[CVE-2026-89549][MODERATE REGULAR] sunrpc: route to a populated pool in svc_pool_for_cpu()

[CVE-2026-89549][MODERATE REGULAR] sunrpc: route to a populated pool in svc_pool_for_cpu()

[AL-KERNEL]

CVE: CVE-2026-89549
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: sunrpc: route to a populated pool in svc_pool_for_cpu()
Commit: 011479cf9a7657d4a3e7cc42a784ac63df594170
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=011479cf9a7657d4a3e7cc42a784ac63df594170
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89549
Analysis date: Fri, 11 Sep 2026 17:48:49 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
SUNRPC can route an incoming transport to an RPC service pool with no worker threads when fewer threads than pools are configured, causing the connection to hang indefinitely and resulting in network-reachable denial of service for affected NFS or SUNRPC deployments.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-89549 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89549

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 REGULAR
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-2026-89549	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89549 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-754;CWE-703;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'SUNRPC svc_pool_for_cpu could select an RPC service pool that has no worker threads when a service has fewer threads than pools, such as nfsd in pernode or percpu mode on a NUMA host. A transport queued to such a threadless pool is never picked up because RPC worker threads only service their own pool, which can leave client connections hanging indefinitely. For the CVSS the PR:N is used because a remote client does not need local privileges on the server beyond network reachability to the exposed NFS or SUNRPC service. The issue is network reachable but depends on a specific server configuration and CPU or NUMA pool routing, so AC:H is appropriate. Impact is denial of service only through stuck transports and degraded RPC service availability, with no evidence of memory corruption, information disclosure, or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	REMOTE READ RACE NETWORK HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2. No floor or cap applied.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* +2 Remote reachable / directly network-triggerable. A remote RPC/NFS client can cause a transport to be enqueued on the server when the vulnerable SUNRPC service is exposed.
* +1 Availability impact realistic. A transport routed to a pool with no worker threads can remain queued indefinitely, causing hung client connections and degraded RPC service availability.
* -1 Concrete special configuration dependency. The issue requires a service configuration where the number of worker threads is smaller than the number of pools, for example nfsd in pernode or percpu mode on a NUMA host. The remote attacker normally cannot create this server-side topology.

Paranoid signals:

* +2 Remote reachable / directly network-triggerable.
* +1 Availability impact realistic.
* No special-configuration penalty in the paranoid score because the supplied input already describes a concrete affected deployment pattern.

Important non-awarded signals:

* No UAF, double free, OOB access, or memory corruption is established.
* No controlled reclaim, arbitrary write, information disclosure, or privilege escalation is demonstrated.
* No extra Availability point is awarded because the stuck transport and service degradation are the same DoS consequence.
* No resource-exhaustion escalation is awarded because the issue is primarily work routing to a threadless pool, not an accumulating persistent leak.
* No ImportantCandidate signal is awarded because the primitive is logic/DoS-only.

## 3. Reachability analysis

The evaluated attacker is a remote RPC/NFS client connecting to an already exposed SUNRPC service. In the vulnerable deployment, the client does not need local code execution or privileges on the server, so HostAdminRequired=NO for the trigger itself.

There is a prior server-side configuration prerequisite: the service must have more pools than worker threads, such as nfsd with fewer threads than NUMA nodes in pernode or percpu mode. That setup is normally controlled by the server administrator, not by the remote attacker.

Namespace-scoped privilege is not relevant to the supplied trigger path. No race is required. No special hardware is required beyond the deployment topology implied by pool mode and CPU or NUMA mapping.

## 4. Primitive and severity interpretation

The exact primitive is a SUNRPC work-routing logic bug. `svc_pool_for_cpu()` can select a service pool with `sp_nrthreads == 0`. `svc_xprt_enqueue()` then queues the transport on that pool and wakes only a worker from that same pool. Since no worker exists there, the transport is never serviced.

This is CLASS_C because the demonstrated behavior is a stuck connection / service DoS without a kernel corruption primitive. It is not an active structured UAF, not fixed stale storage, and not a repeated release/refcount case.

Demonstrated consequence: indefinite hung RPC/NFS connections and degraded service availability.

Speculative consequence: broader service outage if many transports are routed to empty pools in an affected configuration. There is no patch-supported path to LPE, kernel memory corruption, or disclosure.

ImportantCandidate=NO because there is no unresolved corruption, ownership, authorization, or lifetime primitive. AutoDowngradeAllowed=YES because the established issue is DoS-only and configuration-dependent, although network reachability justifies manual review before closure.

Likely operational severity: Low to Moderate, with higher operational concern for exposed NFS/SUNRPC servers using affected pool/thread configurations.

## 5. One-sentence report phrase

SUNRPC can route an incoming transport to an RPC service pool with no worker threads when fewer threads than pools are configured, causing the connection to hang indefinitely and resulting in network-reachable denial of service for affected NFS or SUNRPC deployments.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network reachable and can cause service-level DoS, but the patch does not establish memory corruption, privilege escalation, information disclosure, or an Important-grade primitive.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: sunrpc: route to a populated pool in svc_pool_for_cpu()
Commit: 011479cf9a7657d4a3e7cc42a784ac63df594170
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=011479cf9a7657d4a3e7cc42a784ac63df594170

Commit description:

svc_set_num_threads() spreads the requested threads evenly across the
service's pools (base = nrservs / sv_nrpools).  When a service runs
fewer threads than it has pools -- e.g. an nfsd configured with fewer
threads than the host has NUMA nodes while running in "pernode" or
"percpu" mode -- the trailing pools are left with no threads at all.

svc_xprt_enqueue() selects a pool from the CPU servicing the transport,
queues the transport on that pool's sp_xprts, and only wakes a thread
from the same pool.  Each thread services exclusively its own pool, so a
transport that lands on a threadless pool is enqueued on sp_xprts and
never picked up: the connection hangs indefinitely.

Have svc_pool_for_cpu() skip pools that currently have no threads,
falling back to the next populated pool.  This trades NUMA locality for
a guarantee that the work is actually serviced.  sp_nrthreads is only
updated under the service mutex; the lockless read here is a best-effort
routing hint, so annotate it with data_race().

Fixes: bfd2416 ("[PATCH] knfsd: make rpc threads pools numa aware")
Cc: stable@vger.kernel.org
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260706-sunrpc-pool-mode-v5-1-6c4ee7cd89aa@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=011479cf9a7657d4a3e7cc42a784ac63df594170

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

The original Linux kernel CVE announcement for CVE-2026-89549 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89549

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:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 2
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).