CVE-2026-98978

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

namespace: never expire a locked mount

Locked mounts are special. They protect the underlying files and directories from being revealed. do umount() refuses to unmount locked mounts but shrink submounts() doesn't.

A shrinkable mount can become locked once the owner of a user namespace puts it beneath a locked mount with MOVE MOUNT BENEATH. The locked property now moves to the mount at the bottom making it possible to unmount the top mount.

So umount() of an unlocked ancestor now expires the bottom mount first and the covered directory is revealed:

move mount(c -> x/hidden, MOVE MOUNT BENEATH) = 0 the cover after the lock moved: umount2(x/hidden) = 0 the holder of the lock: umount2(x/hidden) = EINVAL the root of the copy, busy: umount2(x) = EBUSY reads x/hidden/secret: "covered-by-root"

So leave a locked mount alone as it dies together with its parent. A plain umount() of an unlocked mount with a locked one below it is EBUSY from now on. It's the same for any other locked child. A lazy umount still takes the whole tree.

mark mounts for expiry() never sees a locked mount. lock mnt tree() leaves a mount on an expiry list alone and nothing else puts a locked one on such a list. Add an assert for this.