* [CVE-2026-89547][MODERATE 7.0] SUNRPC: Check svc pool percpu counter allocation
@ 2026-09-12 0:32 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 0:32 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89547
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: SUNRPC: Check svc pool percpu counter allocation
Commit: 3a2b7649de76376a69f1d3ed2a539fb15907cd4d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3a2b7649de76376a69f1d3ed2a539fb15907cd4d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89547
Analysis date: Fri, 11 Sep 2026 20:32:01 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89547 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89547
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-2026-89547 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89547 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:H/UI:N/S:U/C:H/I:H/A:H';CWE-252;CWE-665;*CWE-787;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '6.4';DESCR 'SUNRPC service creation ignored percpu_counter_init failures for per pool statistics counters. If a percpu allocation fails during RPC server startup, the service can become live with a NULL percpu counter pointer and uninitialized counter internals. Later hot path counter updates may treat the NULL percpu pointer as offset zero in the current CPU percpu area, which can silently corrupt unrelated percpu memory instead of causing a simple NULL pointer fault. For the CVSS the PR:H is used because reliable creation of the broken service requires local administrator control over RPC server startup together with memory pressure or fault injection. The issue is not directly network reachable because a remote peer cannot create the bad service state on its own, although later RPC traffic may exercise the corrupted counters after the local failure condition exists. Impact is at least local denial of service and may include confidentiality or integrity impact due to percpu memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5) YES REMOTE INIT NETWORK HARDWARE KPANIC YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 4. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate**::
2. Signal breakdown
Conservative signals:
* +2: Concrete kernel memory corruption primitive. Failed percpu_counter_init can leave fbc->counters NULL, and later percpu counter updates can resolve that NULL percpu pointer as offset zero in the current CPU percpu area, corrupting unrelated percpu storage.
* +1: Reliable strong DoS concern. The corrupted or uninitialized counter state can lead to splats under CONFIG_DEBUG_SPINLOCK or lockdep, garbage pool_stats output, and plausible kernel instability.
* +1: Active hot-path corruption surface. The bad counter is not only passive metadata. It is reached from svc_xprt_enqueue, svc_handle_xprt, and svc_pool_wake_idle_thread after the broken service becomes live.
* +1: Post-setup runtime exercise is meaningful. After the bad service state exists, normal RPC service activity can repeatedly hit the corrupted counter paths, although this is not direct remote creation of the vulnerable state.
* -2: Host-admin-equivalent privilege is positively established. The supplied text says creating the broken service requires local administrator control during RPC server startup under memory pressure or fault injection.
Paranoid additional signal:
* +1: Unresolved active percpu corruption leverage. The corrupted target is an unrelated percpu variable at offset zero, not a simple guaranteed NULL dereference, so the exact security consequence is build and layout dependent and should not be treated as automatically harmless.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Local unprivileged trigger not demonstrated.
* Direct remote reachability not awarded because a remote peer cannot create the bad service state on its own.
* Confidentiality and integrity bonuses not separately awarded because no concrete protected data disclosure or attacker-directed protected-state modification is established.
* UAF and repeated-release rules do not apply because this is not a stale lifetime or double-put pattern.
* Extra Availability was not awarded beyond the strong DoS point for the same failure mode.
* CLASS_A was not used because the corruption target and payload are constrained and no controlled replacement, arbitrary R/W, callback control, or LPE chain is established.
3. Reachability analysis
The complete trigger requires RPC server creation while percpu_counter_init fails. The supplied evidence explicitly states that this requires a local administrator under memory pressure or fault injection. Therefore HostAdminRequired=YES.
A remote RPC peer cannot independently induce the initial broken svc_serv state. Remote traffic may later exercise the hot paths once the broken service is already live, but that does not make the vulnerability directly network-triggerable.
No namespace-scoped privilege path is established. There is no evidence that an attacker can create the relevant nfsd, lockd, or NFS callback service and force the percpu allocation failure using only attacker-obtainable namespace privileges.
No special hardware is required. The issue depends on SMP percpu allocation failure and SUNRPC service startup state, not on a rare physical device. This is also not a race in the usual timing-window sense.
4. Primitive and severity interpretation
The exact primitive is unchecked allocation failure leading to an initialized-looking but invalid percpu_counter. The NULL percpu backing pointer is later used by active SUNRPC hot paths and can corrupt unrelated percpu memory rather than faulting immediately.
This is not a UAF. It is also not a simple crash-only NULL dereference, because the described behavior is silent per-CPU memory corruption at offset zero. The object is active kernel service state, and the stale or invalid counter is consumed during normal service operation.
PrimitiveClass=CLASS_B because there is a real kernel corruption primitive requiring exploitation-leverage analysis, but it is constrained by host-admin setup, allocation-failure prerequisites, fixed/low-control target semantics, and lack of demonstrated arbitrary write or LPE.
ImportantCandidate=NO because the complete trigger is positively host-admin-gated and no below-host-admin boundary bypass, exploit chain, or controlled corruption primitive is supplied. ManualReview=REQUIRED and AutoDowngradeAllowed=NO because concrete kernel memory corruption exists and the exact percpu target consequences are unresolved.
Likely operational severity is Actionable Moderate for triage: not an ordinary harmless resource bug, but also not an Important candidate without a less-privileged trigger or stronger exploitation evidence.
5. One-sentence report phrase
SUNRPC service creation can ignore percpu_counter_init failures and leave live RPC service counters with NULL percpu backing storage, causing later hot-path counter updates to corrupt unrelated percpu memory and potentially crash the kernel, but creating this state requires local administrator control during RPC server startup under allocation failure conditions.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Concrete percpu memory corruption is established, and the corrupted target may be an unrelated active percpu variable rather than a guaranteed immediate fault, so automatic downgrade would risk losing a real kernel-corruption issue.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: SUNRPC: Check svc pool percpu counter allocation
Commit: 3a2b7649de76376a69f1d3ed2a539fb15907cd4d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3a2b7649de76376a69f1d3ed2a539fb15907cd4d
Commit description:
__svc_create() initializes three per-pool percpu_counter stats and
ignores every return value. On SMP, percpu_counter_init() fails when
__alloc_percpu_gfp() cannot satisfy the allocation, leaving the failed
counter with fbc->counters == NULL and its embedded raw_spinlock_t,
list_head, and count never initialized. __svc_create() returns the
half-constructed svc_serv to nfsd, lockd, or the NFS callback service
anyway.
Once that service is live, the hot-path increments in
svc_xprt_enqueue(), svc_handle_xprt(), and
svc_pool_wake_idle_thread() reach a counter whose backing pointer is
NULL. The pointer is a per-cpu offset, so the access does not fault:
it resolves to offset zero of the current CPU's per-cpu area and
silently corrupts whatever variable lives there. A
/proc/fs/nfsd/pool_stats read walks the same NULL per-cpu storage and
returns garbage, and on CONFIG_DEBUG_SPINLOCK or lockdep it splats on
the never-initialized lock.
Creating the broken service requires a percpu allocation failure during
RPC server startup, so it is reachable only by a local administrator
under memory pressure or fault injection; a remote peer cannot induce
the bad state on its own.
Check each percpu_counter_init() return value in __svc_create() and
fail when an allocation fails, unwinding the counters already set up
in the current pool and in every pool initialized before it. A
discrete percpu_counter_destroy() per counter at teardown frees each
per-cpu allocation exactly once.
Fixes: ccf08be ("SUNRPC: Replace pool stats with per-CPU variables")
Cc: [email protected]
Reviewed-by: Jeff Layton <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Chuck Lever <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
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=3a2b7649de76376a69f1d3ed2a539fb15907cd4d
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89547 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89547
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:H/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 6.4
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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