* [CVE-2026-89817][MODERATE REGULAR] drm/gud: NUL-terminate TV mode names read from the device
@ 2026-09-16 11:52 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 11:52 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89817
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: drm/gud: NUL-terminate TV mode names read from the device
Commit: 08c8ec28547987e55a90c662f8795e05c2925498
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=08c8ec28547987e55a90c662f8795e05c2925498
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89817
Analysis date: Wed, 16 Sep 2026 07:52:56 -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-2026-89817 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89817
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: NO
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89817 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89817 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:H';CWE-170;CWE-125;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H';BEST CVSS score: '4.9';DESCR 'A malicious or faulty GUD USB display device can return fixed size TV mode name slots without a NUL terminator. The DRM helper later treats each slot as a C string and calls strlen, so parsing can read past the end of one slot and for the last mode past the end of the allocation. For the CVSS the PR:N value is used because the attacker does not need privileges on the victim system when the attack is modeled as a malicious USB device. User interaction is required because the device normally has to be attached or accepted by the target system. The issue is not network reachable. Impact is at least denial of service through a kernel crash or sanitizer detected out-of-bounds read, with only limited confidentiality impact in the paranoid score because the primitive is an out-of-bounds read and not a write or UAF.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) YES READ DANGER SIMPLEFIX HARDWARE LINUS SYZBOT DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
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 was applied.
2. Signal breakdown
Positive signals:
* +1 Firmware/device-mediated external influence: attacker-controlled data comes from a malicious or faulty USB GUD display device, not from a normal local syscall-only path and not from direct network traffic.
* +1 Reliable kernel crash / strong DoS: non-NUL-terminated fixed-size mode names can make strlen() read beyond the slot and, for the last mode, beyond the allocation, supporting KASAN/Oops style DoS.
Non-awarded material signals:
* +0 Read-only OOB concern: the primitive is an out-of-bounds read by strlen(), but no write, UAF, type confusion, callback control, or active object corruption is established.
* Confidentiality not awarded: although adjacent bytes may be read internally, the supplied evidence does not establish a reliable user-visible disclosure path for kernel memory.
* Integrity not awarded: no unauthorized write or state modification primitive is shown.
* Strong LPE / strong exploitation leverage not awarded: no controlled reclaim, arbitrary read/write, object replacement, refcount takeover, or privilege escalation chain is established.
* Remote reachability not awarded: this is USB device input, not network packet input.
* Extra availability not awarded: the same potential crash is already counted once.
* Physical-only penalty not applied: physical USB attachment is already reflected by lack of remote/local-unprivileged bonuses and by device-mediated scoring, and the prompt requires concrete evidence before applying strong hardware rarity penalties.
3. Reachability analysis
The evaluated attacker is a malicious or faulty USB GUD display device, or someone able to present such a device to the host. The vulnerable path is reached when the driver reads TV mode names from the device and passes them as C strings to DRM helper code.
No host administrator privilege is required by the malicious device itself, so HostAdminRequired=NO. User interaction or physical/device acceptance is normally required because the device must be attached or otherwise exposed to the host. This is not namespace-scoped and not network reachable.
The path is hardware/device specific. It requires the GUD USB display driver and a device that can return malformed mode names. No race or special timing requirement is established.
4. Primitive and severity interpretation
The exact primitive is a read-only out-of-bounds access caused by missing NUL termination of fixed-size strings supplied by the USB device. The stale or corrupt object class is not UAF and not lifetime corruption. The affected storage is a fixed buffer of device-provided mode names.
This is CLASS_C because the demonstrated behavior is primarily invalid read / crash behavior without an independently useful corruption primitive. The demonstrated consequence is denial of service through kernel fault or sanitizer-detected OOB read. A speculative consequence is limited accidental exposure of adjacent bytes, but the patch and context do not establish a practical disclosure channel.
ImportantCandidate=NO because there is no Class A primitive and no strong Class B unresolved lifetime/corruption primitive. AutoDowngradeAllowed=YES because the issue is constrained to malformed device input, has no demonstrated write/lifetime/security-boundary primitive, and the likely operational severity is ordinary Moderate.
5. One-sentence report phrase
A malicious GUD USB display device can provide non NUL terminated TV mode names, causing strlen() in the DRM property path to read past the fixed slot and possibly past the allocation, leading primarily to a kernel crash or sanitizer detected DoS.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: the established primitive is limited OOB read from untrusted USB device data and is likely ordinary Moderate, but a quick manual check is useful to confirm whether any DRM property path can expose overread bytes to userspace.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: drm/gud: NUL-terminate TV mode names read from the device
Commit: 08c8ec28547987e55a90c662f8795e05c2925498
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=08c8ec28547987e55a90c662f8795e05c2925498
Commit description:
gud_connector_add_tv_mode() reads a buffer of fixed-size mode names from
the USB device and passes pointers into it to
drm_mode_create_tv_properties_legacy(), which calls strlen() on each one.
Nothing guarantees the device NUL-terminates a name, so strlen() can run
past the end of a slot and, for the last mode, past the end of the
allocation.
Terminate each name at the end of its slot before use.
Fixes: 40e1a70 ("drm: Add GUD USB Display driver")
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=916c888ba5f1a54c9526
Tested-by: [email protected]
Signed-off-by: Deepanshu Kartikey <[email protected]>
Acked-by: Ruben Wauters <[email protected]>
Cc: <[email protected]>
Signed-off-by: Ruben Wauters <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/gpu/drm/gud/gud_connector.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=08c8ec28547987e55a90c662f8795e05c2925498
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89817 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89817
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:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:H
The Best / paranoid CVSS score: 4.9
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: 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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