[CVE-2026-72292][MODERATE REGULAR] KVM: s390: Initialize KVM_S390_GET_CMMA_BITS memory

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-72292
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: s390: Initialize KVM_S390_GET_CMMA_BITS memory
Commit: a4a19941ccb2164edc3491faa5ac5df82e94b363
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a4a19941ccb2164edc3491faa5ac5df82e94b363
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72292
Analysis date: Sat, 15 Aug 2026 17:28:16 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
`KVM_S390_GET_CMMA_BITS can disclose stale kernel memory because gaps in a vmalloc allocated CMMA output buffer may remain uninitialized before copy_to_user(), allowing a local KVM capable user or VMM process to read kernel memory on affected s390 hosts.`

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-72292 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72292

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE REGULAR
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-72292	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N';*CWE-200;CWE-908;*CWE-457;BEST CVSS score: '5.5';DESCR 'KVM_S390_GET_CMMA_BITS can disclose stale kernel memory because kvm_s390_get_cmma_bits() allocated the userspace output buffer with vmalloc(), and some bytes in the reported CMMA range were never initialized before copy_to_user(). In the non-peek migration path, dirty pages are written into the buffer but clean or unmapped gaps between dirty pages can remain unchanged and still be copied back to the VMM. For the CVSS the PR:L is used because an attacker needs local access to KVM s390 VM ioctls, for example through a VMM process or a user allowed to use /dev/kvm, but full administrative privileges are not necessarily required. The issue is not network reachable by itself and requires local KVM access on an s390 host. Impact is confidentiality loss from kernel memory disclosure. No direct integrity or availability impact is supported by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)  SKIP CVE-2026-72292 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: e38c884df92119d96f652d51f82661dd2fc0b885 YES  NO NO unknown 	MAYBE	VIRT LEAK HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user or VMM process with access to KVM s390 VM ioctls can trigger `KVM_S390_GET_CMMA_BITS`. This is not full host root in typical delegated KVM usage.

* Security boundary bypass with sensitive kernel memory exposure: +2
  Uninitialized `vmalloc()` memory is copied to userspace through `copy_to_user()`. This crosses the kernel to userspace confidentiality boundary and may expose stale kernel heap or vmalloc contents.

* Confidentiality impact plausible: +1
  The primitive is a direct kernel memory disclosure from uninitialized bytes in the CMMA output buffer.

* Rare AND difficult-to-reach subsystem/configuration: -1
  The affected path is s390 KVM CMMA migration handling, not a broad default path on general Linux systems.

Paranoid-only adjustment:

* Add +1 practical triage sensitivity for kernel memory disclosure in virtualization context
  The leaked bytes come from stale kernel memory and the caller is a VMM or KVM-capable local process. Even without write corruption, this should not be auto-closed without review.

Not triggered:

* No generic memory corruption.
* No UAF, OOB write, double free, refcount takeover, or type confusion.
* No privilege escalation primitive is shown.
* No integrity or availability impact is supported by the patch.
* No remote or network reachability.

## 3. Reachability analysis

The bug is locally reachable through the s390 KVM `KVM_S390_GET_CMMA_BITS` ioctl path. The attacker needs access to a KVM VM context, for example as a VMM process or a user allowed to use `/dev/kvm`. This is best treated as PR:L rather than PR:H because KVM access can be delegated to non-root users through group membership or service accounts.

Namespaces do not make this broadly reachable by themselves. The path is not network-triggerable. The issue is architecture and feature specific because it affects s390 KVM CMMA migration behavior, so exposure is limited to s390 virtualization hosts using this functionality.

## 4. Severity interpretation

This behaves like an actionable confidentiality issue, not like generic memory corruption. The demonstrated primitive is uninitialized kernel memory disclosure to userspace. There is no evidence of a write primitive, object lifetime corruption, kernel crash, or direct LPE path.

Realistic severity is above ordinary Low because kernel memory disclosure across the KVM boundary may expose sensitive stale data. It is not a Strong Important candidate unless additional evidence shows repeatable disclosure of high-value secrets or an exploit chain.

## 5. One-sentence report phrase

`KVM_S390_GET_CMMA_BITS can disclose stale kernel memory because gaps in a vmalloc allocated CMMA output buffer may remain uninitialized before copy_to_user(), allowing a local KVM capable user or VMM process to read kernel memory on affected s390 hosts.`

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a direct kernel memory disclosure copied to userspace. Even though it is not memory corruption and is limited to s390 KVM CMMA migration paths, confidentiality impact is concrete and should not be auto-closed.

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: KVM: s390: Initialize KVM_S390_GET_CMMA_BITS memory
Commit: a4a19941ccb2164edc3491faa5ac5df82e94b363
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a4a19941ccb2164edc3491faa5ac5df82e94b363

Commit description:

kvm_s390_get_cmma_bits() allocates its output buffer with vmalloc(),
which does not zero the returned pages:

	values = vmalloc(args->count);

In the non-peek (migration) path, dat_get_cmma() reports a byte count
spanning from the first to the last dirty page, but __dat_get_cmma_pte()
writes values[gfn - start] only for pages whose CMMA dirty bit is set.
The walk uses DAT_WALK_IGN_HOLES, so clean and unmapped pages that lie
between two dirty pages within the reported span are visited but never
store their byte.  Those gaps (up to KVM_S390_MAX_BIT_DISTANCE pages
each) stay uninitialized yet fall inside [0, count) and are copied out
by copy_to_user(), disclosing stale kernel memory to user space.

Before the switch to the new gmap implementation the buffer was fully
populated for every gfn in the span, so no uninitialized bytes were
exposed; the dirty-only walk introduced the leak.

Use vzalloc() so the gaps read back as zero.

Fixes: e38c884 ("KVM: s390: Switch to new gmap")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20260611105036.11491-1-borntraeger@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/s390/kvm/kvm-s390.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a4a19941ccb2164edc3491faa5ac5df82e94b363

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-72292 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72292

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 5.5

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 3
  Paranoid ActionableScore: 4

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).