Re: [PATCH] fs: allow clone_private_mount() for a path on real rootfs

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

On Wed, May 14, 2025 at 08:37:54AM +0000, KONDO KAZUMA(近藤 和真) wrote:
> On 2025/05/14 11:43, Al Viro wrote:
> > On Wed, May 14, 2025 at 12:25:58AM +0000, KONDO KAZUMA(近藤 和真) wrote:
> > 
> >> @@ -2482,17 +2482,13 @@ struct vfsmount *clone_private_mount(const struct path *path)
> >>  	if (IS_MNT_UNBINDABLE(old_mnt))
> >>  		return ERR_PTR(-EINVAL);
> >>  
> >> -	if (mnt_has_parent(old_mnt)) {
> >> +	if (!is_mounted(&old_mnt->mnt))
> >> +		return ERR_PTR(-EINVAL);
> >> +
> >> +	if (mnt_has_parent(old_mnt) || !is_anon_ns(old_mnt->mnt_ns)) {
> >>  		if (!check_mnt(old_mnt))
> >>  			return ERR_PTR(-EINVAL);
> >>  	} else {
> >> -		if (!is_mounted(&old_mnt->mnt))
> >> -			return ERR_PTR(-EINVAL);
> >> -
> >> -		/* Make sure this isn't something purely kernel internal. */
> >> -		if (!is_anon_ns(old_mnt->mnt_ns))
> >> -			return ERR_PTR(-EINVAL);
> >> -
> >>  		/* Make sure we don't create mount namespace loops. */
> >>  		if (!check_for_nsfs_mounts(old_mnt))
> >>  			return ERR_PTR(-EINVAL);
> > 
> > Not the right way to do that.  What we want is
> > 
> > 	/* ours are always fine */
> > 	if (!check_mnt(old_mnt)) {
> > 		/* they'd better be mounted _somewhere */
> > 		if (!is_mounted(old_mnt))
> > 			return -EINVAL;
> > 		/* no other real namespaces; only anon */
> > 		if (!is_anon_ns(old_mnt->mnt_ns))
> > 			return -EINVAL;
> > 		/* ... and root of that anon */
> > 		if (mnt_has_parent(old_mnt))
> > 			return -EINVAL;
> > 		/* Make sure we don't create mount namespace loops. */
> > 		if (!check_for_nsfs_mounts(old_mnt))
> > 			return ERR_PTR(-EINVAL);
> > 	}
> 
> Hello Al Viro,
> 
> Thank you for your comment.
> That code can solve my problem, and it seems to be better!

BTW, see https://lore.kernel.org/all/20250506194849.GT2023217@ZenIV/ for
discussion about a week ago when that got noticed:

|| In case of clone_private_mount(), though, there's nothing wrong
|| with "clone me a subtree of absolute root", so it has to be
|| done other way round - check if it's ours first, then in "not
|| ours" case check that it's a root of anon namespace.
||
|| Failing btrfs mount has ended up with upper layer pathname
|| pointing to initramfs directory where btrfs would've been
|| mounted, which had walked into that corner case.  In your
|| case the problem has already happened by that point, but on
|| a setup a-la X Terminal it would cause trouble...

Looks like such setups are less theoretical than I thought.

> So, I will revise my patch and resend it.

Probably worth gathering the comments in one place.  Something like
	/*
	 * Check if the source is acceptable; anything mounted in
	 * our namespace is fine, otherwise it must be the root of
	 * some anon namespace and we need to make sure no namespace
	 * loops get created.
	 */
	if (!check_mnt(old_mnt)) {
		if (!is_mounted(&old_mnt->mnt) ||
		    !is_anon_ns(old_mnt->mnt_ns) ||
		    mnt_has_parent(old_mnt))
			return ERR_PTR(-EINVAL);
		if (!check_for_nsfs_mounts(old_mnt))
			return ERR_PTR(-EINVAL);
	}
might be easier to follow.