dm: convert table_device.count from atomic_t to refcount_t

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 table_device.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>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 4be85324f44dc26177c17bf9ab6acf451dca3fcf..be12f3f12e9ddd81e1d30456a37f29a4b5121718 100644 (file)
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -24,6 +24,7 @@
 #include <linux/delay.h>
 #include <linux/wait.h>
 #include <linux/pr.h>
+#include <linux/refcount.h>
 
 #define DM_MSG_PREFIX "core"
 
@@ -98,7 +99,7 @@ struct dm_md_mempools {
 
 struct table_device {
        struct list_head list;
-       atomic_t count;
+       refcount_t count;
        struct dm_dev dm_dev;
 };
 
@@ -685,10 +686,11 @@ int dm_get_table_device(struct mapped_device *md, dev_t dev, fmode_t mode,
 
                format_dev_t(td->dm_dev.name, dev);
 
-               atomic_set(&td->count, 0);
+               refcount_set(&td->count, 1);
                list_add(&td->list, &md->table_devices);
+       } else {
+               refcount_inc(&td->count);
        }
-       atomic_inc(&td->count);
        mutex_unlock(&md->table_devices_lock);
 
        *result = &td->dm_dev;
@@ -701,7 +703,7 @@ void dm_put_table_device(struct mapped_device *md, struct dm_dev *d)
        struct table_device *td = container_of(d, struct table_device, dm_dev);
 
        mutex_lock(&md->table_devices_lock);
-       if (atomic_dec_and_test(&td->count)) {
+       if (refcount_dec_and_test(&td->count)) {
                close_table_device(td, md);
                list_del(&td->list);
                kfree(td);
@@ -718,7 +720,7 @@ static void free_table_devices(struct list_head *devices)
                struct table_device *td = list_entry(tmp, struct table_device, list);
 
                DMWARN("dm_destroy: %s still exists with %d references",
-                      td->dm_dev.name, atomic_read(&td->count));
+                      td->dm_dev.name, refcount_read(&td->count));
                kfree(td);
        }
 }