[CVE-2026-64419][LOW] mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show() [ Upstream

[CVE-2026-64419][LOW] mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64419
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show() [ Upstream
Commit: de5f69b8dae8698ac5e48dfcd30017887cdf4e5a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=de5f69b8dae8698ac5e48dfcd30017887cdf4e5a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64419
Analysis date: Sat, 25 Jul 2026 15:09:27 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
Reading a memcg-aware shrinker debugfs count file can call a sleeping zswap memcg stats path while an outer RCU read-side section is held, causing an invalid-context warning and possible local DoS only on configurations where such warnings are fatal.

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

The original Linux kernel CVE announcement for CVE-2026-64419 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64419

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-2026-64419	LOW	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:N/I:N/A:L';CWE-667;CWE-703;CWE-362;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '3.3';DESCR 'Reading the shrinker debugfs count file can call a memcg aware shrinker count callback while an outer RCU read side section is held. The zswap count path may flush memcg stats through css_rstat_flush, which can sleep and therefore triggers a sleeping function called from invalid context warning. For the CVSS the PR:L is used in the paranoid score because some deployments may expose debugfs reads to a less privileged local service account or reduced capability root, although typical systems restrict this interface to administrators. The issue is local only and is not network reachable. Impact is limited to availability through warnings or possible local DoS in debug or panic_on_warn configurations, with no supported confidentiality or integrity impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-64419 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	LOCK DANGER DEBUGFS LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2. Reading shrinker debugfs files normally requires privileged debugfs access.
* WARN/invalid-context condition: +1. The bug triggers `BUG: sleeping function called from invalid context`, but the patch does not show UAF, OOB access, double free, arbitrary write, or a reliable crash primitive.
* Availability impact limited: +1. Availability impact is plausible mainly on systems with `panic_on_warn` or debug-heavy configurations.

Conservative total floors to 0.

Paranoid signals:

* Local reduced-privilege trigger: +1. Some deployments may expose debugfs reads to a reduced-privilege service account or container-like administrative context.
* Availability impact realistic in special configurations: +1. If warnings are fatal, reading the file can become a local DoS trigger.
* No memory corruption bonus. The commit explicitly explains why object lifetime remains safe without the outer RCU lock.
* No confidentiality or integrity impact. There is no disclosure, overwrite, stale callback, refcount misuse, or object replacement primitive.

Paranoid total: 2.

## 3. Reachability analysis

The affected path is reached by reading the debugfs `count` file for a memcg-aware shrinker. In normal deployments, debugfs is restricted to root or administrative users, so the conservative interpretation is privileged local only. A paranoid interpretation can consider systems where debugfs is exposed to a reduced-privilege local service account, but this is not the usual security model.

The path is not network reachable. Containers and namespaces do not normally make this interface safely unprivileged unless the host has explicitly exposed debugfs or delegated relevant access. The issue also depends on a shrinker callback such as zswap invoking a memcg stats flush that may sleep.

Call-site confidence: high. The patch and commit describe the direct call chain and the lifetime reasoning for both `mem_cgroup_iter()` and the debugfs-open shrinker object.

## 4. Severity interpretation

This behaves like a Low-like or ordinary Moderate issue, not an Important-class vulnerability. The theoretical risk is an invalid sleep inside an RCU read-side critical section causing a warning and possible DoS in strict configurations. Realistic exploitation evidence does not show memory corruption, privilege escalation, confidentiality impact, or integrity impact.

The patch is a correctness fix for invalid locking context rather than a fix for a concrete UAF or attacker-controlled corruption primitive.

## 5. One-sentence report phrase

Reading a memcg-aware shrinker debugfs count file can call a sleeping zswap memcg stats path while an outer RCU read-side section is held, causing an invalid-context warning and possible local DoS only on configurations where such warnings are fatal.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for urgent security triage because it is local, usually privileged, debugfs-based, and has no supported memory corruption or privilege-escalation primitive.

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

Patch: mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show() [ Upstream
Commit: de5f69b8dae8698ac5e48dfcd30017887cdf4e5a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=de5f69b8dae8698ac5e48dfcd30017887cdf4e5a

Changed files:
  kernel/cgroup/rstat.c
  mm/shrinker_debug.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=de5f69b8dae8698ac5e48dfcd30017887cdf4e5a

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

The original Linux kernel CVE announcement for CVE-2026-64419 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64419

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:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 3.3

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: 0
  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).