[CVE-2026-74481][MODERATE 7.0] mm/page_reporting: use system_freezable_wq to fix UAF during suspend

[CVE-2026-74481][MODERATE 7.0] mm/page_reporting: use system_freezable_wq to fix UAF during suspend

[AL-KERNEL]

CVE: CVE-2026-74481
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mm/page_reporting: use system_freezable_wq to fix UAF during suspend
Commit: f978048326570047e8216e81a67f9c71ef2bb1b1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f978048326570047e8216e81a67f9c71ef2bb1b1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74481
Analysis date: Sat, 15 Aug 2026 14:01:55 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A UAF in mm page reporting during suspend or hibernation can let unfrozen page reporting work access virtio_balloon virtqueues after device teardown, causing a kernel crash and requiring manual review due to real kernel lifetime corruption.

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

The original Linux kernel CVE announcement for CVE-2026-74481 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74481

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 7.0
Manual review required: YES

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

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

CVE-2026-74481	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in mm page reporting during suspend or hibernation because page_reporting_process was queued on system_wq, which is not freezable, and can continue running while virtio_balloon removes virtqueues. If memory freeing or shrinker activity returns pages to the buddy allocator during the PM freeze window, page reporting may call virtballoon_free_page_report on deleted virtqueues and crash in virtqueue_add_split. For the CVSS the PR:L is used for the paranoid score because a local user or guest service may be able to create memory churn and interact with suspend or hibernation policy, although reliable triggering depends on platform power management behavior. The issue is not network reachable and is local to the guest or host where page reporting and virtio_balloon are enabled. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to UAF based memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	OOB DANGER INIT UAF KERNEL_PANIC_PLUS_UAF HARDWARE MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: **3**
* Paranoid score: **5**
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak primitive: **+1**. The commit explicitly describes a UAF on deleted virtqueues, but the shown failure is a GPF/crash and there is no demonstrated reclaim, object replacement, write-after-free, callback control, or arbitrary write.
* Real lifetime corruption: **+1**. The bug is a real lifetime mismatch between PM freezer ordering, page reporting work, and virtqueue teardown.
* Reliable kernel crash / strong DoS: **+1**. The trace shows a crash in `virtqueue_add_split()` via `virtballoon_free_page_report()` and the issue was reproduced through suspend/resume stress testing.
* Hard or unreliable race / special timing required: **-1**. Triggering requires the PM freeze window plus page freeing or shrinker activity while virtqueues are being deleted.
* Requires admin/root/system PM control in typical deployments: **-1**. Full `-2` is not used because local users or services may influence memory pressure and some systems expose suspend through policy-controlled desktop or service paths.

Conservative total: **3**

Paranoid additional signals:

* Local unprivileged trigger contribution: **+1**. A local user may be able to create memory churn while a suspend or hibernation operation is initiated by system policy, even if they cannot directly control all PM steps.
* Privileged kernel/device-management memory corruption path: **+1**. The UAF occurs in virtio/virtqueue device-management state during PM teardown, which is a trusted kernel/device boundary.
* Broad virtualization relevance: **+1**. `virtio_balloon` and page reporting are relevant in virtualized/cloud environments, though not universal.

Paranoid total after timing/privilege constraints: **5**

Call-site confidence: **high**. The commit message provides the direct call path from `page_reporting_process()` to `virtballoon_free_page_report()` and the crash site in `virtqueue_add_split()`.

## 3. Reachability analysis

The issue is local and not network reachable. Realistic triggering requires page reporting with a driver such as `virtio_balloon`, plus S3 suspend or S4 hibernation where the PM freezer runs while virtqueues are deleted. In typical server or VM deployments, initiating suspend/hibernate is privileged or controlled by system policy, so conservative reachability is not fully unprivileged. However, a local user or guest workload may help trigger the vulnerable window by creating memory churn and free-page activity during a policy-driven suspend or hibernation cycle.

Namespaces and containers do not obviously make this directly reachable unless a container or service can influence host or guest PM state. The path is configuration dependent, but not obscure in virtualized environments that use virtio balloon page reporting.

## 4. Severity interpretation

This is more than an ordinary low-priority DoS because the patch fixes a real UAF in kernel device lifetime handling. The realistic demonstrated impact is a local kernel crash during suspend or hibernation. Theoretical privilege escalation is not strongly supported because the patch and trace do not show attacker-controlled reclaim, controlled reuse, type confusion, callback hijack, or write-after-free.

Therefore, conservative handling is **borderline/manual-review Moderate**, while paranoid handling is **Actionable Moderate at minimum** due to real UAF plus device lifetime corruption in a virtualization-relevant path.

## 5. One-sentence report phrase

A UAF in mm page reporting during suspend or hibernation can let unfrozen page reporting work access virtio_balloon virtqueues after device teardown, causing a kernel crash and requiring manual review due to real kernel lifetime corruption.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the bug is an explicit UAF in a kernel device lifetime path. The demonstrated effect is DoS, but the lifetime corruption occurs on virtqueue state during PM teardown, so it should not be auto-closed without manual validation of exploitability and deployment exposure.

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

Patch: mm/page_reporting: use system_freezable_wq to fix UAF during suspend
Commit: f978048326570047e8216e81a67f9c71ef2bb1b1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f978048326570047e8216e81a67f9c71ef2bb1b1

Commit description:

During PM freeze (e.g.  S3 suspend or S4 hibernation), device drivers like
virtio_balloon reset their underlying virtio devices and delete their
virtqueues via vdev->config->del_vqs().

However, page reporting work (page_reporting_process) was scheduled on the
global system_wq.  Because system_wq lacks the WQ_FREEZABLE flag, the PM
freezer skips it, leaving page_reporting_process active during suspend.

If pages are freed into the buddy allocator while suspending (for example,
when core MM invokes the balloon shrinker during S4 hibernation image
saving), page reporting triggers virtballoon_free_page_report() on deleted
virtqueues, resulting in a Use-After-Free / General Protection Fault:

    [  196.795226] general protection fault, probably for non-canonical address 0xaa1436fe70dae6df: 0000 [#1] SMP NOPTI
    [  196.825967] Workqueue: events page_reporting_process
    [  196.831038] RIP: 0010:virtqueue_add_split+0x233/0x4c0 [virtio_ring]
    [  196.927073] virtballoon_free_page_report+0x3a/0xe0 [virtio_balloon]
    [  196.946943] page_reporting_process+0x370/0x4f0

Fix this by switching page reporting work to system_freezable_wq.  This
ensures that the PM freezer pauses page_reporting_process before device
drivers destroy their reporting virtqueues.  Because the reporting worker
is frozen, memory reclamation/freeing (e.g.  via shrinker execution) can
safely return pages to MM during freeze without triggering unfrozen
reporting work on deleted virtqueues.

This aligns with the driver's existing design. The comment in
virtballoon_freeze() states:
    /*
     * The workqueue is already frozen by the PM core before this
     * function is called.
     */

Testing:
I have verified these fixes using Google’s virtualization infrastructure
by running continuous suspend/resume iterations (40+ cycles) while
churning memory using stress-ng (`stress-ng --vm 4 --vm-bytes 60%
--timeout 1`) to constantly create free pages for the buddy allocator.  We
also set the `page_reporting_order` parameter to 0 to make the page
reporting worker highly sensitive, forcing it to pick up any 4K free
pages.  This confirmed that the UAF crashes are no longer reproducible.

Link: https://lore.kernel.org/20260721005603.1710551-1-linkl@google.com
Fixes: 36e66c5 ("mm: introduce Reported pages")
Signed-off-by: Link Lin <linkl@google.com>
Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Jiaqi Yan <jiaqiyan@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  mm/page_reporting.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=f978048326570047e8216e81a67f9c71ef2bb1b1

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

The original Linux kernel CVE announcement for CVE-2026-74481 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74481

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:H/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 5

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 7.0

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).