[CVE-2026-64418][MODERATE 7.0] mm: shrinker: fix shrinker_info teardown race with expansion

[CVE-2026-64418][MODERATE 7.0] mm: shrinker: fix shrinker_info teardown race with expansion

[AL-KERNEL]

CVE: CVE-2026-64418
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: mm: shrinker: fix shrinker_info teardown race with expansion
Commit: b9a280a9a454ed514636351d53fe2a233dc5054b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b9a280a9a454ed514636351d53fe2a233dc5054b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64418
Analysis date: Sat, 25 Jul 2026 11:37:50 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in `mm/shrinker` can free a published `shrinker_info` while `expand_shrinker_info()` still observes it, creating a local UAF or double-free condition that can crash the kernel and may warrant manual review for possible memory-corruption impact.

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

The original Linux kernel CVE announcement for CVE-2026-64418 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64418

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-2026-64418	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-362;*CWE-416;CWE-415;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in mm shrinker_info teardown can occur when a partially online memcg is visible to expand_shrinker_info while its published shrinker_info array is being freed on an error path. This can leave expand_one_shrinker_info using an old shrinker_info pointer while teardown frees the same object, creating a use-after-free or double-free condition in kernel memory. For the CVSS the PR:L value is used for the paranoid score because local cgroup or memcg control can be delegated to an unprivileged user, container, or service account in some deployments. The attack is local and not network reachable. AC:H is appropriate because reliable triggering depends on a narrow race window and error or unwind timing during memcg online and shrinker expansion. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality and integrity impact due to kernel memory corruption. For the paranoid score, choose the highest still-defensible interpretation supported by the UAF and double-free bug class, with preference for manual-review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64418 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 307bececcd1205bcb67a3c0d53a69db237ccc9d4 YES  NO NO unknown 	MAYBE	LOCK DANGER INIT RACE UAF HARDWARE LINUS KPANIC MEMORY 	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative scoring:

* +1 Local unprivileged or low-privileged trigger plausible: memcg or cgroup creation can be delegated to containers, service accounts, or non-root users in some deployments.
* +1 Memory corruption, weak race-dependent primitive: the patch explicitly describes UAF or double-free, but no attacker-controlled reclaim, overwrite, callback dispatch, or type confusion is shown.
* +1 Real lifetime corruption: published `shrinker_info` can be freed while still visible to `expand_shrinker_info()`.
* +1 Reliable kernel crash / strong DoS: UAF or double-free in kernel memory can realistically crash the system.
* +1 Broad/default subsystem exposure: memcg and shrinker infrastructure are core MM/cgroup mechanisms on many systems.
* -1 Hard race / special timing: triggering requires a teardown vs expansion race and likely an allocation failure or unwind path timing.

Paranoid adjustment:

* +1 extra by treating the explicit double-free/UAF as a stronger corruption candidate for manual-review sensitivity.
* Paranoid score is capped at 5 because no concrete reclaim, write primitive, type confusion, or LPE chain is demonstrated.

Not awarded:

* No +2 strong LPE plausibility: the patch does not show attacker-controlled reuse, arbitrary write, refcount takeover, callback control, or type confusion.
* No remote/network score: this is a local MM/cgroup lifecycle path.
* No confidentiality/integrity scoring as proven impact in the conservative result.

## 3. Reachability analysis

The bug is locally reachable through memcg/cgroup lifecycle activity combined with shrinker expansion. In strict deployments, creating or managing cgroups may require elevated privileges, but many containerized or delegated-cgroup environments can expose relevant control to lower-privileged users or service accounts. The affected path is not network-triggerable. Realistic exploitation requires a narrow race window involving partially online memcgs, published `shrinker_info`, teardown, and concurrent shrinker expansion, so reliability is uncertain.

Call-site confidence: high, because the commit message provides the relevant interleaving and affected functions.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because the patch explicitly fixes a kernel lifetime race that can lead to UAF or double-free. It is not a clear Important-class issue on the conservative reading because there is no demonstrated reclaim strategy, overwrite primitive, object replacement, or privilege-escalation path. The best practical bucket is Actionable Moderate at minimum, with manual review required due to explicit memory lifetime corruption.

## 5. One-sentence report phrase

A race in `mm/shrinker` can free a published `shrinker_info` while `expand_shrinker_info()` still observes it, creating a local UAF or double-free condition that can crash the kernel and may warrant manual review for possible memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly describes use-after-free and double-free in core kernel memory-management code, even though practical privilege escalation is not demonstrated.

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

Patch: mm: shrinker: fix shrinker_info teardown race with expansion
Commit: b9a280a9a454ed514636351d53fe2a233dc5054b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b9a280a9a454ed514636351d53fe2a233dc5054b

Commit description:

expand_shrinker_info() iterates all visible memcgs under shrinker_mutex,
including memcgs that have not finished ->css_online() yet.

Once pn->shrinker_info has been published, teardown must stay serialized
with expand_shrinker_info() until that memcg is either fully online or no
longer visible to iteration.  Today alloc_shrinker_info() breaks that rule
by dropping shrinker_mutex before freeing a partially initialized
shrinker_info array, which may cause the following race:

CPU0                   CPU1
====                   ====

css_create
--> list_add_tail_rcu(&css->sibling, &parent_css->children);
    online_css
    --> mem_cgroup_css_online
        --> alloc_shrinker_info
            --> alloc node0 info
                rcu_assign_pointer(C->node0->shrinker_info, old0)
                alloc node1 info -> FAIL -> goto err
                mutex_unlock(shrinker_mutex)

                       shrinker_alloc()
                       --> shrinker_memcg_alloc
                           --> mutex_lock(shrinker_mutex)
                               expand_shrinker_info
                               --> mem_cgroup_iter see the memcg
                                   expand_one_shrinker_info
                                   --> old0 = C->node0->shrinker_info
                                       memcpy(new->unit, old0->unit, ...);

                free_shrinker_info
                --> kvfree(old0);

                                       /* double free !! */
                                       kvfree_rcu(old0, rcu);

The same problem exists later in mem_cgroup_css_online().  If
alloc_shrinker_info() succeeds but a subsequent objcg allocation fails,
the free_objcg -> free_shrinker_info() unwind path tears down the already
published pn->shrinker_info arrays without shrinker_mutex.  The
expand_one_shrinker_info() can race with that teardown in the same way,
leading to use-after-free or double-free of the old shrinker_info.

Fix this by serializing shrinker_info teardown with shrinker_mutex, and by
keeping alloc_shrinker_info() error cleanup inside the locked section.

Link: https://lore.kernel.org/20260617085658.27096-1-qi.zheng@linux.dev
Fixes: 307bece ("mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred}")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Muchun Song <muchun.song@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
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/shrinker.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=b9a280a9a454ed514636351d53fe2a233dc5054b

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

The original Linux kernel CVE announcement for CVE-2026-64418 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64418

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:L/UI:N/S:U/C:L/I:L/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: 4
  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: YES
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).