CVE-2026-98992: 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

maycopytree() 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 checkmnt() nor ismounted() 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 MSREC or ATRECURSIVE copytree() 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 haslockedchildren() only reads the direct children which don't leave the parent's list without namespacesem. The internal mounts carry MNTNSINTERNAL instead of NULL and are still walked. Copying an unmounted tree recursively fails with EINVAL.