* [CVE-2026-90209][LOW] s390/debug: Fix deadlock during unregister [ Upstream
@ 2026-09-18 2:27 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-18 2:27 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90209
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: s390/debug: Fix deadlock during unregister [ Upstream
Commit: a214c3dacc779b2afcb0ac9c8001e45ee4ffb2ed
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a214c3dacc779b2afcb0ac9c8001e45ee4ffb2ed
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90209
Analysis date: Thu, 17 Sep 2026 22:27:06 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90209 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90209
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-2026-90209 LOW https://www.kernelcve.org/list/?q=CVE-2026-90209 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:N/I:N/A:H';*CWE-833;CWE-667;CWE-362;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.1';DESCR 'A deadlock can occur in the s390 debug subsystem when a debugfs file is written while the associated debug area is being unregistered. The unregister path holds debug_mutex and waits for debugfs file users to drain, while the write path can hold a debugfs active reference and then block trying to take the same debug_mutex. This can leave module removal or debugfs operations stuck and cause a local denial of service. For the CVSS the PR:H is selected because reliable triggering normally requires privileged control over debugfs entries and module removal through rmmod or an equivalent administrative operation. The issue is not network reachable. Impact is availability only with no evidence of information disclosure or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0) YES WRITE OOB DEADLOCK DANGER INIT SIMPLEFIX HARDWARE DEBUGFS LINUS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 0
* Conservative score: 0
* Raw paranoid score: 0
* Paranoid score: 0
* Final recommended bucket: **Ordinary Moderate**::
No floor was applied. No cap was needed. Arithmetic uses +1 for a demonstrated local deadlock / availability issue and -2 because the complete reliable trigger requires host-admin-equivalent module removal such as rmmod.
2. Signal breakdown
Positive signals:
* +1: Local denial of service through a real lock-order deadlock. The unregister path holds debug_mutex and waits in debugfs_remove for active debugfs users, while the write path holds a debugfs active reference and waits on debug_mutex.
* +0: No kernel memory corruption is established.
* +0: No UAF, double free, refcount corruption, stale object reuse, or repeated lifetime mutation is established.
* +0: No confidentiality or integrity primitive is established.
* +0: No network reachability is established.
Negative signals:
* -2: Host-admin-equivalent privilege is positively established for the complete reliable trigger because rmmod or equivalent module unregister control is required.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Remote reachability not established.
* UAF not established.
* Concrete repeated release/refcount mutation not established.
* Extra availability point not awarded because the same deadlock is the only demonstrated impact.
* C/I not awarded because there is no disclosure, memory corruption, authorization bypass, or protected object modification.
3. Reachability analysis
The demonstrated trigger requires a concurrent debugfs write to an s390dbf file and unregistering the associated debug area. The example uses writing to a vmur debugfs file while running rmmod vmur.
The write side may depend on debugfs file permissions, but the unregister side requires module removal or an equivalent host administrative operation. The complete reliable trigger is therefore host-admin-equivalent.
There is no evidence that this operation is namespace scoped. There is no network trigger. The issue is architecture/subsystem specific to s390 debug support, but no separate rare-hardware penalty is needed because the host-admin requirement already strongly reduces practical attacker reachability.
The race is a specific concurrent ordering, but no additional race penalty is applied because the host-admin privilege requirement already dominates and the issue is not being scored as an exploitable memory corruption race.
4. Primitive and severity interpretation
The exact primitive is a lock-order deadlock between debugfs_remove waiting for active debugfs users and the debugfs write path waiting for debug_mutex. This is Class C: demonstrated crash/DoS/invalid-state behavior without an independently useful corruption primitive.
This is not an active structured UAF and not fixed stale storage. It is a synchronization deadlock. There is no concrete repeated release/refcount operation. There is no first lifetime operation that may free an object followed by a second lifetime mutation through the same dangling reference.
The demonstrated consequence is local availability impact, mainly stuck module removal or stuck debugfs operations. The speculative consequence is broader administrative inconvenience if the stuck task blocks related cleanup, but there is no evidence of privilege escalation, information disclosure, or kernel memory corruption.
ImportantCandidate=NO because no Important-grade primitive is established and the complete trigger requires host-admin-equivalent control. AutoDowngradeAllowed=YES because the bug is a constrained host-admin-triggered deadlock with availability-only impact.
Likely operational severity is low / ordinary Moderate.
5. One-sentence report phrase
A lock-order deadlock in s390 debug unregister can occur when a debugfs file is written while the associated debug area is being removed, causing a local availability issue that normally requires host administrative module removal privileges.
6. Manual review recommendation
NO MANUAL CHECK NEEDED
Reason: availability-only deadlock, no memory corruption, no UAF, no authorization bypass, no network exposure, and complete reliable triggering requires host-admin-equivalent control.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: s390/debug: Fix deadlock during unregister [ Upstream
Commit: a214c3dacc779b2afcb0ac9c8001e45ee4ffb2ed
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a214c3dacc779b2afcb0ac9c8001e45ee4ffb2ed
Changed files:
arch/s390/kernel/debug.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=a214c3dacc779b2afcb0ac9c8001e45ee4ffb2ed
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90209 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90209
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:N/I:N/A:L
The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 4.1
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: unknown
Paranoid ActionableScore: 0
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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