fs, nilfs: convert nilfs_root.count from atomic_t to refcount_t

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

Link: http:https://lkml.kernel.org/r/[email protected]
Signed-off-by: Elena Reshetova <[email protected]>
Signed-off-by: Ryusuke Konishi <[email protected]>
Suggested-by: Kees Cook <[email protected]>
Reviewed-by: David Windsor <[email protected]>
Reviewed-by: Hans Liljestrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/nilfs2/the_nilfs.c

@@ -737,7 +737,7 @@ struct nilfs_root *nilfs_lookup_root(struct the_nilfs *nilfs, __u64 cno)
  } else if (cno > root->cno) {
  n = n->rb_right;
  } else {
- atomic_inc(&root->count);
+ refcount_inc(&root->count);
  spin_unlock(&nilfs->ns_cptree_lock);
  return root;
  }
@@ -776,7 +776,7 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64 cno)
  } else if (cno > root->cno) {
  p = &(*p)->rb_right;
  } else {
- atomic_inc(&root->count);
+ refcount_inc(&root->count);
  spin_unlock(&nilfs->ns_cptree_lock);
  kfree(new);
  return root;
@@ -786,7 +786,7 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64 cno)
  new->cno = cno;
  new->ifile = NULL;
  new->nilfs = nilfs;
- atomic_set(&new->count, 1);
+ refcount_set(&new->count, 1);
  atomic64_set(&new->inodes_count, 0);
  atomic64_set(&new->blocks_count, 0);
 
@@ -806,7 +806,7 @@ nilfs_find_or_create_root(struct the_nilfs *nilfs, __u64 cno)
 
 void nilfs_put_root(struct nilfs_root *root)
 {
- if (atomic_dec_and_test(&root->count)) {
+ if (refcount_dec_and_test(&root->count)) {
  struct the_nilfs *nilfs = root->nilfs;
 
  nilfs_sysfs_delete_snapshot_group(root);

fs/nilfs2/the_nilfs.h

@@ -27,6 +27,7 @@
 #include <linux/blkdev.h>
 #include <linux/backing-dev.h>
 #include <linux/slab.h>
+#include <linux/refcount.h>
 
 struct nilfs_sc_info;
 struct nilfs_sysfs_dev_subgroups;
@@ -246,7 +247,7 @@ struct nilfs_root {
  __u64 cno;
  struct rb_node rb_node;
 
- atomic_t count;
+ refcount_t count;
  struct the_nilfs *nilfs;
  struct inode *ifile;
 
@@ -299,7 +300,7 @@ void nilfs_swap_super_block(struct the_nilfs *);
 
 static inline void nilfs_get_root(struct nilfs_root *root)
 {
- atomic_inc(&root->count);
+ refcount_inc(&root->count);
 }
 
 static inline int nilfs_valid_fs(struct the_nilfs *nilfs)