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Subject[PATCH] fs, nilfs: convert nilfs_root.count from atomic_t to refcount_t
Date
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.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
---
fs/nilfs2/the_nilfs.c | 8 ++++----
fs/nilfs2/the_nilfs.h | 5 +++--
2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/fs/nilfs2/the_nilfs.c b/fs/nilfs2/the_nilfs.c
index 2dd75bf..afebb50 100644
--- a/fs/nilfs2/the_nilfs.c
+++ b/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);
diff --git a/fs/nilfs2/the_nilfs.h b/fs/nilfs2/the_nilfs.h
index b305c6f..883d732 100644
--- a/fs/nilfs2/the_nilfs.h
+++ b/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)
--
2.7.4
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