[CVE-2026-89878][MODERATE REGULAR] media: s2255: check firmware size before reading trailing marker

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

CVE: CVE-2026-89878
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: media: s2255: check firmware size before reading trailing marker
Commit: 6f6a5b0b0a84c2de0e152f2841e57bc226db924f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6f6a5b0b0a84c2de0e152f2841e57bc226db924f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89878
Analysis date: Wed, 16 Sep 2026 13:06:33 -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-2026-89878 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89878

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.

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AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89878	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89878 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:N/A:H';CWE-125;CWE-191;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.1';DESCR 's2255_probe() reads a 4 byte marker and version from the last 8 bytes of the firmware blob. If the firmware file is shorter than 8 bytes, fw_size - 8 underflows and the driver reads before the firmware buffer. This is a local firmware parsing issue and is not network reachable. For the CVSS the PR:H is used because a reliable trigger normally requires control over the local firmware file or driver load environment, which is administrative in typical deployments. Impact is primarily denial of service or a failed probe. In the paranoid interpretation, the out of bounds kernel read may have limited confidentiality impact and possible high availability impact, so it should be reviewed manually rather than treated as a pure warning.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)	YES	OOB USB SIMPLEFIX ERRORPATH HARDWARE LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
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 is clamped at 0 because the only positive primitive signal is outweighed by the positively established host-admin-equivalent trigger requirement.

2. Signal breakdown

Positive signals:

* +1. Limited read-only OOB access. The firmware size is used in `fw_size - 8` before validating that the firmware blob is at least 8 bytes, so a malformed too-small firmware file can make the driver read before the firmware buffer.
* +0. Availability is possible but not counted as a separate reliable strong DoS signal. The patch supports an invalid kernel read and failed probe risk, but it does not show a reliable host-wide panic or persistent outage.
* +0. Confidentiality is not awarded. The OOB bytes are used internally for marker/version validation and no user-visible disclosure path is established.
* +0. Integrity is not awarded. The patch shows no write, overwrite, lifetime mutation, or object corruption.

Negative signals:

* -2. Reliable triggering requires host-admin-equivalent control in normal deployments. The attacker must normally control the local firmware file or driver load/probe environment. This is not a network, namespace, or ordinary unprivileged interface.

Material non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Real UAF not demonstrated.
* Repeated release/refcount mutation not present.
* Remote reachability not established.
* Firmware-mediated external remote influence not established because the malformed blob is a local host firmware file rather than attacker-controlled network input.
* Extra availability not awarded because a reliable host-wide crash is not shown.
* C/I not awarded merely because the kernel performs an OOB read.

Repeated release/refcount checklist:

* First release can end lifetime: no.
* Exact same reference survives: no.
* Second operation mutates lifetime/refcount: no.
* Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage awarded: no.
* FLOOR 5 applied: no.

3. Reachability analysis

The vulnerable path is in `s2255_probe()` while validating the local firmware blob for the USB media driver. A reliable trigger normally requires replacing or supplying a malformed firmware file shorter than 8 bytes, or otherwise controlling the host firmware loading environment. That is normally host administrator control.

An ordinary local user is not shown to be able to provide the firmware blob. No namespace-scoped capability path is shown. The operation is not controlled through a network protocol. USB device presence may trigger probing, but the attacker-controlled component in this bug is the host firmware blob size, not packet traffic or ordinary device data.

Prior host setup is material because the malformed firmware file must be present in the host firmware search path. Special hardware is involved only in the sense that the s2255 USB device/driver path must be used, but no additional score penalty is needed beyond the host-admin trigger requirement. No race or special timing is involved.

4. Primitive and severity interpretation

The exact primitive is a read-only out-of-bounds access caused by unsigned underflow in firmware-size-based indexing. This is not a UAF, not a double free, not a refcount issue, and not an active structured-object lifetime violation.

The demonstrated consequence is invalid kernel memory read during firmware marker validation, likely resulting in failed probe or possible diagnostic/OOB report depending on build and memory layout. A kernel crash is possible in principle if the invalid address faults or sanitizers are active, but the patch does not establish a reliable crash primitive.

This is PrimitiveClass=CLASS_C because the behavior is primarily invalid read / validation failure / possible DoS without an independently useful corruption primitive. ImportantCandidate=NO because there is no controlled write, disclosure path, reclaim, authorization bypass, lifetime corruption, or privilege-escalation evidence. AutoDowngradeAllowed=YES because the issue is constrained, host-admin-triggered, and does not leave meaningful unresolved exploitation leverage.

Likely operational severity is Low-like or ordinary Moderate, mainly useful to fix for robustness and correctness rather than urgent exploitation risk.

5. One-sentence report phrase

A too-small local firmware blob for the s2255 driver can make `s2255_probe()` underflow `fw_size - 8` and read before the firmware buffer, but reliable triggering normally requires host-admin control of the firmware file and no disclosure, write, UAF, or privilege-escalation primitive is established.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a real kernel OOB read and should be reviewed for any product-specific firmware-loading exposure, but the supplied evidence supports a constrained host-admin local firmware parsing issue with no demonstrated C/I impact or strong DoS.

7. Call-site confidence

Call-site confidence: high

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

Patch: media: s2255: check firmware size before reading trailing marker
Commit: 6f6a5b0b0a84c2de0e152f2841e57bc226db924f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6f6a5b0b0a84c2de0e152f2841e57bc226db924f

Commit description:

s2255_probe() reads a 4-byte marker and version from the last 8 bytes
of the firmware blob (fw->data[fw_size - 8] and [fw_size - 4]). If the
firmware file is shorter than 8 bytes, fw_size - 8 underflows and the
access reads out of bounds. Validate the firmware size before indexing.

Fixes: 14d9626 ("V4L/DVB (8752): s2255drv: firmware improvement patch")
Cc: stable@vger.kernel.org
Signed-off-by: Lei Huang <huanglei@kylinos.cn>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/media/usb/s2255/s2255drv.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=6f6a5b0b0a84c2de0e152f2841e57bc226db924f

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

The original Linux kernel CVE announcement for CVE-2026-89878 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89878

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:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 5.1

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: 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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).