Medium [CVE-2026-72412] Fix handling of _PAGE_UNUSED pte bit
This medium-severity Red Hat Linux advisory covers CVE-2026-72412.
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Summary
In the Linux kernel, the following vulnerability has been resolved: s390/mm: Fix handling of _PAGE_UNUSED pte bit The _PAGE_UNUSED softbit should not really be lying around. Its sole purpose is to signal to try_to_unmap_one() and try_to_migrate_one() 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. 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 _PAGE_UNUSED 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. A flaw was found in the Linux kernel's memory management for s390 systems.
An internal flag, intended to mark guest memory pages for discard after unmapping by the Kernel-based Virtual Machine (KVM), was not consistently cleared when those pages were subsequently reused. This improper handling could cause actively used guest memory to be prematurely discarded.
The most significant consequence is data corruption within guest virtual machines. Red Hat severity: Moderate — CVSS 5.5 (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H).
Weakness: CWE-911.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · medium-confidence parse· fetched 1 hour ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · medium-confidence parse· fetched 1 hour ago·verify at source
Mitigation
The source record does not include mitigation steps. That is not a statement that no fix exists — read the vendor advisory below for the authoritative guidance.
Official advisory · medium-confidence parse· fetched 1 hour ago·verify at source
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