[CVE-2026-74514][LOW] KVM: s390: pci: Fix memory accounting for pinned/unpinned pages

[CVE-2026-74514][LOW] KVM: s390: pci: Fix memory accounting for pinned/unpinned pages

[AL-KERNEL]

CVE: CVE-2026-74514
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: s390: pci: Fix memory accounting for pinned/unpinned pages
Commit: 70871b121f81d08879363cb1238a4c85c5c2800c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=70871b121f81d08879363cb1238a4c85c5c2800c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74514
Analysis date: Sat, 15 Aug 2026 14:05:16 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-74514 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74514

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: LOW
Manual review required: NO

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

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74514	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-772;*CWE-401;**CWE-400;*CWE-664;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.5';DESCR 'KVM s390 PCI interrupt forwarding accounted pinned pages against the current user and current mm at account and unaccount time, but the unaccount path may run in a different process context. This can leave user_struct references unreleased and can subtract pinned memory from the wrong mm or user accounting object. For the CVSS the PR:L is used for the paranoid score because a VM management process or reduced capability service account with access to the relevant KVM s390 PCI device may be able to trigger repeated enable and disable cycles without full host root privileges. The issue is not network reachable and requires local control of the KVM PCI configuration path. Impact is mainly denial of service or resource accounting bypass through leaked references or incorrect locked memory counters, not proven memory corruption or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	VIRT NOMEMCHK HARDWARE LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=2
ActionableScoreLower=1

1. ActionableScore
* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

2. Signal breakdown
* Real lifetime / reference accounting bug: +1. `get_uid()` increased `user_struct` refcount without matching `free_uid()`, and the fix adds persistent `user_struct` / `mm_struct` ownership with `free_uid()` and `mmdrop()`.
* Availability / resource exhaustion plausible: +1. Repeated enable / disable cycles can leak references or leave incorrect pinned memory accounting, potentially causing resource exhaustion or accounting-related DoS.
* Cross-boundary accounting concern: +1. The old code could unaccount against `current->mm`, which may differ from the original pinning process, so resource accounting may be applied to the wrong task context.
* Requires privileged KVM / PCI device-management access in typical deployments: -2. This path is not available to ordinary local users and normally requires host administrator, hypervisor, or privileged VM management control.
* Rare hardware / configuration path: -1. The issue is specific to s390 KVM PCI interrupt forwarding / zPCI usage, not a broad default kernel path.
* Paranoid adjustment: use reduced privilege penalty rather than full admin penalty when a VM management service account or delegated device-management process can trigger the path without full interactive host root.

3. Reachability analysis
The trigger is local and tied to KVM s390 PCI interrupt forwarding enable / disable paths. It is not network reachable. Ordinary containers or unprivileged namespaces should not reach this unless KVM device-management authority is explicitly delegated. Realistic exploitation requires an s390 host using KVM zPCI passthrough or related interrupt forwarding, plus the ability to drive repeated accounting and unaccounting cycles.

Call-site confidence: high. The patch shows both relevant call sites, `kvm_s390_pci_aif_enable()` and `kvm_s390_pci_aif_disable()`.

4. Severity interpretation
This behaves like an ordinary Moderate or Low-like resource accounting issue, not an Important-class vulnerability. The patch supports reference leaks and wrong accounting object usage, but does not show UAF, OOB access, arbitrary write, callback control, type confusion, or a practical privilege-escalation primitive. The main realistic impact is local privileged DoS or resource accounting inconsistency.

5. One-sentence report phrase
KVM s390 PCI pinned-page accounting could leak `user_struct` references and unaccount pinned memory against the wrong `mm_struct` when disable runs in a different process context, causing a local privileged resource-accounting DoS rather than proven memory corruption.

6. Manual review recommendation
NO MANUAL CHECK NEEDED.
No network exposure, no unprivileged default trigger, no concrete memory corruption primitive, and no plausible LPE path are supported by the patch.

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

Patch: KVM: s390: pci: Fix memory accounting for pinned/unpinned pages
Commit: 70871b121f81d08879363cb1238a4c85c5c2800c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=70871b121f81d08879363cb1238a4c85c5c2800c

Commit description:

The account_mem() and unaccount_mem() functions call get_uid() which
increments the reference count of struct user_struct on every invocation.
But we don't decrement the count by calling free_uid(). It also
accounted/unaccounted the pages against the current->mm. But its possible
the unaccount_mem() can be called from a different process context than the
one that originally pinned the pages.

Let's fix this by storing the pinning process user_struct and mm_struct
when accounting for pinned pages, and subsequently free these resources
when the pages are unpinned.

Fixes: 3c5a1b6 ("KVM: s390: pci: provide routines for enabling/disabling interrupt forwarding")
Cc: stable@vger.kernel.org
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
[borntraeger@linux.ibm.com: Fixed whitespace]
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/s390/kvm/pci.c
  arch/s390/kvm/pci.h

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=70871b121f81d08879363cb1238a4c85c5c2800c

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

The original Linux kernel CVE announcement for CVE-2026-74514 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74514

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: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).