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dm: convert table_device.count from atomic_t to refcount_t

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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>
This commit is contained in:
Elena Reshetova 2017-10-20 10:37:39 +03:00 committed by Mike Snitzer
parent 2a0b4682e0
commit b0b4d7c675
1 changed files with 7 additions and 5 deletions
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12
drivers/md/dm.c
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@ -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);
}
}