CVE-2026-72412

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

s390/mm: Fix handling of PAGEUNUSED pte bit

The PAGEUNUSED softbit should not really be lying around. Its sole

purpose is to signal to trytounmapone() and trytomigrateone()

that the page can be discarded instead of being moved / swapped.

KVM has no way to know why a page is being unmapped, so it sets the bit

on userspace ptes corresponding to unused guest pages every time they

get unmapped. KVM has no reasonable way to clear the bit once the page

is in use again.

While set_ptes() checks and clears the bit, other paths that set new

ptes did not. This led to used pages being thrown out as if they were

unused, causing guest corruption.

Fix the issue by clearing the PAGEUNUSED bit for present ptes in

set_pte(), i.e. whenever a present pte is getting set. The check in

set_ptes() is then redundant and can be removed.

Also fix gmaphelpertrysetpte_unused() to only set the bit if the

pte is present; the PAGEUNUSED bit is only defined for present ptes

and thus should not be set for non-present ptes.