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CVE-2026-72412
s390/mm: Fix handling of _PAGE_UNUSED pte bit
Published 2026-08-15 (2 months ago) by the Linux kernel CNA. Subsystem arch/s390. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-72412 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | not affected | 6.12.113-1 | Nothing to do. trixie never shipped the vulnerable code. |
| Debian 14forky | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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. 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 _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. Also fix gmap_helper_try_set_pte_unused() to only set the bit if the pte is present; the _PAGE_UNUSED bit is only defined for present ptes and thus should not be set for non-present ptes.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
arch/s390/include/asm/pgtable.h
arch/s390/mm/gmap_helpers.c
- What this is
- S390 MMMAINTAINERS section matched on arch/s390/include/asm/pgtable.h · linux-s390@vger.kernel.org · Supported
Show whether Debian builds this, per release
Not established. This site could not find the build rule for the affected file in the kernel trees Debian ships, so it has no answer about whether Debian compiles this code. Read that as unknown, never as absent.
- bookworm, trixie, forky, sid?
- ?
- not established - no Makefile entry for the affected file
Where a release above reads "not established", no build rule for the affected file was found in that release's kernel tree. That is a gap in this lookup, not a finding that the code is absent.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS
Signals
- Published
- 2026-08-15 (2 months ago)source
- CVSS
- 9.3 critical
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.18%, higher than 7.0% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- arch/s390
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 7.0 | 7.1.5 | fda07c8e4b54 |
| 7.0 | 7.2 | d4bb00704a66 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- The bug is only reachable on s390 KVM hosts when a guest drives CMMA/ESSA page-state changes and gmap unmaps that reach gmap_helper_try_set_pte_unused() and host try_to_unmap_one()/try_to_migrate_one() on kvm->mm; there is no network or physical-bus entry point.
- AC:LAttack complexity, low
- A guest can deterministically mark pages unused via ESSA, remap them through PTE-install paths that left stale _PAGE_UNUSED set, then induce host reclaim/migration with memory pressure; no attacker-uncontrollable race or rare layout is required on CMMA-enabled s390 KVM.
- PR:NPrivileges required, none
- A malicious KVM guest tenant on an IBM Z/LinuxONE host with CMMA-enabled VMs needs no host root, capabilities, or /dev/kvm access—only guest supervisor memory management that issues ESSA and normal guest page reuse to trigger the stale-bit discard path.
- UI:NUser interaction, none
- No victim interaction is required; once a CMMA-enabled s390 KVM guest is running, the attacker directly drives ESSA unused marking, guest remap/fault activity, and memory-pressure workloads to trigger premature host-side page discard.
- S:CScope, changed
- The flaw corrupts host-kernel management of kvm->mm backing pages from guest-controlled CMMA state, discarding live present pages outside the guest security authority and enabling cross-boundary host memory corruption/disclosure primitives.
- C:HConfidentiality impact, high
- Prematurely discarding present backing pages while guest mappings remain can free and reallocate folios, exposing prior page contents on refault and providing UAF-class read primitives consistent with memory-corruption confidentiality impact.
- I:HIntegrity impact, high
- Stale _PAGE_UNUSED causes try_to_unmap_one()/try_to_migrate_one() to drop in-use guest backing pages instead of swapping/migrating them, enabling arbitrary guest memory corruption and host mm metadata corruption exploitable for control-flow hijacking.
- A:HAvailability impact, high
- Incorrect discard during host reclaim, migration, or unmap can panic/oops the kernel or repeatedly corrupt guest RAM, causing guest or host denial of service that the attacker can retrigger with memory-pressure workloads.