namespace: don't copy an unmounted mount tree

In the Linux kernel, the following vulnerability has been resolved:

namespace: don't copy an unmounted mount tree

may_copy_tree() accepts a mount whose root is an nsfs or pidfs file
before it checks whether that mount is mounted. This exception exists
for nsfs and pidfs mounts. They are part of no mount namespace so
neither check_mnt() nor is_mounted() would let them through and
bind-mounting a nsfsfd or a pidfd wouldn't work.

Problem is that exception also admits a bind mount of such a file when
it was lazily unmounted and is only kept alive by a file descriptor:

mount --bind /proc/self/ns/net /tmp/x
exec 3< /tmp/x
umount -l /tmp/x
mount --rbind /proc/self/fd/3 /tmp/y

With MS_REC or AT_RECURSIVE copy_tree() doesn't stop at the unmounted
mount. It copies every mount that stayed attached to it as well. So if
you have a bunch of overmounts over the nsfsfd or the pidfd the whole
unmounted stack is copied. That's a bug.

So refuse to walk an unmounted tree. Copying its root alone stays
possible. So a bind mount of a namespace that is gone can still be
recreated from a file descriptor on the old one. The file descriptor
pins that mount and its root and __has_locked_children() only reads the
direct children which don't leave the parent's list without
namespace_sem. The internal mounts carry MNT_NS_INTERNAL instead of NULL
and are still walked. Copying an unmounted tree recursively fails with
EINVAL.