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Commit 6bdd0796 authored by Elena Reshetova's avatar Elena Reshetova Committed by Mike Snitzer
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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: default avatarKees Cook <keescook@chromium.org>
Reviewed-by: default avatarDavid Windsor <dwindsor@gmail.com>
Reviewed-by: default avatarHans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: default avatarElena Reshetova <elena.reshetova@intel.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent b0b4d7c6

drivers/md/dm-cache-metadata.c

+5 −4
Original line number Diff line number Diff line
@@ -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);