[CVE-2026-90335][LOW] tty: skip cdev_del() when no cdev is registered [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-90335
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: tty: skip cdev_del() when no cdev is registered [ Upstream
Commit: 3dbf85ef40b6366ba15e882f7536cdc98bea579a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3dbf85ef40b6366ba15e882f7536cdc98bea579a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90335
Analysis date: Thu, 17 Sep 2026 16:55:39 -0400
ActionableScore: 2
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-2026-90335 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90335

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-2026-90335	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90335 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:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-703;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A NULL pointer dereference can occur in tty_unregister_device because cdev_del() may be called with a NULL cdev slot after TTY device registration failed before a cdev was allocated. The serial core may keep the port available for later operations, and a later device removal can reach the unregister path with driver cdevs index still NULL. For the CVSS the PR:L is used in the paranoid score because some environments may delegate device management, reset, bind, or unbind operations to a local service or reduced capability root context. The typical practical trigger still requires local control over the TTY device lifecycle or a driver error path during registration and removal. The issue is not network reachable. Impact is denial of service via kernel crash, with no clear UAF, OOB access, information disclosure, or privilege escalation primitive indicated by the code change.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	PRINTF-TTY SIMPLEFIX HARDWARE LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score.

2. Signal breakdown

Positive signals:

* +1: Invalid object pointer / NULL pointer dereference path. tty_unregister_device() can pass a NULL driver->cdevs[index] pointer to cdev_del().
* +1: Reliable kernel crash / DoS. The commit explicitly states that the NULL cdev slot causes a NULL-pointer dereference.

Negative signals:

* 0: No host-admin penalty applied because host-admin-equivalent privilege is likely in practical device lifecycle scenarios, but not fully proven by the supplied evidence.
* 0: No race penalty. This is an ordered error/lifecycle state issue, not a timing race.
* 0: No rare hardware penalty. TTY/serial is common enough, and the patch does not establish a rare physical-only prerequisite.

Important non-awarded signals:

* RealUAF not awarded. The bug is a NULL pointer dereference, not post-free use.
* Concrete repeated release/refcount not awarded. There is no first release that may end lifetime followed by a second lifetime mutation on the same dangling reference.
* Structured stale-object leverage not awarded. No stale active object reuse or attacker-shaped replacement is shown.
* Weak LPE not awarded. The patch supports crash-only behavior.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity not awarded.
* Remote reachability not established.
* Extra availability point not awarded because the same kernel crash is already counted once.

3. Reachability analysis

The vulnerable path is inside TTY device unregister handling after an earlier TTY device registration failure before cdev allocation. A realistic trigger likely requires local influence over device registration and later removal, such as driver/device lifecycle control, bind/unbind, reset, or a failing serial/TTY registration path.

The supplied evidence does not prove that an ordinary unprivileged user can complete the trigger. It also does not prove that host-admin-equivalent privilege is strictly required in all cases. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.

No namespace-scoped privilege path is established. This is not a network-triggered issue. No prior special sysctl, boot option, or rare hardware prerequisite is established from the supplied input. This is not a race, so race repeatability is not relevant.

4. Primitive and severity interpretation

The exact primitive is a NULL pointer dereference in tty_unregister_device() when driver->cdevs[index] is NULL but cdev_del() is still called.

This is not an active structured UAF and not fixed stale storage. It is a crash-oriented invalid-state bug in an unregister path. The demonstrated consequence is kernel crash / denial of service. The speculative consequence does not extend to privilege escalation because no UAF, OOB access, controlled write, object replacement, refcount takeover, callback control, disclosure, or authorization bypass is indicated.

PrimitiveClass is CLASS_C because the established behavior is invalid dereference / DoS-only. ImportantCandidate is NO because there is no meaningful unresolved exploitation primitive beyond crash. AutoDowngradeAllowed is YES because the issue appears suitable for ordinary Moderate / Low-like handling and does not preserve an Important-grade primitive.

5. One-sentence report phrase

A failed TTY device registration can leave a NULL cdev slot that is later passed to cdev_del() during unregister, causing a local kernel crash and denial of service without evidence of UAF, information disclosure, or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: crash-only NULL pointer dereference, no memory corruption primitive, no authorization bypass, no network reachability, and no evidence of LPE.

7. Call-site confidence

Call-site confidence: high

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

Patch: tty: skip cdev_del() when no cdev is registered [ Upstream
Commit: 3dbf85ef40b6366ba15e882f7536cdc98bea579a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3dbf85ef40b6366ba15e882f7536cdc98bea579a

Changed files:
  drivers/tty/tty_io.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=3dbf85ef40b6366ba15e882f7536cdc98bea579a

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

The original Linux kernel CVE announcement for CVE-2026-90335 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90335

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:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 2

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).