[CVE-2026-89874][LOW] media: v4l2-async: avoid deleting unlinked ASC entry on link error

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

CVE: CVE-2026-89874
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: media: v4l2-async: avoid deleting unlinked ASC entry on link error
Commit: d273571e531405426983c6a45ec635c7d0415eed
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d273571e531405426983c6a45ec635c7d0415eed
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89874
Analysis date: Wed, 16 Sep 2026 14:53:50 -0400
ActionableScore: 1
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-2026-89874 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89874

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: LOW
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-2026-89874	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89874 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-754;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.4';DESCR 'An error path in v4l2_async_match_notify() can call list_del() for asc_subdev_entry before that entry has been inserted into sd asc_list. Because async connection objects are zero initialized, the unlinked entry has NULL next and prev pointers and list_del() can cause a NULL pointer dereference, resulting in an Oops or kernel panic. For the CVSS the PR:H reflects that reliable triggering normally requires administrative control over V4L2 device registration, driver loading, driver rebinding, or the affected media hardware configuration. The issue is not network reachable and ordinary access to a V4L2 device node does not appear sufficient to reach this notifier registration error path. Impact is denial of service only. There is no supported UAF, out of bounds access, information leak, or attacker controlled memory corruption primitive that would justify confidentiality or integrity impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)  SKIP CVE-2026-89874 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	HARDWARE 	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash or strong DoS. `list_del()` receives an unlinked, zero-initialized entry and can dereference its NULL `next` or `prev` pointer, producing an Oops or panic.
* +0 Weak corruption concern. This is a fixed invalid list operation with an immediate NULL dereference, not a useful list-corruption primitive.
* No UAF signal. The ASC object has not been freed, released, or used after its lifetime ended.
* No repeated release or refcount signal. `list_del()` does not release the ASC object or mutate its lifetime refcount.
* No controlled reclaim, arbitrary write, disclosure, callback control, or security-boundary bypass is demonstrated.
* No remote or ordinary unprivileged reachability point is awarded.
* No privilege penalty is applied because host-admin-equivalent privilege is not positively established by the supplied call path.
* No extra Availability point is awarded because it would duplicate the same kernel crash.
* No paranoid point is added because a stronger consequence would require an unsupported primitive.

3. Reachability analysis

The vulnerable operation occurs while `v4l2_async_match_notify()` binds and registers V4L2 asynchronous subdevices and creates ancillary media links. The immediate prerequisite is a link-creation failure before `asc_subdev_entry` is inserted into `sd->asc_list`.

Ordinary access to a V4L2 device node is not shown to reach this registration error path. Driver loading, rebinding, hardware discovery, or media-topology setup may involve an administrator or kernel-managed device initialization, but the supplied evidence does not positively prove that host-admin privilege is required for the complete trigger. Therefore `HostAdminRequired` remains `UNKNOWN`.

No namespace-scoped capability chain is shown. Prior device or driver setup is required, but no unusual hardware requirement or attacker-controlled race is established. This is an ordered error path, not a timing race.

4. Primitive and severity interpretation

The exact primitive is `list_del()` on a list entry whose `next` and `prev` fields remain NULL because insertion has not occurred. The expected result is an immediate invalid pointer dereference during rollback.

This is not stale storage, an active-object UAF, a double release, or a second lifetime mutation. No useful attacker-controlled operation remains after excluding the crash. It is therefore `CLASS_C`, with denial of service as the demonstrated consequence.

`ImportantCandidate=NO` because there is no lifetime violation, controlled corruption, authorization bypass, or unresolved exploitation leverage. `AutoDowngradeAllowed=YES` because the behavior is constrained to an invalid cleanup operation with crash-only impact.

5. One-sentence report phrase

A V4L2 asynchronous binding error path can call list_del() on an unlinked zero-initialized ASC list entry, causing a NULL pointer dereference and potential kernel crash without establishing a UAF or independently useful corruption primitive.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a constrained NULL-dereference DoS with no UAF, repeated lifetime mutation, controlled corruption, or security-boundary consequence.

7. Call-site confidence

Call-site confidence: high

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

Patch: media: v4l2-async: avoid deleting unlinked ASC entry on link error
Commit: d273571e531405426983c6a45ec635c7d0415eed
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d273571e531405426983c6a45ec635c7d0415eed

Commit description:

v4l2_async_match_notify() creates ancillary media links before adding
asc->asc_subdev_entry to sd->asc_list.

If ancillary link creation fails, the function jumps to
err_call_unbind while asc_subdev_entry has not been linked yet. Async
connections are zero-allocated, so the list entry still has NULL next
and prev pointers on this path. Calling list_del() on it can therefore
dereference NULL instead of returning the original link creation error.

Do not delete asc_subdev_entry from err_call_unbind. There is no list
insertion to undo on this path; the bound callback and sub-device
registration are the operations that need to be rolled back.

Fixes: 28a1295 ("media: v4l: async: Allow multiple connections between entities")
Cc: stable@vger.kernel.org
Signed-off-by: Xu Rao <raoxu@uniontech.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/media/v4l2-core/v4l2-async.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=d273571e531405426983c6a45ec635c7d0415eed

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

The original Linux kernel CVE announcement for CVE-2026-89874 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89874

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.4

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: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).