Re: [PATCH] vfs: move_mount: reject moving kernel internal mounts

[Al Viro]

On Sat, Jun 29, 2019 at 09:39:16PM +0100, Al Viro wrote:
> On Sat, Jun 29, 2019 at 01:27:44PM -0700, Eric Biggers wrote:
> 
> > @@ -2600,7 +2600,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
> >  	if (attached && !check_mnt(old))
> >  		goto out;
> >  
> > -	if (!attached && !(ns && is_anon_ns(ns)))
> > +	if (!attached && !(ns && ns != MNT_NS_INTERNAL && is_anon_ns(ns)))
> >  		goto out;
> >  
> >  	if (old->mnt.mnt_flags & MNT_LOCKED)
> 
> *UGH*
> 
> Applied, but that code is getting really ugly ;-/

FWIW, it's too ugly and confusing.  Look:
        /* The mountpoint must be in our namespace. */
        if (!check_mnt(p))
                goto out;

        /* The thing moved should be either ours or completely unattached. */
        if (attached && !check_mnt(old))
                goto out;

        if (!attached && !(ns && ns != MNT_NS_INTERNAL && is_anon_ns(ns)))
                goto out;

OK, the first check is sane and understandable.  But let's look at what's
coming after it.  We have two cases:
	1) attached.  IOW, old->mnt_parent != old.  In that case we
require old->mnt_ns == current mnt_ns.  Anything else is rejected.
	2) !attached.  old->mnt_parent == old.  In that case we
require old->mnt_ns to be an anon namespace.

Let's reorder that a bit:
        /* The mountpoint must be in our namespace. */
        if (!check_mnt(p))
                goto out;

	/* The thing moved must be mounted... */
	if (!is_mounted(old_path->mnt))
		goto out;

        /* ... and either ours or the root of anon namespace */
	if (!(attached ? check_mnt(old) : is_anon_ns(ns)))
		goto out;

IMO that looks saner and all it costs us is a redundant check
in attached case.  Objections?