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Subject[PATCH 2/4] dm cache: convert dm_cache_metadata.ref_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_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>
---
drivers/md/dm-cache-metadata.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/md/dm-cache-metadata.c b/drivers/md/dm-cache-metadata.c
index 4a4e9c7..0d72124 100644
--- 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);
--
2.7.4
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