[PATCH 4/4] dm: convert table_device.count from atomic_t to refcount_t
From: Elena Reshetova <elena.reshetova@intel.com>
Date: 2017-10-20 07:38:56
Also in:
dm-devel, linux-bcache, lkml
Subsystem:
device-mapper (lvm), the rest · Maintainers:
Alasdair Kergon, Mike Snitzer, Mikulas Patocka, Benjamin Marzinski, Linus Torvalds
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 <redacted> Reviewed-by: David Windsor <redacted> Reviewed-by: Hans Liljestrand <redacted> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com> --- drivers/md/dm.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 4be8532..be12f3f 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c@@ -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); } }
--
2.7.4