* [CVE-2026-80923][MODERATE REGULAR] xhci: dbgtty: Fix unregister on tty_register_driver() failure
@ 2026-09-09 17:10 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-09 17:10 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-80923
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: xhci: dbgtty: Fix unregister on tty_register_driver() failure
Commit: 01b7bc0938061f2fd46e0094f6483d8c6c02f7d3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=01b7bc0938061f2fd46e0094f6483d8c6c02f7d3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80923
Analysis date: Wed, 09 Sep 2026 13:10:59 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-80923 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80923
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 REGULAR
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-2026-80923 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-80923 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:L/I:L/A:H';*CWE-416;CWE-825;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.2';DESCR 'A use-after-free can occur in the xhci dbgtty cleanup path when tty_register_driver() fails during initialization. The driver reference is dropped but the global dbc_tty_driver pointer remains non NULL, so later module unload can call tty_unregister_driver() on an already freed tty driver object. For the CVSS the PR:H is used because reliable triggering typically requires administrative control over module load or unload and a registration failure condition. The issue is local and is not network reachable. Impact is at least denial of service via kernel crash. In the paranoid score, limited confidentiality or integrity impact is kept because the bug class is UAF, but the patch context does not show a stronger arbitrary write or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) YES USB UAF DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 1
* Conservative score: 3
* Applied conservative floor/cap: confirmed kernel UAF calibration floor 1 -> 3
* Raw paranoid score: 2
* Paranoid score: 4
* Applied paranoid floor/cap: confirmed active-object cleanup UAF calibration floor 2 -> 4
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF / lifetime violation. `tty_register_driver()` failure drops the reference via `tty_driver_kref_put()`, but `dbc_tty_driver` remains dangling.
* +1 Reliable kernel crash / DoS. Later module unload can call `tty_unregister_driver()` on an already freed `tty_driver`.
* -2 Host-admin-equivalent requirement. The demonstrated trigger is module unload / module lifecycle control, normally requiring host root or `CAP_SYS_MODULE`.
Paranoid additional signal:
* +1 Unresolved active cleanup operation on a freed structured object. `tty_unregister_driver()` consumes a nontrivial freed `tty_driver` object, so the stale use is stronger than a pure NULL dereference, but no controlled reclaim or write primitive is shown.
Non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Privilege escalation not demonstrated.
* Remote reachability not established.
* Extra Availability not awarded because the crash is already counted.
* C/I not awarded conservatively because the supplied evidence shows UAF and crash, not disclosure or unauthorized modification.
* Concrete repeated release/refcount +1 not awarded. First release can end lifetime and the exact global reference survives, but the supplied visible path only proves later `tty_unregister_driver()` stale use, not a clearly visible second `kref_put()` or equivalent refcount mutation on the same object.
Repeated-release/refcount check:
* First release can end lifetime: yes, `tty_driver_kref_put()` may free the driver.
* Exact same reference survives: yes, global `dbc_tty_driver` remains non-NULL.
* Second operation is a real lifetime/refcount mutation: not sufficiently established from supplied context.
* Dedicated +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.
3. Reachability analysis
The demonstrated path requires `tty_register_driver()` to fail during xHCI dbgtty initialization and then requires module unload or module exit cleanup. That is a local host control-plane scenario, not packet-driven or remotely reachable. In normal deployments, loading and unloading the xHCI module is host-admin-equivalent and requires root or `CAP_SYS_MODULE`.
No evidence shows namespace-scoped reachability. No evidence shows that an ordinary local user can trigger the complete sequence. No network trigger is present. The bug is not a timing race. It is an ordered failure-path and cleanup-path lifetime bug. The failure condition is unusual, but the UAF mechanics are concrete once that path is reached.
4. Primitive and severity interpretation
The exact primitive is a dangling global pointer to a freed `tty_driver` object. The stale object is an active structured kernel object, but the demonstrated stale use occurs in module cleanup / unregister processing. This is Class B because it is a real kernel lifetime violation / UAF, not merely invalid state. It is not Class A because no controlled replacement, arbitrary read/write, callback control, refcount takeover, LPE, or security-boundary bypass is established.
This is closer to a constrained cleanup UAF than an Important-preserving exploitation primitive. The likely operational consequence is local privileged kernel crash / DoS. Paranoid scoring keeps it borderline because active structured UAFs deserve review, but the evidence does not justify treating it as an Important candidate.
ImportantCandidate=NO because the trigger is host-admin-gated and the demonstrated consequence is cleanup-path crash only. AutoDowngradeAllowed=YES because no meaningful unresolved exploitation primitive beyond the constrained UAF is established.
5. One-sentence report phrase
A use-after-free in the xhci dbgtty cleanup path can occur when `tty_register_driver()` fails, leaves `dbc_tty_driver` dangling after `tty_driver_kref_put()`, and later module unload calls `tty_unregister_driver()` on the freed driver, causing a local privileged kernel crash.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: this is a real kernel UAF, but current evidence points to a constrained host-admin cleanup-path DoS without controlled reclaim, arbitrary write, privilege escalation, or remote reachability.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: xhci: dbgtty: Fix unregister on tty_register_driver() failure
Commit: 01b7bc0938061f2fd46e0094f6483d8c6c02f7d3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=01b7bc0938061f2fd46e0094f6483d8c6c02f7d3
Commit description:
If tty_register_driver() fails, it drops the reference, but fails to set
the global dbc_tty_driver to NULL, causing the unregister to be called
again when module exits.
On module unload dbc_tty_exit() only gates its cleanup on the driver
pointer being non-NULL, so it operates on the already-freed driver:
module_init(xhci_hcd_init)
xhci_hcd_init()
xhci_dbc_init() [return value ignored]
dbc_tty_init()
tty_register_driver() fails
tty_driver_kref_put() -> driver freed
(dbc_tty_driver left dangling)
...
module_exit(xhci_hcd_fini)
xhci_hcd_fini()
xhci_dbc_exit()
dbc_tty_exit()
if (dbc_tty_driver) -> true (dangling)
tty_unregister_driver() -> use-after-free
Fixes: 4521f16 ("xhci: dbctty: split dbc tty driver registration and unregistration functions.")
Cc: [email protected] # v5.10
Cc: Mathias Nyman <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Lucas De Marchi <[email protected]>
Signed-off-by: Mathias Nyman <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/usb/host/xhci-dbgtty.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=01b7bc0938061f2fd46e0094f6483d8c6c02f7d3
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-80923 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80923
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:H/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS score: 5.2
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: 3
Paranoid ActionableScore: 4
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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR
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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