[CVE-2026-89714][LOW] NFS: fix delegation_hash_table leak when nfs4_server_common_setup() fails

[CVE-2026-89714][LOW] NFS: fix delegation_hash_table leak when nfs4_server_common_setup() fails

[AL-KERNEL]

CVE: CVE-2026-89714
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: NFS: fix delegation_hash_table leak when nfs4_server_common_setup() fails
Commit: f3adf1643517357221422c05986d6de5df7b9913
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f3adf1643517357221422c05986d6de5df7b9913
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89714
Analysis date: Fri, 11 Sep 2026 18:20:45 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89714 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89714

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: LOW
Manual review required: NO

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

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89714	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89714 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.4';DESCR 'A resource leak in the NFSv4 client setup path occurs when delegation_hash_table is allocated but a later setup step fails before the server destroy callback is installed. Each failed NFSv4 mount can leak approximately 4 KiB of unreclaimable kernel memory. Repeated mount retries can exhaust slab memory and cause a host denial of service. For the CVSS the PR:H is used because creating and repeatedly retrying NFS mounts normally requires administrative mount privileges. The issue is not directly network reachable. A remote NFS server can cause a mount setup failure, but an attacker still needs a privileged local mount or retry configuration to sustain the leak. Impact is denial of service through resource exhaustion. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)  SKIP CVE-2026-89714 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	DISK INIT LEAK ERRORPATH NETWORK LINUS 	-	-	checked

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

ActionableScore=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Concrete persistent kernel resource leak. The patch directly establishes that delegation_hash_table is allocated and is not released on several nfs4_server_common_setup failure paths.
* +1 Persistent host resource-exhaustion consequence. The supplied evidence demonstrates repeatable accumulation of unreclaimable slab memory, including observed multi-GiB growth under repeated failed NFSv4 mounts.
* +1 Broadly deployed exposure. NFSv4 client mounting is a broadly used kernel feature and mount retry behavior is realistic in storage and backup deployments.
* -2 Host-admin-equivalent privilege requirement. Creating NFS mounts normally requires CAP_SYS_ADMIN for the relevant host mount operation. The supplied evidence does not establish that an unprivileged attacker can perform the complete NFS mount sequence in an attacker-controlled namespace.

Important non-awarded signals:

* No UAF, stale dereference, double free, refcount misuse, OOB access, or active-object lifetime violation is established.
* No controlled reclaim, arbitrary write, information disclosure, integrity violation, authorization bypass, or privilege escalation primitive is established.
* No remote trigger is established. A remote NFS server can cause a setup failure, but cannot independently cause unlimited mount retries without a locally privileged client configuration or retrying process.
* No extra crash point is awarded. The demonstrated consequence is gradual resource exhaustion rather than an immediate kernel crash.
* No paranoid bonus is awarded. The unbounded leak is already represented by the persistent resource-exhaustion signal, and no stronger corruption mechanism is shown.

3. Reachability analysis

The vulnerable operation is reached by attempting an NFSv4 mount that fails after delegation_hash_table allocation and before server->destroy is assigned. Repeating the failed mount leaks approximately 4 KiB of unreclaimable kernel memory per attempt.

The supplied reproducer uses mount -t nfs4 in a loop. A normal NFS mount operation requires host-administrative mount authority, normally CAP_SYS_ADMIN. NFS client mounts are not established here as a complete attacker-reachable unprivileged user-namespace operation. Therefore HostAdminRequired is YES and NamespaceScopedPrivilege is NO.

A malicious or incompatible remote NFS endpoint can contribute to mount setup failure, for example by being NFSv3-only or rejecting an NFSv4 operation. However, the remote endpoint does not independently control the repeated local mount attempts. Sustained exploitation requires a locally privileged retry loop, mount-management service, backup service, or equivalent configuration.

No special hardware, boot option, race condition, or rare subsystem configuration is required beyond NFSv4 client use and repeated failed mount attempts.

4. Primitive and severity interpretation

The exact primitive is a persistent memory leak caused by a missing kfree of server->delegation_hash_table on intermediate nfs4_server_common_setup error paths. The normal nfs_free_server cleanup path skips nfs4_destroy_server because server->destroy has not yet been initialized.

This is not an active structured-object UAF and not a stale-object dereference. The allocation becomes unreachable after the failed setup path returns. There is no second release, no refcount mutation through freed storage, no use-after-free, and no attacker-controlled post-free state.

The demonstrated consequence is host memory pressure and eventual denial of service after sustained retries. The supplied production observation of approximately 3.4 GiB of unreclaimable slab growth per day confirms that the accumulation is operationally relevant, but it remains a resource-exhaustion issue.

PrimitiveClass is NONE because no meaningful corruption, lifetime, ownership, authorization, or similar security primitive beyond the leak itself is established. ImportantCandidate is NO because there is no unresolved corruption or boundary-bypass mechanism. AutoDowngradeAllowed is YES because the issue is a privilege-gated resource leak without evidence of memory corruption, confidentiality impact, integrity impact, or privilege escalation.

5. One-sentence report phrase

A locally privileged process can repeatedly issue failed NFSv4 mount attempts to leak delegation_hash_table allocations and eventually exhaust unreclaimable kernel slab memory, causing denial of service.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a concrete but privilege-gated resource leak with sustained DoS potential. No UAF, memory corruption, authorization bypass, or privilege-escalation primitive is present.

7. Call-site confidence

Call-site confidence: medium

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

Patch: NFS: fix delegation_hash_table leak when nfs4_server_common_setup() fails
Commit: f3adf1643517357221422c05986d6de5df7b9913
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f3adf1643517357221422c05986d6de5df7b9913

Commit description:

nfs4_server_common_setup() allocates server->delegation_hash_table
first, but server->destroy - the only path that frees the table via
nfs4_destroy_server() - is not assigned until the very end of the
function. If any intermediate step fails (the is_ds_only_client()
check, nfs4_init_session(), nfs4_get_rootfh(), or nfs_probe_server()),
the function returns with server->destroy still NULL, so the caller's
nfs_free_server() skips the destroy callback and the hash table is
leaked (4 KiB per attempt with the default delegation watermark).

This is trivially reachable from userspace: every failed NFSv4 mount
leaks one allocation. A client that persistently retries a mount that
cannot succeed leaks kernel memory without bound. Observed in
production where a Longhorn backup poller retried mount.nfs4 against
an NFSv3-only server roughly 10 times per second, leaking ~3.4 GiB of
unreclaimable slab (kmalloc-rnd-13-4k) per day; the node accumulated
12 GiB of leaked slab before the source was identified via the
kmem:kmalloc tracepoint (call_site=nfs4_delegation_hash_alloc).

Reproducer:

  # server exports NFSv3 only (or export path absent for v4)
  while :; do mount -t nfs4 <server>:/missing /mnt; done
  # watch SUnreclaim in /proc/meminfo grow 4 KiB per iteration

Free the table on the error paths between the allocation and the
assignment of server->destroy.

Fixes: f5b3108 ("NFS: use a hash table for delegation lookup")
Cc: stable@vger.kernel.org
Signed-off-by: Nate Prodromou <nate@prodromou.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfs/nfs4client.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=f3adf1643517357221422c05986d6de5df7b9913

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

The original Linux kernel CVE announcement for CVE-2026-89714 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89714

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.4

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: unknown
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).