Re: [PATCH linux-next] mqueue: fix IPC namespace use-after-free

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Dec 19, 2017 at 03:32:25PM +0000, Al Viro wrote:
> +	m = mq_internal_mount();
> +	if (IS_ERR(m))
> +		return ERR_CAST(m);
> +	atomic_inc(&m->mnt_sb->s_active);
> +	down_write(&m->mnt_sb->s_umount);
> +	return dget(m->mnt_root);

Note: this is stripped down mount_subtree(m, ""), of course;
it might make sense to recognize that case and bypass the
create_mnt_ns/vfs_path_lookup/put_mnt_ns business in
mount_subtree() when the relative pathname is empty, replacing
it with path.mnt = mntget(mnt); path.dentry = dget(mnt->mnt_root);
in such case.  That'd allow to simply call mount_subtree() here.
It would work as-is, but it's ridiculously heavy for such use
right now.

>  static int __init init_mqueue_fs(void)
>  {
> +	struct vfsmount *m;
>  	int error;
>  
>  	mqueue_inode_cachep = kmem_cache_create("mqueue_inode_cache",
> @@ -1577,6 +1606,10 @@ static int __init init_mqueue_fs(void)
>  	if (error)
>  		goto out_filesystem;
>  
> +	m = kern_mount_data(&mqueue_fs_type, &init_ipc_ns);
> +	if (IS_ERR(m))
> +		goto out_filesystem;
> +	init_ipc_ns.mq_mnt = m;
>  	return 0;
>  
>  out_filesystem:

Unrelated issue, but register_filesystem() should be the last thing
module_init() of a filesystem driver does.  It's a separate story,
in any case...