[CVE-2026-93216][LOW] mm/page_owner: use memcg_data snapshot to avoid TOCTOU in print_page_owner_memcg()

[CVE-2026-93216][LOW] mm/page_owner: use memcg_data snapshot to avoid TOCTOU in print_page_owner_memcg()

[AL-KERNEL]

CVE: CVE-2026-93216
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm/page_owner: use memcg_data snapshot to avoid TOCTOU in print_page_owner_memcg()
Commit: 46761406e14381dc35f498c247a87565b87ea2ef
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=46761406e14381dc35f498c247a87565b87ea2ef
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93216
Analysis date: Thu, 24 Sep 2026 13:42:09 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93216 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93216

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-2026-93216	LOW	https://www.kernelcve.org/list/?q=CVE-2026-93216 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-367;CWE-617;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A TOCTOU race in print_page_owner_memcg can occur because page memcg_data is read once and then read again without synchronization while the page may be freed and reallocated. If the page changes into a THP tail page or a slab page during this window, CONFIG_DEBUG_VM assertions in the memcg handling path can trigger a kernel BUG or panic. For the CVSS the PR:L vector is used for the paranoid score because a low privileged local process could trigger the issue if access to the page owner debug interface is delegated by a privileged service or permissive debugfs policy. Under normal configurations access is restricted to administrators. The issue is not network reachable. Impact is local denial of service on debug enabled kernels. No information disclosure integrity impact or privilege escalation primitive is supported by the race or by the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)	YES	PRINTF-TTY UAF KERNEL_PANIC_PLUS_UAF LINUS MEMORY  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: -2
* Conservative score: 0
* Raw paranoid score: -2
* Paranoid score: 0
* Applied score bound: -2 -> 0 because ActionableScore is non-negative.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash / strong DoS. The patch and commit explicitly state that a concurrent page reuse can trigger VM_BUG_ON assertions and crash the kernel on CONFIG_DEBUG_VM builds.
* -2 Host-admin-equivalent access requirement. The page_owner output is a privileged debugfs diagnostic interface and is normally created with root-only access. Reading the relevant page_owner data is required to enter print_page_owner_memcg.
* -1 Rare and difficult-to-reach configuration. The vulnerable behavior requires page-owner diagnostics and CONFIG_DEBUG_VM, which are debug-oriented kernel configurations not normally enabled for ordinary production workloads.

Important non-awarded signals:

* No real UAF was established. struct page storage remains valid, but its state may change because the page is freed and reused. The demonstrated defect is a TOCTOU state reinterpretation leading to an assertion, not dereference of a freed object.
* No controlled reclaim, replacement-object control, arbitrary write, callback influence, refcount corruption, or post-free lifetime mutation is established.
* No confidentiality or integrity impact is established. The patch does not show data disclosure, unauthorized modification, or a security-boundary bypass.
* No remote reachability is established. The vulnerable path is local page_owner diagnostic output.
* No separate race penalty was applied. The patch shows a TOCTOU window, but does not establish that the timing is unusually narrow, one-shot, or materially difficult to repeat for an already privileged caller.
* No extra Availability point was awarded because the crash is already represented by the Reliable kernel crash signal.
* No Local unprivileged trigger point was awarded. A low-privilege process can generate allocation and freeing activity, but cannot normally access the root-only page_owner debugfs interface needed to execute the vulnerable reader.
* The previously suggested PR:L scenario depends on non-default delegated debugfs access or a privileged service exposing the interface. That possibility is not established by the supplied patch or call path and is not used as an ActionableScore positive signal.

3. Reachability analysis

The relevant attacker model is a local host administrator or a process with equivalent access to the privileged page_owner debugfs interface. The vulnerable function is reached while formatting page-owner diagnostic data, not through normal page allocation or normal user memory operations alone.

The complete trigger requires page-owner support to be enabled and the page_owner diagnostic file to be accessible. A separate concurrent task can create allocation and freeing pressure that makes page reuse possible, but that does not independently grant access to the diagnostic reader.

The relevant privilege is not namespace-scoped for the complete operation. A user namespace or network namespace does not provide access to the host page_owner debugfs file. No network exposure, special hardware, device input, or firmware-mediated trigger is involved.

A debug-oriented build configuration is also required. CONFIG_DEBUG_VM is explicitly needed for the VM_BUG_ON assertion behavior described in the commit. Page-owner diagnostics are similarly not a normal default production exposure.

4. Primitive and severity interpretation

The exact primitive is a lockless TOCTOU read of page memcg_data while the page can transition from one allocation role to another. The first memcg_data snapshot can describe an ordinary page, while later lockless checks observe the same struct page after reuse as a THP tail page or a slab page.

This is not active structured-object UAF and not fixed stale storage UAF. The patch does not establish that a freed object is dereferenced after its lifetime ends. Instead, it establishes inconsistent interpretation of reused page state that reaches CONFIG_DEBUG_VM assertions.

The demonstrated consequence is a local kernel BUG or panic on debug-enabled kernels. A speculative stronger consequence such as privilege escalation is not supported. No attacker-controlled overwrite, stale-object replacement, active-object field consumption after reuse, information leak, or authorization bypass is shown.

The issue is therefore CLASS_C. ImportantCandidate is NO because the established behavior is assertion-triggered DoS in a privileged diagnostic path with debug configuration requirements. AutoDowngradeAllowed is YES because no unresolved meaningful kernel-corruption or security-boundary primitive remains after excluding the crash.

Likely operational severity is low. Systems without page-owner diagnostics, without CONFIG_DEBUG_VM, or without privileged access to page_owner are not practically exposed to the documented crash path.

5. One-sentence report phrase

A privileged local reader of the page_owner debugfs interface can race page reuse with memcg diagnostic formatting and trigger a CONFIG_DEBUG_VM kernel assertion resulting in a local denial of service.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a debug-only TOCTOU assertion crash with root-restricted diagnostic access and no demonstrated UAF, write primitive, disclosure, policy bypass, or privilege-escalation mechanism.

7. Call-site confidence

Call-site confidence: high

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

Patch: mm/page_owner: use memcg_data snapshot to avoid TOCTOU in print_page_owner_memcg()
Commit: 46761406e14381dc35f498c247a87565b87ea2ef
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=46761406e14381dc35f498c247a87565b87ea2ef

Commit description:

print_page_owner_memcg() reads page->memcg_data via READ_ONCE() at the
start to guard against tail pages and NULL data.  However, it later
re-reads page->memcg_data locklessly in two places:

1: page_memcg_check(page)

2: PageMemcgKmem(page) (via folio_memcg_kmem(), which includes
   VM_BUG_ON assertions for tail pages and MEMCG_DATA_OBJEXTS)

If the page is concurrently freed and reallocated as a THP tail page or
slab page between these calls, the VM_BUG_ON assertions can trigger on
CONFIG_DEBUG_VM=y builds, crashing the kernel.

Fix both TOCTOU issues by using the memcg_data snapshot throughout.

Link: https://lore.kernel.org/20260714015117.78351-10-ye.liu@linux.dev
Fixes: fcf8935 ("mm/page_owner: print memcg information")
Signed-off-by: Ye Liu <ye.liu@linux.dev>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
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_owner.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=46761406e14381dc35f498c247a87565b87ea2ef

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

The original Linux kernel CVE announcement for CVE-2026-93216 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93216

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:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 0

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).