[CVE-2026-89548][MODERATE 7.0] SUNRPC: always drain cache_cleaner before destroying a cache_detail

[CVE-2026-89548][MODERATE 7.0] SUNRPC: always drain cache_cleaner before destroying a cache_detail

[AL-KERNEL]

CVE: CVE-2026-89548
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: SUNRPC: always drain cache_cleaner before destroying a cache_detail
Commit: 9d44836f60c8c29bcdb1471fd9202387c642a890
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9d44836f60c8c29bcdb1471fd9202387c642a890
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89548
Analysis date: Sat, 12 Sep 2026 04:41:55 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89548 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89548

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-2026-89548	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89548 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:H/I:H/A:H';*CWE-416;CWE-362;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 SUNRPC cache teardown can leave the global cache_cleaner work item running while a per-netns cache_detail is being destroyed. The delayed worker may keep a stale pointer after sunrpc_destroy_cache_detail unlinks the object, and later cache_destroy_net can free it while cache_clean() still references it. This creates a use-after-free in freed kernel memory, including cache_dequeue() taking cd queue_lock and cache_put() reading cd cache_put as a function pointer from freed slab. For the CVSS the PR:L is used for the paranoid score because a local user may be able to create and tear down network namespaces on systems with unprivileged user namespaces or delegated namespace control. The issue is not directly network reachable and requires local namespace or SUNRPC lifecycle activity. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	REMOTE DANGER UAF NETWORK KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	NO	NO	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 4
* Paranoid score: 5
* Applied conservative floor/cap: FLOOR 5 applied because a real active-object UAF consumes a function pointer field from freed slab memory.
* Applied paranoid floor/cap: FLOOR 5 applied for the same established lifetime primitive. No additional speculative paranoid point was added.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Real UAF / active lifetime violation. The commit directly states that cache_destroy_net() can kfree cache_detail while cache_clean() still holds a dangling pointer to it.
* +1 Reliable kernel crash / strong DoS. The stale worker can take cd->queue_lock on freed memory and later dereference freed object state, which realistically supports kernel fault or crash.
* +1 Weak LPE concern / unresolved structured stale-object use. The stale path includes cache_put() reading cd->cache_put as a function pointer from freed slab memory. This is more meaningful than a simple NULL dereference, but controlled reclaim or attacker-controlled callback redirection is not demonstrated.
* FLOOR 5 applied. The UAF is not fixed stale storage only. The stale object is an active structured SUNRPC cache_detail and the stale use can consume a callback-like function pointer field.

Non-awarded or withheld signals:

* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Confidentiality and integrity bonuses not awarded. The established primitive is UAF with possible callback-field consumption, not a demonstrated information leak or unauthorized write.
* Strong exploitation leverage +2 not awarded. The evidence does not show attacker-shaped replacement contents, callback control, refcount takeover, or demonstrated LPE.
* Concrete repeated release/refcount +1 not awarded. The patch shows stale dereference and function-pointer field use, not a concrete second put/release/refcount mutation on the exact same dangling reference.
* Paranoid repeated-refcount +1 not awarded because no second stale lifetime/refcount mutation is established.
* Extra availability point not awarded because the crash/DoS is already counted once.
* Remote reachability +2 not awarded. This is SUNRPC code, but the supplied evidence describes per-netns teardown and delayed work, not direct network packet triggering.
* Race/timing penalty not applied. The issue is a race with delayed work, but the supplied evidence does not prove a narrow one-shot timing window that is externally uncontrollable.
* Host-admin penalty not applied. Host-admin-equivalent privilege is not positively established from the supplied evidence.
* Namespace-scoped unprivileged bonus not awarded. Per-netns teardown is involved, but the complete trigger chain being reachable from attacker-created user/network namespaces is not fully established.

Repeated-release/refcount checklist:

* First release can end lifetime: not applicable to the dedicated repeated-release rule.
* Exact same reference survives for second release: not established.
* Second operation is a real lifetime/refcount mutation: not established.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: yes, but due active structured UAF with function-pointer field consumption, not due repeated release/refcount.

3. Reachability analysis

The reachable path is described as SUNRPC per-netns teardown racing with the global cache_cleaner delayed_work. The attacker model is therefore local lifecycle activity, not direct remote packet input.

HostAdminRequired is UNKNOWN. The supplied evidence does not prove that host root or host-admin-equivalent capability is required for the complete trigger. It also does not prove that an ordinary user can perform the complete trigger below host admin.

NamespaceScopedPrivilege is UNKNOWN. Network namespace teardown is part of the scenario, but the prompt does not establish that all required SUNRPC cache creation, teardown, and delayed-worker interaction can be performed inside an attacker-created namespace without host-admin-equivalent setup.

Prior privileged setup is unresolved. SUNRPC/NFS-related cache objects may depend on system configuration or namespace lifecycle paths, but the supplied evidence does not identify a mandatory host-root-only prerequisite for the vulnerable operation.

No special hardware requirement is established. No direct network reachability is established. The race is between teardown and delayed work, and the evidence supports an asynchronous lifetime bug rather than a purely deterministic local syscall sequence.

4. Primitive and severity interpretation

The exact primitive is a real use-after-free of struct cache_detail. The object is an active structured kernel object, not a fixed stale buffer. The stale use is not limited to a shallow NULL dereference: cache_dequeue() may lock freed memory and cache_put() may read cd->cache_put as a function pointer from freed slab memory.

This remains PrimitiveClass=CLASS_B rather than CLASS_A because the evidence does not establish controlled reclaim, attacker-controlled replacement object contents, arbitrary read/write, callback hijack, refcount takeover, or demonstrated privilege escalation.

The demonstrated consequence is local kernel crash or host DoS. The speculative consequence is possible stronger exploitation if freed cache_detail storage can be meaningfully reclaimed or if the stale function-pointer consumption can be influenced, but that is not proven by the supplied patch.

ImportantCandidate=YES because a real active-object UAF with stale function-pointer field consumption leaves meaningful Important-vs-Moderate exploitation leverage unresolved.

AutoDowngradeAllowed=NO because automatic downgrade would discard an established kernel lifetime-corruption primitive with unresolved function-pointer-field consumption. This is not merely a memory leak, warning, or ordinary crash-only NULL dereference.

Likely operational severity is Actionable Moderate at minimum, with Strong Important review warranted if below-host-admin reachability or controlled reclaim can be established.

5. One-sentence report phrase

A SUNRPC cache teardown race can free a cache_detail while cache_cleaner still uses it, causing a kernel use-after-free where freed object fields including a function pointer may be consumed and requiring manual review beyond ordinary crash-only DoS.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch establishes a real kernel UAF in an active structured object and the stale path can consume a function pointer field from freed slab memory. Controlled exploitation is not demonstrated, but the primitive is too meaningful for automatic downgrade.

7. Call-site confidence

Call-site confidence: high

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

Patch: SUNRPC: always drain cache_cleaner before destroying a cache_detail
Commit: 9d44836f60c8c29bcdb1471fd9202387c642a890
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9d44836f60c8c29bcdb1471fd9202387c642a890

Commit description:

sunrpc_destroy_cache_detail() only cancels the global cache_cleaner
delayed_work when cache_list is empty.  During per-netns teardown
cache_list is never empty because init_net's caches remain registered,
so the cancel never fires.  After unlink, the caller proceeds to
cache_destroy_net() which kfrees the cache_detail while cache_clean()
may still hold a dangling pointer to it.  The result is a
use-after-free: cache_dequeue() takes cd->queue_lock on freed memory,
and cache_put() dereferences cd->cache_put as a function pointer from
freed slab.

Drop the list_empty guard so that cancel_delayed_work_sync() always
runs, ensuring any in-flight cache_clean() completes before the
cache_detail is freed.  Re-arm the cleaner afterwards if other caches
are still registered.

Fixes: 820f944 ("SUNRPC: split cache creation and PipeFS registration")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260526-cache_cleaner_vs_destroy_no_sync-v1-1-a707a6fcfd32@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sunrpc/cache.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=9d44836f60c8c29bcdb1471fd9202387c642a890

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

The original Linux kernel CVE announcement for CVE-2026-89548 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89548

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: unknown
  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).