[CVE-2026-89695][MODERATE 7.0] nfsd: cap decoded POSIX ACL count to bound sort cost

[CVE-2026-89695][MODERATE 7.0] nfsd: cap decoded POSIX ACL count to bound sort cost

[AL-KERNEL]

CVE: CVE-2026-89695
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: cap decoded POSIX ACL count to bound sort cost
Commit: ea14d71d6ecb925673761bcf79f781f7dc9042cc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea14d71d6ecb925673761bcf79f781f7dc9042cc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89695
Analysis date: Sat, 12 Sep 2026 04:26:25 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
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-2026-89695 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89695

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-2026-89695	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89695 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-400;CWE-770;CWE-407;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'nfsd4_decode_posixacl accepted a client chosen POSIX ACL entry count from the wire without enforcing NFS_ACL_MAX_ENTRIES. The count is used for allocation and then passed to sort_pacl_range which performs O(n2) sorting, so a crafted NFSv4 compound request can force excessive CPU work in the server processing path. For the CVSS the PR:N is used for the paranoid score because an exposed nfsd endpoint may process the malformed request from a network client before any meaningful authorization decision limits the decode cost. The base score uses PR:L because many real deployments require access to an exported filesystem or valid NFS client credentials. The issue is network reachable when NFSv4 POSIX ACL support is enabled and reachable by the attacker. Impact is denial of service through CPU exhaustion or request processing stalls. No concrete memory corruption primitive is indicated by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)  SKIP CVE-2026-89695 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DISK NETWORK HARDWARE KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap changed the conservative score. No floor or cap changed the paranoid score.

2. Signal breakdown

Conservative positive signals:

* +2 Remote reachable / directly network-triggerable: the decoded POSIX ACL count is read from NFS XDR data supplied by a network client to nfsd.
* +1 Reliable strong DoS: a client-controlled count can drive O(n2) CPU cost in sort_pacl_range during compound request processing.

Conservative negative signals:

* No negative privilege penalty: host-admin-equivalent privilege is not required for the remote client side of the trigger.
* No rare-subsystem penalty: NFS server exposure depends on deployment, but the supplied evidence does not establish a genuinely rare or difficult-to-reach configuration.

Paranoid additional signal:

* +1 Additional resource-pressure concern: the uncapped count is also passed to posix_acl_alloc before validation, so large requests may combine CPU amplification with allocation pressure. This is still resource exhaustion, not memory corruption.

Non-awarded signals:

* RealUAF not awarded: no use-after-free or stale lifetime object use is established.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity not awarded: no disclosure, protected-object read, unauthorized write, or ownership violation is established.
* Important-grade exploitation leverage not awarded: this is algorithmic complexity and allocation pressure, not a kernel memory corruption primitive.
* Extra availability beyond the paranoid allocation-pressure point is not awarded.
* Concrete resource/reference leak not awarded: the patch prevents excessive allocation but does not show a persistent leaked reference.
* Security-boundary bypass not awarded: no authorization ALLOW-vs-DENY change is shown.

3. Reachability analysis

The trigger is a network-supplied NFSv4 POSIX ACL decode path in nfsd. A remote NFS client that can reach the server and submit the relevant compound operation can influence the ACL entry count. The conservative interpretation assumes the attacker may need valid NFS client access or credentials for the relevant export, but not host administrator privilege.

HostAdminRequired=NO because the attacker is a remote client, not a local host administrator. NamespaceScopedPrivilege=NO because namespace-scoped local capabilities are not material to this network server path.

There is prior privileged setup in the operational sense that an administrator must run nfsd and expose NFS service, but this is normal service deployment rather than a privilege requirement for the attacker. No special hardware, race, timing window, or local-only interface is required. The vulnerability is not public Internet by default in many environments, but NFS is commonly deployed on internal storage or data-center networks.

4. Primitive and severity interpretation

The exact primitive is attacker-controlled algorithmic complexity plus possible allocation pressure: nfsd4_decode_posixacl accepts an uncapped u32 count from the wire, allocates based on it, and then passes it into an O(n2) sort.

PrimitiveClass=CLASS_C because the demonstrated behavior is DoS/resource exhaustion without an independently useful corruption, lifetime, ownership, authorization, or semantic-reinterpretation primitive.

The demonstrated consequence is denial of service through excessive CPU consumption and request processing stalls. The paranoid consequence is stronger availability impact if allocation pressure combines with the O(n2) sorting cost. No practical privilege escalation, arbitrary kernel read/write, information disclosure, or security-boundary bypass is supported by the supplied patch.

ImportantCandidate=NO because there is no memory corruption, UAF, repeated lifetime mutation, COW ownership violation, or authorization bypass requiring Important-vs-Moderate exploitation review. AutoDowngradeAllowed=YES because the unresolved risk is resource exhaustion only, although manual review is still recommended due to network reachability.

5. One-sentence report phrase

A network client can supply an excessive POSIX ACL entry count to nfsd4_decode_posixacl, causing unbounded O(n2) sorting work and possible allocation pressure in the NFS server compound path, resulting in denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is network reachable in nfsd and may produce high availability impact on exposed NFS servers, but the patch supports resource exhaustion only and does not establish memory corruption or privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: cap decoded POSIX ACL count to bound sort cost
Commit: ea14d71d6ecb925673761bcf79f781f7dc9042cc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ea14d71d6ecb925673761bcf79f781f7dc9042cc

Commit description:

nfsd4_decode_posixacl() reads a u32 entry count off the wire and passes
it straight to posix_acl_alloc() and sort_pacl_range(). The latter is
an O(n^2) bubble sort, so a client-chosen count drives unbounded CPU in
the server's compound processing path.

    nfsd4_decode_posixacl()
      xdr_stream_decode_u32(&count)       /* uncapped u32 */
      posix_acl_alloc(count, GFP_KERNEL)
      sort_pacl_range(*acl, 0, count - 1) /* O(n^2) bubble sort */

The encoder side in the same file already rejects ACLs whose a_count
exceeds NFS_ACL_MAX_ENTRIES, but the decoder introduced in commit
5fc51df ("NFSD: Add support for XDR decoding POSIX draft ACLs")
omitted the symmetric check.

Fix by rejecting a wire count greater than NFS_ACL_MAX_ENTRIES with
nfserr_inval, before any allocation, so the sort is bounded by
NFS_ACL_MAX_ENTRIES^2 comparisons.

While we're in here, also fix the nfserr_resource return if
posix_acl_alloc() fails. That's not a legal error code for v4.1+. Change
it to return nfserr_jukebox as that's more appropriate for memory
allocation failures.

Fixes: 5fc51df ("NFSD: Add support for XDR decoding POSIX draft ACLs")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Chris Mason <clm@meta.com>
Link: https://patch.msgid.link/20260530-nfsd-fixes-v2-8-f27e8eb4d974@kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfs4xdr.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=ea14d71d6ecb925673761bcf79f781f7dc9042cc

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

The original Linux kernel CVE announcement for CVE-2026-89695 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89695

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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: 3
  Paranoid ActionableScore: 4

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).