[CVE-2026-89741][MODERATE 7.0] Revert "media: v4l2-dev: fix error handling in __video_register_device()"

[CVE-2026-89741][MODERATE 7.0] Revert "media: v4l2-dev: fix error handling in __video_register_device()"

[AL-KERNEL]

CVE: CVE-2026-89741
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Revert "media: v4l2-dev: fix error handling in __video_register_device()"
Commit: 712ca1cf756de2557a116e0df55791bac0c9f99e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=712ca1cf756de2557a116e0df55791bac0c9f99e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89741
Analysis date: Sat, 12 Sep 2026 02:02:56 -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-2026-89741 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89741

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-2026-89741	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89741 CHECK WITH IMPACT FROM ORIG NN LOW	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-415;CWE-703;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A double free can occur in the V4L2 device registration error path when device_register() fails after the earlier error handling change. The problematic path may call put_device() and free the video_device object, while the V4L2 API still expects the caller to call video_device_release() after video_register_device() failure, freeing the same object again. For the CVSS the PR:L is used in the paranoid score because reduced capability root, container root, or a service account that can trigger media device registration may be enough in some deployments. The issue is local and not directly network reachable. Impact is at least denial of service via kernel memory corruption and in the worst case may allow confidentiality or integrity impact including privilege escalation. Reliable exploitation is likely difficult because it depends on making device_register() fail during a V4L2 registration path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)  SKIP CVE-2026-89741 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	INIT LEAK ERRORPATH UAF HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4	-	-	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
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

Conservative arithmetic: +2 real lifetime violation / double free, +1 concrete second lifetime operation on the same object, +1 realistic kernel crash or allocator corruption. Applied conservative FLOOR 5 because the patch establishes a concrete repeated free of the same object after the first operation may already have ended the object lifetime.

Paranoid arithmetic: conservative signals plus +1 paranoid repeated-free leverage, because the second operation is not only a stale read but a second kfree/release-style lifetime mutation through storage that may already be outside its original lifetime. No additional floor or cap changed the paranoid score.

2. Signal breakdown

Positive signals:

* +2 Real UAF / lifetime violation: `put_device(&vdev->dev)` may release the `video_device` object, while the V4L2 API still requires the caller to invoke `video_device_release()` on failure.
* +1 Concrete repeated release/free: the same `video_device` reference can reach a later `video_device_release()` / `kfree()` after the earlier `put_device()` may already have freed it.
* +1 Reliable kernel crash / strong DoS: double free in kernel object lifetime handling can realistically lead to allocator corruption, BUG, panic, or KASAN-detected crash.
* +1 Paranoid repeated-free leverage: the second free is a concrete lifetime-mutating operation on potentially freed storage, leaving unresolved post-lifetime allocator corruption leverage.

Repeated-release checks:

* First release can end lifetime: yes. `put_device()` can drop the final device reference and invoke the registered release callback.
* Exact same reference survives: yes. The API contract says the caller still calls `video_device_release()` after `video_register_device()` failure.
* Second operation mutates lifetime/refcount/free state: yes. `video_device_release()` frees the same `video_device` object.
* +1 Concrete repeated release/refcount awarded: yes.
* Paranoid repeated-refcount/free leverage +1 awarded: yes, because the second operation itself mutates allocator/lifetime state after possible object lifetime end.
* FLOOR 5 applied: yes.

Non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Callback or function-pointer control not demonstrated.
* Local unprivileged trigger not established.
* Remote reachability not established.
* Confidentiality and integrity bonuses not awarded as separate ActionableScore signals. The patch supports memory corruption risk, but not a concrete disclosure or unauthorized write primitive.
* Extra Availability point not awarded because the same crash/free corruption is already counted once.
* Weak LPE not separately awarded because the repeated free fact is already counted by the dedicated repeated lifetime mutation rule.

Negative signals:

* No -2 host-admin penalty applied. Host-admin-equivalent privilege is not positively established from the supplied evidence.
* No -1 race/timing penalty applied. This is an ordered error path, not a demonstrated hard race.
* No rare-hardware penalty applied. V4L2 is hardware-related, but the patch does not establish rare or physical-only attacker requirements.
* No generic “device_register failure is unlikely” subtraction applied as a standalone score penalty. It is noted as a practical reachability constraint, but not enough to erase the concrete double-free primitive.

3. Reachability analysis

The vulnerable path is in V4L2 device registration error handling. The direct caller is kernel driver code, not a normal userspace syscall interface. A practical trigger requires reaching `__video_register_device()` and making `device_register()` fail, after which the V4L2 caller follows the documented failure contract and calls `video_device_release()`.

Ordinary unprivileged local reachability is not established. Host-admin-equivalent reachability is also not positively established as the only possible model, because failures might involve probe paths, hotplug, driver-managed registration, fault injection, memory pressure, or service-mediated device management depending on deployment.

Namespace-scoped privilege is not supported by the supplied evidence. This is not a network namespace or user namespace scoped operation.

No direct network trigger is established. Prior privileged setup may exist in real deployments, such as loading or probing a V4L2 driver, but the supplied context does not fully resolve the complete attacker privilege boundary.

The trigger is likely difficult in practice because `device_register()` failure during V4L2 registration is described as very unlikely. That affects operational likelihood, but the established primitive remains a real kernel double free.

4. Primitive and severity interpretation

The exact primitive is a concrete double free / repeated lifetime operation on `struct video_device`.

This is an active structured kernel object, not a fixed stale DMA buffer. The vulnerable operation is not merely an ordinary stale cleanup dereference. It is category B from the prompt: a second lifetime/free operation through a potentially dangling object.

The demonstrated consequence is kernel memory corruption with likely crash or allocator failure. The speculative consequence is local privilege escalation or broader confidentiality/integrity impact if an attacker can make the failure path reliable and influence allocator reuse, but that stronger exploitation primitive is not demonstrated by the patch.

PrimitiveClass is CLASS_B, not CLASS_A. There is a real kernel lifetime corruption primitive, but no controlled reclaim, arbitrary write, callback control, or demonstrated LPE.

ImportantCandidate is YES because a concrete double-free lifetime mutation remains unresolved and should not be collapsed into ordinary crash-only cleanup behavior.

AutoDowngradeAllowed is NO because automatic downgrade would discard a real repeated free of a potentially already-freed kernel object. The issue is at least Actionable Moderate and needs manual Important-vs-Moderate review.

Likely operational severity is Actionable Moderate, with Strong Important candidate review justified by the double-free primitive.

5. One-sentence report phrase

A V4L2 device registration error path can double free a video_device object if device_register() fails, because put_device() may already release the object while the API still requires the caller to call video_device_release().

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch establishes a concrete kernel double free through the same object lifetime path, and post-lifetime allocator corruption leverage remains unresolved even though controlled reclaim or LPE is not demonstrated.

7. Call-site confidence

Call-site confidence: high

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

Patch: Revert "media: v4l2-dev: fix error handling in __video_register_device()"
Commit: 712ca1cf756de2557a116e0df55791bac0c9f99e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=712ca1cf756de2557a116e0df55791bac0c9f99e

Commit description:

This reverts commit 2a934fd .

The intentions of that patch were good, but it doesn't work.

The idea is that if device_register fails, you have to do a put_device
to let the ref counter release resources.

However, the V4L2 API says that if video_register_device() fails, then
you have to call video_device_release(), which kfree()s the video_device
struct.

But the put_device() will already have freed the struct, so you end
up in a double-free scenario.

There is not really a good way of fixing this without breaking
video_register_device() into two parts, one that initializes everything,
and one that does the actual device_register, and then converting all
V4L2 drivers to this new model.

That is a massive job, and it is very unlikely that device_register
will fail.

So rather than ending up in a double-free scenario, just revert this
patch, and in that case we'll have a small memory leak. Which is a lot
more robust.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Fixes: 2a934fd ("media: v4l2-dev: fix error handling in __video_register_device()")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/linux-media/20260520090624.1071139-1-lgs201920130244@gmail.com/
Link: https://lore.kernel.org/all/2026042058-charm-storable-4ad8@gregkh/
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/media/v4l2-core/v4l2-dev.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=712ca1cf756de2557a116e0df55791bac0c9f99e

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

The original Linux kernel CVE announcement for CVE-2026-89741 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89741

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