CVE-2026-98993: namespace: don't inherit MNT_UMOUNT in clone_mnt()

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

namespace: don't inherit MNTUMOUNT in clonemnt()

MNTUMOUNT gets raised on a mount that umounttree() or umountone() has taken out of its namespace. It's never cleared. detachmounts() unhooks a mount that carries it instead of unmounting it.

disconnectmount() lets an unmounted child stay attached only if its parent carries it. propagateumount() takes it as the sign that a mount is going to be unmounted when it decides what else can be unmounted, where a chain of locked mounts ends and where a for reparenting an overmount.

clonemnt() copies the flags of the source and only masks MNTINTERNALFLAGS which doesn't include MNTUMOUNT. A mount that was lazily unmounted but is kept alive by a file descriptor can still be copied when its root is an nsfs or pidfs file because maycopytree() accepts those before checking whether the source is mounted at all:

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

The mount on /tmp/y is attached to the caller's namespace with MNTUMOUNT already set and every copy of it inherits the flag again.

That causes a number of bugs:

- Unlink its mountpoint from a mount namespace where it isn't one. detachmounts() takes it for a mount that was unmounted earlier, unhooks it from its parent and drops the reference the namespace holds on it. It stays in the namespace with one reference too few and as its own parent, so it can't be unmounted through the file descriptor anymore and the namespace never finds it again. When the namespace goes away it's leaked and points to a freed struct mntnamespace that maychangepropagation() and anonnsroot() read on the next propagation change or mountsetattr() through the file descriptor.

- Make it shared and unmount a peer. tracetransfers() follows the flag from the peer into it and strips it from its peer group.

- Let a propagated umount reach it. gathercandidates() marks it with TUMOUNTCANDIDATE but never puts it on the candidate list, so the mark is never cleared and every later propagated umount skips it. If it's the parent of a locked mount handlelocked() takes it as the cutoff and commits the locked child that should have stayed and disconnectmount() leaves that child attached to it, unmounted under a mounted parent, which is what disconnectmount() exists to prevent. When the namespace goes away umounttree() visits the child a second time. And once detachmounts() has made it its own parent reparent() never finds the end of the chain of unmounted parents.

A clone is a new mount that has never been unmounted. Add MNTUMOUNT to MNTINTERNALFLAGS so clonemnt() never copies it. And warn when umounttree() reaches a mount that already has it set.