dm cache: convert dm_cache_metadata.ref_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 dm_cache_metadata.ref_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-cache-metadata.c b/drivers/md/dm-cache-metadata.c
index 4a4e9c75fc4cddca59c30df2f18ebaa6f0baaa61..0d7212410e2156addae298dfdc5e5a0ba4cbf690 100644 (file)
--- a/drivers/md/dm-cache-metadata.c
+++ b/drivers/md/dm-cache-metadata.c
@@ -13,6 +13,7 @@
 #include "persistent-data/dm-transaction-manager.h"
 
 #include <linux/device-mapper.h>
+#include <linux/refcount.h>
 
 /*----------------------------------------------------------------*/
 
@@ -100,7 +101,7 @@ struct cache_disk_superblock {
 } __packed;
 
 struct dm_cache_metadata {
-       atomic_t ref_count;
+       refcount_t ref_count;
        struct list_head list;
 
        unsigned version;
@@ -753,7 +754,7 @@ static struct dm_cache_metadata *metadata_open(struct block_device *bdev,
        }
 
        cmd->version = metadata_version;
-       atomic_set(&cmd->ref_count, 1);
+       refcount_set(&cmd->ref_count, 1);
        init_rwsem(&cmd->root_lock);
        cmd->bdev = bdev;
        cmd->data_block_size = data_block_size;
@@ -791,7 +792,7 @@ static struct dm_cache_metadata *lookup(struct block_device *bdev)
 
        list_for_each_entry(cmd, &table, list)
                if (cmd->bdev == bdev) {
-                       atomic_inc(&cmd->ref_count);
+                       refcount_inc(&cmd->ref_count);
                        return cmd;
                }
 
@@ -862,7 +863,7 @@ struct dm_cache_metadata *dm_cache_metadata_open(struct block_device *bdev,
 
 void dm_cache_metadata_close(struct dm_cache_metadata *cmd)
 {
-       if (atomic_dec_and_test(&cmd->ref_count)) {
+       if (refcount_dec_and_test(&cmd->ref_count)) {
                mutex_lock(&table_lock);
                list_del(&cmd->list);
                mutex_unlock(&table_lock);