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Commit b0b4d7c6 authored by Elena Reshetova's avatar Elena Reshetova Committed by Mike Snitzer
Browse files

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: 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 2a0b4682

drivers/md/dm.c

+7 −5
Original line number Diff line number Diff line
@@ -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);

	}

}