drivers/md/dm.h
+++ b/drivers/md/dm.h
@@ -19,6 +19,7 @@
#include <linux/hdreg.h>
#include <linux/completion.h>
#include <linux/kobject.h>
+#include <linux/refcount.h>

#include "dm-stats.h"

@@ -38,7 +39,7 @@
*/
struct dm_dev_internal {
struct list_head list;
- atomic_t count;
+ refcount_t count;
struct dm_dev *dm_dev;
};

--
2.7.4
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