Linux kernel: mount propagation lets a user namespace reveal covered files

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

namespace: keep the lock on a mount that a propagated copy is moved beneath

attach_recursive_mnt() transfers MNT_LOCKED from the top mount to the mount that is moved beneath it with MOVE_MOUNT_BENEATH. This allows the owner of a user namespace to replace its locked /proc or its root. The mount beneath takes on the job of covering the underlying mountpoint allowing the top mount to be unmounted.

Consider two mount namespaces:

(H) The host H has a shared mount P with a secret in P/d (Z) Z is a user namespace made from M. Its copy of P receives propagation from the host's P and its copy of M's cover on P/d is locked Z's owner must not get to see P/d.

Now (H) mounts X on P/d. This propagates into (Z). The copy of X lands beneath (Z)'s locked cover. The locked property is transfered from (Z)'s cover to the copy of X propagated beneath it. The cover is now unlocked.

Now (H) unmounts X again. The copy of X in (Z) gets unmounted and the covering mount is left unlocked on top of P/d. (Z) can now unmount it:

Z: umount2(P/d)             = EINVAL            /* the cover is locked */
H: mount X on P/d, umount X                     /* both propagate into Z /*
Z: umount2(P/d)             = 0                 /* the cover is now unlocked */
Z: read P/d/secret          = "covered-by-root" /* secret revealed */

So only transfer the locked property to the mount beneath for mounts the caller has placed. A propagated copy that lands beneath a locked mount is locked as well so that the mount at the bottom of the stack carries a lock the way every check expects. The mount on top of it remains locked to ensure that it keeps covering even if the propagated mount is unmounted again.