hfs_fill_super returns success even if no root inode (CVE-2006-6056)

http://kernelfun.blogspot.com/2006/11/mokb-14-11-2006-linux-26x-selinux.html

mount that image...
fs: filesystem was not cleanly unmounted, running fsck.hfs is recommended.  mounting read-only.
hfs: get root inode failed.
BUG: unable to handle kernel NULL pointer dereference at virtual address 00000018
 printing eip
...
EIP is at superblock_doinit+0x21/0x767
...
 [] selinux_sb_kern_mount+0xc/0x4b
 [] vfs_kern_mount+0x99/0xf6
 [] do_kern_mount+0x2d/0x3e
 [] do_mount+0x5fa/0x66d
 [] sys_mount+0x77/0xae
 [] syscall_call+0x7/0xb
DWARF2 unwinder stuck at syscall_call+0x7/0xb

hfs_fill_super() returns success even if
  root_inode = hfs_iget(sb, &fd.search_key->cat, &rec);
or
  sb->s_root = d_alloc_root(root_inode);

fails.  This superblock finds its way to superblock_doinit() which does:

        struct dentry *root = sb->s_root;
        struct inode *inode = root->d_inode;

and boom.  Need to make sure the error cases return an error, I think.

[akpm@osdl.org: return -ENOMEM on oom]
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

fs/hfs/super.c

@@ -390,11 +390,13 @@ static int hfs_fill_super(struct super_block *sb, void *data, int silent)
 		hfs_find_exit(&fd);
 		goto bail_no_root;
 	}
+	res = -EINVAL;
 	root_inode = hfs_iget(sb, &fd.search_key->cat, &rec);
 	hfs_find_exit(&fd);
 	if (!root_inode)
 		goto bail_no_root;
 
+	res = -ENOMEM;
 	sb->s_root = d_alloc_root(root_inode);
 	if (!sb->s_root)
 		goto bail_iput;