[PATCH 15/15] bdi: convert bdi_writeback_congested.refcnt from atomic_t to refcount_t

[Elena Reshetova]

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 bdi_writeback_congested.refcnt 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>
---
 include/linux/backing-dev-defs.h |  3 ++-
 include/linux/backing-dev.h      |  4 ++--
 mm/backing-dev.c                 | 14 ++++++++------
 3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index b7c7be6..429fe3b 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -4,6 +4,7 @@
 #include <linux/list.h>
 #include <linux/radix-tree.h>
 #include <linux/rbtree.h>
+#include <linux/refcount.h>
 #include <linux/spinlock.h>
 #include <linux/percpu_counter.h>
 #include <linux/percpu-refcount.h>
@@ -75,7 +76,7 @@ enum wb_reason {
  */
 struct bdi_writeback_congested {
 	unsigned long state;		/* WB_[a]sync_congested flags */
-	atomic_t refcnt;		/* nr of attached wb's and blkg */
+	refcount_t refcnt;		/* nr of attached wb's and blkg */
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 	struct backing_dev_info *__bdi;	/* the associated bdi, set to NULL
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index e6f5037..f3b38c4 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -401,13 +401,13 @@ static inline bool inode_cgwb_enabled(struct inode *inode)
 static inline struct bdi_writeback_congested *
 wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
 {
-	atomic_inc(&bdi->wb_congested->refcnt);
+	refcount_inc(&bdi->wb_congested->refcnt);
 	return bdi->wb_congested;
 }
 
 static inline void wb_congested_put(struct bdi_writeback_congested *congested)
 {
-	if (atomic_dec_and_test(&congested->refcnt))
+	if (refcount_dec_and_test(&congested->refcnt))
 		kfree(congested);
 }
 
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 74b52df..e92a20f 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -440,14 +440,17 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
 			node = &parent->rb_left;
 		else if (congested->blkcg_id > blkcg_id)
 			node = &parent->rb_right;
-		else
-			goto found;
+		else {
+			refcount_inc(&congested->refcnt);
+ 			goto found;
+		}
 	}
 
 	if (new_congested) {
 		/* !found and storage for new one already allocated, insert */
 		congested = new_congested;
 		new_congested = NULL;
+		refcount_set(&congested->refcnt, 1);
 		rb_link_node(&congested->rb_node, parent, node);
 		rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree);
 		goto found;
@@ -460,13 +463,12 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp)
 	if (!new_congested)
 		return NULL;
 
-	atomic_set(&new_congested->refcnt, 0);
+	refcount_set(&new_congested->refcnt, 0);
 	new_congested->__bdi = bdi;
 	new_congested->blkcg_id = blkcg_id;
 	goto retry;
 
 found:
-	atomic_inc(&congested->refcnt);
 	spin_unlock_irqrestore(&cgwb_lock, flags);
 	kfree(new_congested);
 	return congested;
@@ -483,7 +485,7 @@ void wb_congested_put(struct bdi_writeback_congested *congested)
 	unsigned long flags;
 
 	local_irq_save(flags);
-	if (!atomic_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
+	if (!refcount_dec_and_lock(&congested->refcnt, &cgwb_lock)) {
 		local_irq_restore(flags);
 		return;
 	}
@@ -793,7 +795,7 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
 	if (!bdi->wb_congested)
 		return -ENOMEM;
 
-	atomic_set(&bdi->wb_congested->refcnt, 1);
+	refcount_set(&bdi->wb_congested->refcnt, 1);
 
 	err = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
 	if (err) {
-- 
2.7.4