f2fs: fix use-after-free issue when accessing sbi->stat_info

iput() on sbi->node_inode can update sbi->stat_info
in the below context, if the f2fs_write_checkpoint()
has failed with error.

f2fs_balance_fs_bg+0x1ac/0x1ec
f2fs_write_node_pages+0x4c/0x260
do_writepages+0x80/0xbc
__writeback_single_inode+0xdc/0x4ac
writeback_single_inode+0x9c/0x144
write_inode_now+0xc4/0xec
iput+0x194/0x22c
f2fs_put_super+0x11c/0x1e8
generic_shutdown_super+0x70/0xf4
kill_block_super+0x2c/0x5c
kill_f2fs_super+0x44/0x50
deactivate_locked_super+0x60/0x8c
deactivate_super+0x68/0x74
cleanup_mnt+0x40/0x78

Fix this by moving f2fs_destroy_stats() further below iput() in
both f2fs_put_super() and f2fs_fill_super() paths.

Signed-off-by: Sahitya Tummala <[e-mail]>
Reviewed-by: Chao Yu <[e-mail]>
Signed-off-by: Jaegeuk Kim <[e-mail]>

fs/f2fs/super.c

···
 		f2fs_write_checkpoint(sbi, &cpc);
 	}
 
-	/* f2fs_write_checkpoint can update stat informaion */
-	f2fs_destroy_stats(sbi);
-
 	/*
 	 * normally superblock is clean, so we need to release this.
 	 * In addition, EIO will skip do checkpoint, we need this as well.
···
 
 	iput(sbi->node_inode);
 	iput(sbi->meta_inode);
+
+	/*
+	 * iput() can update stat information, if f2fs_write_checkpoint()
+	 * above failed with error.
+	 */
+	f2fs_destroy_stats(sbi);
 
 	/* destroy f2fs internal modules */
 	f2fs_destroy_node_manager(sbi);
···
 
 	f2fs_build_gc_manager(sbi);
 
+	err = f2fs_build_stats(sbi);
+	if (err)
+		goto free_nm;
+
 	/* get an inode for node space */
 	sbi->node_inode = f2fs_iget(sb, F2FS_NODE_INO(sbi));
 	if (IS_ERR(sbi->node_inode)) {
 		f2fs_msg(sb, KERN_ERR, "Failed to read node inode");
 		err = PTR_ERR(sbi->node_inode);
-		goto free_nm;
+		goto free_stats;
 	}
-
-	err = f2fs_build_stats(sbi);
-	if (err)
-		goto free_node_inode;
 
 	/* read root inode and dentry */
 	root = f2fs_iget(sb, F2FS_ROOT_INO(sbi));
 	if (IS_ERR(root)) {
 		f2fs_msg(sb, KERN_ERR, "Failed to read root inode");
 		err = PTR_ERR(root);
-		goto free_stats;
+		goto free_node_inode;
 	}
 	if (!S_ISDIR(root->i_mode) || !root->i_blocks ||
 			!root->i_size || !root->i_nlink) {
 		iput(root);
 		err = -EINVAL;
-		goto free_stats;
+		goto free_node_inode;
 	}
 
 	sb->s_root = d_make_root(root); /* allocate root dentry */
···
 free_root_inode:
 	dput(sb->s_root);
 	sb->s_root = NULL;
-free_stats:
-	f2fs_destroy_stats(sbi);
 free_node_inode:
 	f2fs_release_ino_entry(sbi, true);
 	truncate_inode_pages_final(NODE_MAPPING(sbi));
 	iput(sbi->node_inode);
+free_stats:
+	f2fs_destroy_stats(sbi);
 free_nm:
 	f2fs_destroy_node_manager(sbi);
 free_sm: