[PATCH 5/6] block: convert bsg_device.ref_count from atomic_t to refcount_t

From: Elena Reshetova
Date: Fri Oct 20 2017 - 04:16:40 EST


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 bsg_device.ref_count is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
---
block/bsg.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/bsg.c b/block/bsg.c
index ee1335c..6c98422 100644
--- a/block/bsg.c
+++ b/block/bsg.c
@@ -21,6 +21,7 @@
#include <linux/idr.h>
#include <linux/bsg.h>
#include <linux/slab.h>
+#include <linux/refcount.h>

#include <scsi/scsi.h>
#include <scsi/scsi_ioctl.h>
@@ -38,7 +39,7 @@ struct bsg_device {
struct list_head busy_list;
struct list_head done_list;
struct hlist_node dev_list;
- atomic_t ref_count;
+ refcount_t ref_count;
int queued_cmds;
int done_cmds;
wait_queue_head_t wq_done;
@@ -710,7 +711,7 @@ static int bsg_put_device(struct bsg_device *bd)

mutex_lock(&bsg_mutex);

- do_free = atomic_dec_and_test(&bd->ref_count);
+ do_free = refcount_dec_and_test(&bd->ref_count);
if (!do_free) {
mutex_unlock(&bsg_mutex);
goto out;
@@ -768,7 +769,7 @@ static struct bsg_device *bsg_add_device(struct inode *inode,

bsg_set_block(bd, file);

- atomic_set(&bd->ref_count, 1);
+ refcount_set(&bd->ref_count, 1);
mutex_lock(&bsg_mutex);
hlist_add_head(&bd->dev_list, bsg_dev_idx_hash(iminor(inode)));

@@ -788,7 +789,7 @@ static struct bsg_device *__bsg_get_device(int minor, struct request_queue *q)

hlist_for_each_entry(bd, bsg_dev_idx_hash(minor), dev_list) {
if (bd->queue == q) {
- atomic_inc(&bd->ref_count);
+ refcount_inc(&bd->ref_count);
goto found;
}
}
--
2.7.4