Code:  static bool add_free_nid(struct f2fs_sb_info *sbi, nid_t nid, bool build)
 {
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
	struct free_nid *i;
 	struct nat_entry *ne;
	int err;
 
 	/* 0 nid should not be used */
 	if (unlikely(nid == 0))
 		return false;
 
	if (build) {
		/* do not add allocated nids */
		ne = __lookup_nat_cache(nm_i, nid);
		if (ne && (!get_nat_flag(ne, IS_CHECKPOINTED) ||
				nat_get_blkaddr(ne) != NULL_ADDR))
			return false;
	}
 	i = f2fs_kmem_cache_alloc(free_nid_slab, GFP_NOFS);
 	i->nid = nid;
 	i->state = NID_NEW;
 
	if (radix_tree_preload(GFP_NOFS)) {
		kmem_cache_free(free_nid_slab, i);
		return true;
	}
 
 	spin_lock(&nm_i->nid_list_lock);
 	err = __insert_nid_to_list(sbi, i, FREE_NID_LIST, true);
 	spin_unlock(&nm_i->nid_list_lock);
 	radix_tree_preload_end();
	if (err) {
 		kmem_cache_free(free_nid_slab, i);
		return true;
	}
	return true;
 }

Vulnerability Type: DoS 
CWE ID: CWE-362
Summary: The add_free_nid function in fs/f2fs/node.c in the Linux kernel before 4.12 does not properly track an allocated nid, which allows local users to cause a denial of service (race condition) or possibly have unspecified other impact via concurrent threads.
Commit Message: f2fs: fix race condition in between free nid allocator/initializer

In below concurrent case, allocated nid can be loaded into free nid cache
and be allocated again.

Thread A				Thread B
- f2fs_create
 - f2fs_new_inode
  - alloc_nid
   - __insert_nid_to_list(ALLOC_NID_LIST)
					- f2fs_balance_fs_bg
					 - build_free_nids
					  - __build_free_nids
					   - scan_nat_page
					    - add_free_nid
					     - __lookup_nat_cache
 - f2fs_add_link
  - init_inode_metadata
   - new_inode_page
    - new_node_page
     - set_node_addr
 - alloc_nid_done
  - __remove_nid_from_list(ALLOC_NID_LIST)
					     - __insert_nid_to_list(FREE_NID_LIST)

This patch makes nat cache lookup and free nid list operation being atomical
to avoid this race condition.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>