* [CVE-2026-90236][MODERATE 7.0] NFSD: Release the export reference when reaping open stateids [ Upstream
@ 2026-09-18 3:02 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-18 3:02 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90236
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSD: Release the export reference when reaping open stateids [ Upstream
Commit: 7458727fd7cb79d09e5120b6fecc1ad11cdb0946
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7458727fd7cb79d09e5120b6fecc1ad11cdb0946
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90236
Analysis date: Thu, 17 Sep 2026 23:02:37 -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.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90236 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90236
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-2026-90236 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90236 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-911;CWE-362;*CWE-416;*CWE-401;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'NFSD open and lock stateid teardown can leak references to svc_export because the reap path calls sc_free directly instead of using the normal reference release path. This can pin an export and block unmount operations, resulting in denial of service. A separate race exists because nfs4_put_stid can read sc_export before taking cl_lock while drop_stid_export clears the pointer and releases the reference under that lock. Concurrent execution can therefore release the same export reference twice and may prematurely free svc_export, creating a potential use after free condition with possible confidentiality, integrity, and availability impact. For the CVSS the PR:N value is used for the paranoid score because a network client may create and release NFS stateids without privileges on the server when the export is reachable and permits client access. AC:H is used because the more severe condition depends on a narrow race with concurrent stateid revocation. The issue is network reachable through an exposed NFSD service, although reliable exploitation may require a concurrent administrative or server side revocation event. Impact is at least denial of service through a blocked unmount or kernel memory lifetime failure. In the worst still defensible case the double reference release may result in use after free and privilege escalation or broader host compromise.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) SKIP CVE-2026-90236 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE LOCK DANGER DISK UAF NETWORK LINUS KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* FLOOR 5 conditions are satisfied by the concrete second `exp_put()` on the exact reference after the first release may have ended the export lifetime. The raw conservative score was already 5, so the floor did not change it.
* No cap was applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF or equivalent lifetime violation. `drop_stid_export()` may perform the final `exp_put()` while `nfs4_put_stid()` retains the previously read `sc_export` pointer and later calls `exp_put()` again.
* +1 Concrete repeated release or refcount operation. The first `exp_put()` can end the `svc_export` lifetime, the exact pointer survives in the local `exp` variable, and the second `exp_put()` performs another write-like lifetime mutation.
* +1 Concrete persistent reference leak. The reap-list path bypasses the normal export-reference release, permanently pinning the export.
* +1 Strong availability impact. The leaked reference concretely blocks export unmount, while the double release also provides a credible kernel-fault path.
* Total: 5.
Paranoid-only signal:
* +1 Unresolved post-free refcount corruption. If the first `exp_put()` destroys the export, the second operation can modify lifetime metadata through storage outside the original object lifetime. This is grounded in the concrete second refcount operation, not generic heap-reuse speculation.
* Total: 6.
Repeated-release checks:
* First release can end lifetime: yes.
* Exact same pointer survives: yes, in the local `exp` variable read before `cl_lock`.
* Second operation mutates lifetime or refcount: yes, through `exp_put(exp)`.
* Concrete repeated release bonus awarded: yes, +1.
* Paranoid repeated-refcount leverage awarded: yes, +1.
* FLOOR 5 applicable: yes, but no numeric adjustment was needed.
Not awarded:
* Direct remote reachability +2 was not awarded because an NFS client can influence stateid creation and teardown, but the complete double-release sequence also requires concurrent revocation whose attacker control is unresolved.
* No hard-race penalty was applied. Concurrency is established, but the evidence does not show that attempts are necessarily rare, one-shot, or externally uncontrollable.
* Controlled reclaim, refcount takeover, arbitrary write, callback control, disclosure, and demonstrated privilege escalation are not established.
* Structured stale-object leverage and Weak LPE were not separately awarded because that would duplicate the concrete second refcount mutation.
* No additional Availability point was awarded for a second description of the same kernel-fault consequence.
* Confidentiality and integrity bonuses were not awarded merely because post-free refcount corruption could theoretically affect reclaimed memory.
3. Reachability analysis
An NFSv4 client with access to an exported filesystem can create open and lock stateids and influence their normal teardown. The server must already have NFSD and an export configured, which is prior administrative deployment setup but does not establish that the remote client has host-administrator privileges.
The stronger double-release path requires `nfs4_put_stid()` to overlap with `drop_stid_export()`. The supplied evidence establishes concurrent revocation but does not resolve whether a remote client alone can reliably initiate every required stage or whether a server-side administrative or lifecycle event is necessary. Therefore `HostAdminRequired=UNKNOWN`.
No user-namespace or network-namespace capability chain is supplied, so `NamespaceScopedPrivilege=UNKNOWN`. No special hardware, boot mode, or unusual kernel device is required. Race repeatability is unresolved, and no timing penalty is justified solely from the existence of the race.
4. Primitive and severity interpretation
The patch establishes two defects. The reap-list path leaks an export reference and can indefinitely pin the export. Separately, reading `sc_export` before acquiring `cl_lock` allows revocation to clear the field and release the reference while the old pointer remains in `nfs4_put_stid()`.
This is category B, not an ordinary stale cleanup dereference. The second stale operation is another `exp_put()` that actively mutates the lifetime refcount of a potentially freed `svc_export`. The first put may destroy the object, and the exact saved pointer is then used for the second put.
The primitive is CLASS_B because concrete lifetime corruption exists, but controlled reclaim, directed corruption, and an independently useful read or write primitive are not demonstrated. The established consequences are blocked unmount and possible kernel failure. Post-free refcount mutation and possible privilege escalation remain unresolved rather than demonstrated.
`ImportantCandidate=YES` because automatically reducing this to a resource-leak or crash-only issue would discard the concrete repeated lifetime mutation. `AutoDowngradeAllowed=NO` because meaningful post-lifetime corruption leverage remains unresolved. The likely operational floor is Actionable Moderate, with Important classification requiring manual analysis of `svc_export` destruction, allocator reuse, and remote control over the revocation race.
5. One-sentence report phrase
A race between NFSD stateid teardown and export revocation can release the same `svc_export` reference twice after one release may have destroyed it, while a separate missing release pins exports and blocks unmount.
6. Manual review recommendation
MANUAL CHECK REQUIRED
The exact same export pointer can receive a second lifetime-mutating `exp_put()` after the first put may have ended its lifetime, so post-free refcount effects and attacker control over the revocation sequence require source-level review.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: NFSD: Release the export reference when reaping open stateids [ Upstream
Commit: 7458727fd7cb79d09e5120b6fecc1ad11cdb0946
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7458727fd7cb79d09e5120b6fecc1ad11cdb0946
Changed files:
fs/nfsd/nfs4state.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=7458727fd7cb79d09e5120b6fecc1ad11cdb0946
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90236 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90236
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:L/UI:N/S:U/C:L/I:L/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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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