fs, nfs: convert nfs_cache_defer_req.count from atomic_t to refcount_t

ereshetova · amschuma-ntap · commit 0896cade1224 · 2017-11-17T13:48:00.000-05:00
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 nfs_cache_defer_req.count is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook &lt;keescook@chromium.org&gt;
Reviewed-by: David Windsor &lt;dwindsor@gmail.com&gt;
Reviewed-by: Hans Liljestrand &lt;ishkamiel@gmail.com&gt;
Signed-off-by: Elena Reshetova &lt;elena.reshetova@intel.com&gt;
Signed-off-by: Anna Schumaker &lt;Anna.Schumaker@Netapp.com&gt;
diff --git a/fs/nfs/cache_lib.c b/fs/nfs/cache_lib.c
@@ -66,7 +66,7 @@ int nfs_cache_upcall(struct cache_detail *cd, char *entry_name)
  */
 void nfs_cache_defer_req_put(struct nfs_cache_defer_req *dreq)
 {
-	if (atomic_dec_and_test(&dreq->count))
+	if (refcount_dec_and_test(&dreq->count))
 		kfree(dreq);
 }
 
@@ -86,7 +86,7 @@ static struct cache_deferred_req *nfs_dns_cache_defer(struct cache_req *req)
 
 	dreq = container_of(req, struct nfs_cache_defer_req, req);
 	dreq->deferred_req.revisit = nfs_dns_cache_revisit;
-	atomic_inc(&dreq->count);
+	refcount_inc(&dreq->count);
 
 	return &dreq->deferred_req;
 }
@@ -98,7 +98,7 @@ struct nfs_cache_defer_req *nfs_cache_defer_req_alloc(void)
 	dreq = kzalloc(sizeof(*dreq), GFP_KERNEL);
 	if (dreq) {
 		init_completion(&dreq->completion);
-		atomic_set(&dreq->count, 1);
+		refcount_set(&dreq->count, 1);
 		dreq->req.defer = nfs_dns_cache_defer;
 	}
 	return dreq;
diff --git a/fs/nfs/cache_lib.h b/fs/nfs/cache_lib.h
@@ -15,7 +15,7 @@ struct nfs_cache_defer_req {
 	struct cache_req req;
 	struct cache_deferred_req deferred_req;
 	struct completion completion;
-	atomic_t count;
+	refcount_t count;
 };
 
 extern int nfs_cache_upcall(struct cache_detail *cd, char *entry_name);

Original file line number	Diff line number	Diff line change
`@@ -66,7 +66,7 @@ int nfs_cache_upcall(struct cache_detail cd, char entry_name)`
`66`	`66`	`*/`
`67`	`67`	`void nfs_cache_defer_req_put(struct nfs_cache_defer_req *dreq)`
`68`	`68`	`{`
`69`		`- if (atomic_dec_and_test(&dreq->count))`
	`69`	`+ if (refcount_dec_and_test(&dreq->count))`
`70`	`70`	`kfree(dreq);`
`71`	`71`	`}`
`72`	`72`
`@@ -86,7 +86,7 @@ static struct cache_deferred_req nfs_dns_cache_defer(struct cache_req req)`
`86`	`86`
`87`	`87`	`dreq = container_of(req, struct nfs_cache_defer_req, req);`
`88`	`88`	`dreq->deferred_req.revisit = nfs_dns_cache_revisit;`
`89`		`- atomic_inc(&dreq->count);`
	`89`	`+ refcount_inc(&dreq->count);`
`90`	`90`
`91`	`91`	`return &dreq->deferred_req;`
`92`	`92`	`}`
`@@ -98,7 +98,7 @@ struct nfs_cache_defer_req *nfs_cache_defer_req_alloc(void)`
`98`	`98`	`dreq = kzalloc(sizeof(*dreq), GFP_KERNEL);`
`99`	`99`	`if (dreq) {`
`100`	`100`	`init_completion(&dreq->completion);`
`101`		`- atomic_set(&dreq->count, 1);`
	`101`	`+ refcount_set(&dreq->count, 1);`
`102`	`102`	`dreq->req.defer = nfs_dns_cache_defer;`
`103`	`103`	`}`
`104`	`104`	`return dreq;`