fuse: convert fuse_file.count from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -58,7 +58,7 @@ struct fuse_file *fuse_file_alloc(struct fuse_conn *fc)
 	}
 
 	INIT_LIST_HEAD(&ff->write_entry);
-	atomic_set(&ff->count, 1);
+	refcount_set(&ff->count, 1);
 	RB_CLEAR_NODE(&ff->polled_node);
 	init_waitqueue_head(&ff->poll_wait);
 
@@ -77,7 +77,7 @@ void fuse_file_free(struct fuse_file *ff)
 
 static struct fuse_file *fuse_file_get(struct fuse_file *ff)
 {
-	atomic_inc(&ff->count);
+	refcount_inc(&ff->count);
 	return ff;
 }
 
@@ -88,7 +88,7 @@ static void fuse_release_end(struct fuse_conn *fc, struct fuse_req *req)
 
 static void fuse_file_put(struct fuse_file *ff, bool sync)
 {
-	if (atomic_dec_and_test(&ff->count)) {
+	if (refcount_dec_and_test(&ff->count)) {
 		struct fuse_req *req = ff->reserved_req;
 
 		if (ff->fc->no_open) {
@@ -293,7 +293,7 @@ static int fuse_release(struct inode *inode, struct file *file)
 
 void fuse_sync_release(struct fuse_file *ff, int flags)
 {
-	WARN_ON(atomic_read(&ff->count) != 1);
+	WARN_ON(refcount_read(&ff->count) > 1);
 	fuse_prepare_release(ff, flags, FUSE_RELEASE);
 	/*
 	 * iput(NULL) is a no-op and since the refcount is 1 and everything's

fs/fuse/fuse_i.h

@@ -24,6 +24,7 @@
 #include <linux/workqueue.h>
 #include <linux/kref.h>
 #include <linux/xattr.h>
+#include <linux/refcount.h>
 
 /** Max number of pages that can be used in a single read request */
 #define FUSE_MAX_PAGES_PER_REQ 32
@@ -137,7 +138,7 @@ struct fuse_file {
 	u64 nodeid;
 
 	/** Refcount */
-	atomic_t count;
+	refcount_t count;
 
 	/** FOPEN_* flags returned by open */
 	u32 open_flags;