[CVE-2026-89546][MODERATE 7.0] SUNRPC: close backchannel before destroying callback service

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-89546
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: SUNRPC: close backchannel before destroying callback service
Commit: 6debde9e3e6ae21dcca75837b4247b13ca4ea2b8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6debde9e3e6ae21dcca75837b4247b13ca4ea2b8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89546
Analysis date: Sat, 12 Sep 2026 04:55:10 -0400
ActionableScore: 6
ActionableScore lower bound: 5
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-2026-89546 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89546

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89546	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89546 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-362;*CWE-416;*CWE-772;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'A race in the SUNRPC NFS backchannel teardown path can allow a backchannel receive to complete while the callback service is being shut down. The request can be committed and then either leaked when xprt bc_serv is already NULL, or queued on sv_cb_list after callback threads have stopped while the svc_serv object is close to destruction. For the CVSS the PR:N is used because a malicious or compromised NFS peer can send backchannel traffic over an existing transport without local privileges on the victim. The issue is network reachable in deployments where the victim NFS client is connected to an attacker controlled or compromised NFS server, but practical exploitation depends on racing callback service teardown and is not an Internet wide exposure in typical environments. Impact is at least denial of service through leaked requests, reference leaks, stalled teardown, or kernel crash. The paranoid impact treats the stale queueing during service destruction as a plausible use-after-free candidate that may affect confidentiality and integrity and therefore requires manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)  SKIP CVE-2026-89546 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	LOCK DISK INIT RACE LEAK NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF / active lifetime violation. A backchannel request can be queued on `sv_cb_list` after callback threads are stopped while the `svc_serv` is about to be freed.
* +2 Remote reachable / network peer reachable. A malicious or compromised NFS peer can influence backchannel receive traffic over an existing NFSv4.1 backchannel transport.
* +1 Reliable DoS / strong availability concern. The bug can leak committed requests and references, leave stale list state, stall teardown, or crash later callback/backchannel handling.
* +1 Concrete persistent resource/reference leak. The commit explicitly says the committed request is not enqueued or freed and the `xprt_get()` reference is leaked.
* -1 Conservative teardown race timing. The strongest path depends on a receive completing while callback service teardown is in progress.

Paranoid difference:

* The paranoid score withholds the -1 timing reduction because the supplied context does not prove that the race is one-shot or practically unretryable for an attacker controlling the NFS peer or connection behavior.

Important non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary kernel read/write is not demonstrated.
* Callback or function pointer control is not demonstrated.
* Confidentiality and integrity are not awarded as direct ActionableScore signals. The UAF-like stale queueing makes them unresolved, not proven.
* Concrete repeated release/refcount mutation is not awarded. The patch shows a leak and stale queueing risk, not a second `put` or `release` on the exact same dangling reference.
* Structured stale-object leverage is not separately awarded because there is no distinct demonstrated stale field consumption beyond the request being left linked to a service nearing destruction.
* Extra Availability is not awarded separately because the leak/crash/stall effects are already counted as DoS and reference leak.

3. Reachability analysis

The evaluated attacker is a malicious or compromised NFS server or peer that can send backchannel traffic over an already established NFSv4.1 callback/backchannel transport. This does not require local privileges on the victim host, so `HostAdminRequired=NO`.

The issue is not a generic Internet-wide socket bug. Practical exposure depends on the victim being connected to an attacker-controlled or compromised NFS service, or to a trusted storage network where the peer becomes malicious. Prior NFS mount or callback setup is needed, but that is part of the normal deployment model rather than an attacker host-admin prerequisite in this peer-threat model.

No namespace-scoped privilege chain is relevant here, so `NamespaceScopedPrivilege=NO`.

The race occurs during callback service teardown or callback-start failure. The exact timing is nontrivial, but the peer can influence backchannel request arrival. The conservative score treats this as a hard teardown race. The paranoid score treats repeatability as unresolved rather than proven hard.

4. Primitive and severity interpretation

The primitive is a SUNRPC/NFS backchannel lifetime race. `xprt_complete_bc_request()` can commit a request and pass it toward `xprt_enqueue_bc_request()` while the callback service is being shut down. If `bc_serv` is already NULL, the old path can lose the committed request and leak the transport reference. If the producer wins earlier, it can enqueue the request onto `sv_cb_list` after callback threads have stopped, leaving the request attached to a `svc_serv` object that is close to destruction.

This is an active structured lifetime issue, not fixed stale storage. The relevant objects are `rpc_rqst`, `rpc_xprt`, `svc_serv`, and the callback list. The demonstrated consequence is DoS through leaked references, leaked or stranded requests, teardown inconsistency, and possible kernel crash.

This is `CLASS_B`, not `CLASS_A`. The patch supports a real lifetime/stale-object primitive, but it does not establish controlled reclaim, directed corruption, arbitrary read/write, callback hijack, or demonstrated privilege escalation.

ImportantCandidate is YES because this is a network-peer-reachable active-object lifetime race involving objects linked to a service being destroyed. The post-lifetime behavior is not sufficiently constrained to treat it as ordinary crash-only cleanup.

AutoDowngradeAllowed is NO because automatically closing it as ordinary Moderate would discard unresolved lifetime-corruption leverage in a network-exposed NFS/SUNRPC backchannel path.

5. One-sentence report phrase

A race in SUNRPC NFS backchannel teardown can leave a committed backchannel request leaked or linked to a callback service that is being destroyed, allowing a malicious NFS peer on an existing backchannel transport to cause DoS and requiring manual review for unresolved UAF-style lifetime risk.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-peer reachability plus active structured lifetime corruption around `svc_serv` teardown leaves unresolved UAF-style exploitation leverage, even though controlled reclaim or arbitrary read/write is not demonstrated.

7. Call-site confidence

Call-site confidence: medium

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

Patch: SUNRPC: close backchannel before destroying callback service
Commit: 6debde9e3e6ae21dcca75837b4247b13ca4ea2b8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6debde9e3e6ae21dcca75837b4247b13ca4ea2b8

Commit description:

A backchannel receive can complete a request while the NFS callback
service is being torn down.  xprt_complete_bc_request() removes the
request from bc_pa_list, drops bc_alloc_count, marks the request in use,
and then asks xprt_enqueue_bc_request() to hand it to the callback
service.

If teardown has already cleared xprt->bc_serv, xprt_enqueue_bc_request()
currently returns without enqueueing or freeing the committed request.
The xprt_get() taken on entry is leaked as well.  If the producer wins
the race before bc_serv is cleared, it can also enqueue onto sv_cb_list
after nfs_callback_down() has stopped the callback threads, leaving the
request linked to a svc_serv that is about to be freed.

Close the producer side before callback threads are stopped.  Add
xprt_svc_shutdown_bc() to clear xprt->bc_serv under bc_pa_lock, and call
it on callback shutdown and callback-start failure before stopping the
service threads.  Requests that lose the NULL transition in
xprt_enqueue_bc_request() are released through the normal backchannel
free path after balancing bc_slot_count.  Finally, drain any remaining
sv_cb_list requests after the callback threads have stopped and before
svc_destroy() frees the service.

Fixes: 441244d ("SUNRPC: cleanup common code in backchannel request")
Fixes: 9e9fdd0 ("NFSv4.1: protect destroying and nullifying bc_serv structure")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260528-tier2-v1-6-d026a1415e0b@oracle.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfs/callback.c
  include/linux/sunrpc/bc_xprt.h
  net/sunrpc/backchannel_rqst.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=6debde9e3e6ae21dcca75837b4247b13ca4ea2b8

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

The original Linux kernel CVE announcement for CVE-2026-89546 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89546

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 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).