[CVE-2026-89545][IMPORTANT] sunrpc: defer rq_argp and rq_resp free until after RCU grace period

[CVE-2026-89545][IMPORTANT] sunrpc: defer rq_argp and rq_resp free until after RCU grace period

[AL-KERNEL]

CVE: CVE-2026-89545
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: sunrpc: defer rq_argp and rq_resp free until after RCU grace period
Commit: 08bc49e0541260bf294962cf889a32a2d2713ad6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=08bc49e0541260bf294962cf889a32a2d2713ad6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89545
Analysis date: Sat, 12 Sep 2026 01:10:51 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A SUNRPC RCU lifetime bug frees rq_argp and rq_resp before all RCU readers of the parent svc_rqst have drained, allowing a local status dump reader to dereference freed request data and causing at least kernel crash DoS with unresolved UAF exploitation risk.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-89545 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89545

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: IMPORTANT
Manual review required: YES

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

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89545	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89545 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-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the SUNRPC service request teardown path because rq_argp and rq_resp were freed with kfree while the parent svc_rqst object remained visible to pre existing RCU readers until the grace period ended. An RCU reader such as nfsd_nl_rpc_status_get_dumpit can still traverse the service thread list after list_del_rcu and dereference rqstp rq_argp after it has already been freed, which can cause a kernel crash or stale memory exposure. For the CVSS the PR:L is used for the paranoid score because a local user context is needed to reach the status dump path, while reliable triggering also depends on racing with service thread teardown. The issue is not directly network reachable through NFS traffic alone. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to a kernel use-after-free, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	REMOTE INIT UAF NETWORK LINUS KPANIC 	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=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: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

Applied conservative calibration: RCU referenced-object lifetime mismatch with real UAF and host-admin requirement not positively established raises 4 -> 5.
Applied paranoid calibration: unresolved active RCU reader behavior over freed referenced buffers raises 5 -> 6.

## 2. Signal breakdown

Conservative signals:

* +2 Real UAF / lifetime violation: `rqstp` remains visible to pre-existing RCU readers, while `rqstp->rq_argp` and `rqstp->rq_resp` are freed synchronously.
* +1 Concrete RCU referenced-object lifetime mismatch: container object `svc_rqst` is RCU-protected, but referenced objects `rq_argp` and `rq_resp` were freed before the same grace period.
* +1 Reliable kernel crash / strong DoS: an RCU reader such as `nfsd_nl_rpc_status_get_dumpit()` can dereference already freed memory.
* 0 No race penalty: this is an RCU lifetime invariant violation, not merely a speculative hard timing race. The reader window is explicitly described.
* Conservative raw total: 4.
* Conservative floor/calibration: 4 -> 5 due to concrete RCU referenced-object UAF where host-admin-equivalent privilege is not positively established.

Paranoid additional signal:

* +1 Paranoid unresolved active RCU reader concern: the stale objects are request/response buffers reached through an active service request object and consumed by a status dump reader. Exact downstream field use is not fully visible, so exploitation leverage beyond crash requires manual review.
* Paranoid raw total before calibration: 5.
* Paranoid calibration: 5 -> 6 for active RCU referenced-object lifetime mismatch with unresolved post-lifetime reader behavior.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Local privilege escalation not demonstrated.
* Remote reachability through NFS traffic alone is not established.
* Confidentiality and integrity points are not awarded as direct signals because no concrete read-back of sensitive freed data or unauthorized modification primitive is proven.
* Concrete repeated release/refcount signal is not awarded. The bug is a premature `kfree()` followed by stale dereference, not a second lifetime/refcount mutation on the same dangling reference.
* Structured stale-object leverage is not separately awarded beyond the RCU calibration, because no distinct controlled replacement or attacker-shaped field consumption is shown.
* Extra Availability is not awarded because the same crash is already counted once.

## 3. Reachability analysis

The vulnerable reader identified in the commit is `nfsd_nl_rpc_status_get_dumpit()`, which suggests a local control or status query path rather than direct packet-triggered exploitation. The teardown side occurs when an SUNRPC service thread exits and `svc_exit_thread()` removes the request object from the thread list using `list_del_rcu()` before `svc_rqst_free()`.

The complete privilege boundary for invoking the relevant NFSD/SUNRPC status dump operation is not fully established by the supplied patch. Therefore `HostAdminRequired=UNKNOWN`, not `YES` or `NO`. No evidence proves that the complete trigger chain is available through attacker-created namespaces, so `NamespaceScopedPrivilege=UNKNOWN`.

Remote NFS traffic may influence service activity or request contents, but the supplied evidence does not establish that remote packets alone can trigger the stale RCU reader path. The evaluated attacker model is therefore local or unresolved local-control-plane access plus concurrent service-thread teardown.

No special hardware requirement is present. The race is an RCU grace-period lifetime window explicitly created by freeing child objects before all legitimate readers of the parent object have drained.

## 4. Primitive and severity interpretation

The exact primitive is a real RCU use-after-free: `rq_argp` and `rq_resp` are freed while `rqstp` remains legitimately observable by RCU readers. This is not fixed stale storage only, because the parent `svc_rqst` is an active structured service request object exposed through an RCU-visible thread list.

This is not a concrete repeated release/refcount case. The first lifetime operation is `kfree(rq_argp/rq_resp)`, and the later stale operation is a dereference by an RCU reader, not a second `put`, `release`, `free`, or refcount mutation on the same object.

Primitive class is `CLASS_B`: a real kernel lifetime primitive exists, but controlled reclaim, arbitrary read/write, callback control, or demonstrated LPE is not established. Demonstrated consequence is kernel crash or stale-object access during status dumping. Speculative consequence is stronger exploitation through post-lifetime memory reuse, but that is not proven.

`ImportantCandidate=YES` because this is a concrete RCU lifetime bug involving an active kernel object and referenced buffers, with unresolved exploitation leverage beyond a simple NULL dereference. `AutoDowngradeAllowed=NO` because automatically closing it as ordinary Moderate would discard a real post-lifetime RCU primitive that still needs Important-vs-Moderate review.

Likely operational severity: actionable Moderate at minimum, with Strong Important review justified if downstream reader behavior or allocator reuse makes stale data exposure or corruption practical.

## 5. One-sentence report phrase

A SUNRPC RCU lifetime bug frees rq_argp and rq_resp before all RCU readers of the parent svc_rqst have drained, allowing a local status dump reader to dereference freed request data and causing at least kernel crash DoS with unresolved UAF exploitation risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real RCU use-after-free in kernel service-request lifetime management, and the post-lifetime reader behavior is not sufficiently constrained to safely auto-downgrade.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: sunrpc: defer rq_argp and rq_resp free until after RCU grace period
Commit: 08bc49e0541260bf294962cf889a32a2d2713ad6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=08bc49e0541260bf294962cf889a32a2d2713ad6

Commit description:

svc_rqst_free() frees rqstp->rq_argp and rqstp->rq_resp synchronously
via kfree(), but defers the rqstp struct free via kfree_rcu().  After
svc_exit_thread() calls list_del_rcu() and svc_rqst_free(), there is
a window where RCU readers that started before list_del_rcu() can still
traverse the thread list and find the rqstp.  These readers (e.g.
nfsd_nl_rpc_status_get_dumpit()) dereference rqstp->rq_argp, which has
already been freed — a use-after-free.

Fix this by moving the kfree of rq_argp and rq_resp into an explicit
call_rcu() callback alongside the struct free.  Resources not accessed
by RCU readers (bvec, buffer pages, scratch folio, auth_data) remain
synchronously freed.

Fixes: 8124438 ("sunrpc: add a rcu_head to svc_rqst and use kfree_rcu to free it")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260611-nfsd-testing-v2-4-5b90e276f2d9@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sunrpc/svc.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=08bc49e0541260bf294962cf889a32a2d2713ad6

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

The original Linux kernel CVE announcement for CVE-2026-89545 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89545

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: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: 5
  Paranoid ActionableScore: 6

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).