* [CVE-2026-90334][MODERATE 7.0] tty: clear cdev pointer after cdev_add() failure [ Upstream
@ 2026-09-18 0:53 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-18 0:53 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90334
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tty: clear cdev pointer after cdev_add() failure [ Upstream
Commit: fb1de260033c215dba438973d82b48562bdc86bb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb1de260033c215dba438973d82b48562bdc86bb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90334
Analysis date: Thu, 17 Sep 2026 20:53:00 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
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-2026-90334 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90334
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-2026-90334 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90334 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:H/I:H/A:H';*CWE-416;*CWE-672;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use after free can occur in the tty device registration path when tty_cdev_add drops the cdev reference after cdev_add fails but leaves driver cdevs index pointing to freed memory. A later tty_unregister_device call can pass this stale pointer to cdev_del, which turns the failed registration path into a kernel memory safety issue. For the CVSS the PR:L is used in the paranoid score because a local user may be able to influence failure timing indirectly through memory pressure or delegated device management, although reliable triggering normally requires privileged driver or device lifecycle control. The issue is not network reachable and is triggered through local device registration and unregister paths. Impact is at least local denial of service via kernel crash and in the worst defensible case may include confidentiality and integrity impact or privilege escalation due to kernel UAF memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES PRINTF-TTY LEAK UAF LINUS KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 5
* Applied conservative floor/cap: REAL UAF FLOOR 5 applied because the first kobject_put can end the cdev lifetime, the exact same driver->cdevs[index] reference survives, and tty_unregister_device can later perform a second lifetime operation through cdev_del on that stale reference.
* Raw paranoid score: 5
* Paranoid score: 5
* Applied paranoid floor/cap: no additional floor or cap beyond the same floor 5 calibration.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Positive signals:
* +2 Real UAF / active lifetime violation: tty_cdev_add calls kobject_put after cdev_add failure while leaving driver->cdevs[index] pointing to the potentially freed cdev object. Later tty_unregister_device can use that stale pointer.
* +1 Reliable kernel crash / strong DoS: passing a freed cdev pointer to cdev_del is a realistic kernel memory safety fault path and can lead to crash or KASAN UAF.
* +1 Concrete repeated release/refcount operation: the first kobject_put may drop the final reference. The same exact pointer remains stored in driver->cdevs[index]. A later cdev_del on that pointer is a real lifetime/refcount cleanup operation, not just a passive stale read.
* +1 Paranoid repeated-refcount leverage: the second lifetime operation may mutate refcount or kobject lifetime state through storage whose original cdev lifetime may already have ended. This is unresolved post-lifetime corruption leverage, but not enough for CLASS_A.
Negative signals:
* No -2 host-admin penalty: host-admin-equivalent privilege is not positively established from the supplied evidence. TTY driver lifecycle control is often privileged, but the complete failure plus unregister trigger boundary is unresolved.
* No -1 hard race penalty: the supplied bug is an ordered failure path followed by later unregister, not a narrow timing race.
* No rare hardware penalty: TTY is a common kernel subsystem, and no rare physical device requirement is established.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Local unprivileged trigger not awarded because below-host-admin reachability is not positively established.
* Remote reachability not established.
* Structured stale-object leverage not separately awarded because the meaningful extra fact is already counted as concrete repeated release/refcount.
* Weak LPE not separately awarded for the same repeated-release fact.
* Extra Availability not awarded because the crash is already counted once.
* Confidentiality and integrity points are not awarded as separate ActionableScore signals merely because a UAF could theoretically affect reclaimed memory.
Repeated-release checklist:
* First release can end lifetime: yes, kobject_put can drop the final cdev kobject reference.
* Exact same reference survives: yes, driver->cdevs[index] remains non-NULL in the vulnerable code.
* Second operation is a real lifetime/refcount mutation: yes, cdev_del is a lifetime cleanup path for cdev objects and can put the kobject.
* +1 Concrete repeated release/refcount awarded: yes.
* Paranoid repeated-refcount leverage +1 awarded: yes, but only as unresolved post-free lifetime mutation leverage.
* FLOOR 5 applied: yes.
3. Reachability analysis
The vulnerable sequence is local to the TTY device registration and unregister lifecycle. The trigger requires cdev_add failure during tty_cdev_add and a later tty_unregister_device path that consumes the stale driver->cdevs[index] pointer.
An ordinary local user trigger is not proven. A host administrator or privileged driver/device lifecycle actor can more clearly influence module loading, device registration, and unregister paths. A local user may be able to influence failure conditions indirectly through memory pressure or through delegated device-management scenarios, but that is not enough to mark HostAdminRequired=NO.
Namespace-scoped privilege is not established. The patch does not involve a network namespace, user namespace, or namespace-scoped capability path.
No direct network vector is present. No prior special hardware prerequisite is proven beyond use of a TTY driver path. No timing race is required by the supplied control flow.
4. Primitive and severity interpretation
The exact primitive is a stale cdev pointer retained after kobject_put on cdev_add failure, followed by later use of that stale pointer by tty_unregister_device through cdev_del.
This is an active structured object UAF rather than fixed stale storage. The important operation is not only an ordinary stale cleanup dereference. It is closer to a second lifetime/refcount mutation through a potentially dangling object.
A. Ordinary stale cleanup dereference: not the best classification, because cdev_del is not merely reading a stale field before faulting.
B. Second lifetime/refcount mutation through a potentially dangling object: supported. The first kobject_put can end lifetime, the exact same pointer survives, and a later cdev_del can perform another object lifetime operation.
PrimitiveClass is CLASS_B. The demonstrated consequence is kernel memory safety violation and likely crash or DoS. The speculative consequence is stronger exploitation such as privilege escalation if an attacker can influence allocation reuse or refcount state, but controlled reclaim, arbitrary write, callback control, or demonstrated LPE is not established.
ImportantCandidate=YES because a concrete repeated lifetime/refcount mutation on a potentially freed kernel object leaves meaningful Important-vs-Moderate exploitation leverage unresolved.
AutoDowngradeAllowed=NO because automatic closure would discard a real UAF with a concrete second lifetime operation on the same potentially dangling reference.
Likely operational severity is Actionable Moderate, with manual review required to decide whether this remains crash-only in the evaluated product or becomes an Important-class local kernel vulnerability.
5. One-sentence report phrase
A use-after-free in the TTY cdev registration error path can leave driver cdevs index pointing to a freed cdev after cdev_add failure, allowing later unregister cleanup to call cdev_del on the stale object and requiring manual review due to concrete repeated lifetime mutation.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a real UAF with a concrete second lifetime/refcount cleanup operation on the same stale reference. Controlled exploitation is not demonstrated, but the primitive is stronger than a simple crash-only stale dereference.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tty: clear cdev pointer after cdev_add() failure [ Upstream
Commit: fb1de260033c215dba438973d82b48562bdc86bb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb1de260033c215dba438973d82b48562bdc86bb
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=fb1de260033c215dba438973d82b48562bdc86bb
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90334 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90334
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:L/I:L/A:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 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: 5
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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