* [CVE-2026-89897][MODERATE 7.0] media: cec: Serialize exclusive follower delivery
@ 2026-09-16 17:24 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 17:24 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89897
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: media: cec: Serialize exclusive follower delivery
Commit: efc829b4b937f98c4467bffe9243f756b7cfbf7f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=efc829b4b937f98c4467bffe9243f756b7cfbf7f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89897
Analysis date: Wed, 16 Sep 2026 13:24:25 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A race in the HDMI CEC receive path allows `cec_receive_notify()` to use the exclusive follower pointer without adapter locking while follower mode changes or filehandle release occur, creating a possible stale pointer or UAF that can cause local kernel DoS and requires manual review for unresolved memory corruption leverage.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89897 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89897
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.
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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-89897 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89897 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-362;*CWE-416;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in the HDMI CEC receive path can occur because cec_receive_notify reads and uses the exclusive follower pointer without holding the adapter lock. A local user with access to the CEC device can race follower mode changes or filehandle release against incoming CEC message delivery, which may result in a stale cec_follower pointer and a possible use after free. For the CVSS the PR:L is used because reliable triggering requires local access to the CEC device node or an equivalent local process context, not just remote packet delivery. The issue is not reachable over normal IP networking. It is limited to systems with a CEC adapter and exposed CEC userspace access, often tied to HDMI connected devices. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality or integrity impact because the bug class is a race with a plausible UAF during object release.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE LOCK INIT SIMPLEFIX HARDWARE LINUS KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 - - checked
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ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
## 1. ActionableScore
* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
Conservative arithmetic: +2 real lifetime/UAF primitive, +1 realistic kernel crash/DoS. No negative race penalty is applied because the patch shows a race, but does not prove a materially narrow or one-shot race. Conservative floor 4 is applied because a real stale lifetime use is directly implied, the stale pointer is used, this is not fixed stale storage, and host-admin-equivalent privilege is not established.
Paranoid arithmetic: conservative signals plus +1 attacker-influenced repeatable lifetime manipulation, because a local CEC user can plausibly race follower mode changes or filehandle release against receive delivery. Paranoid floor 5 is applied because this is an active structured object used in message delivery, not merely a passive cleanup pointer.
## 2. Signal breakdown
Positive signals:
* +2 Real UAF / active lifetime violation: `cec_receive_notify()` reads and uses `adap->cec_follower` without `adap->lock`, while the commit says this must be serialized against mode changes and release.
* +1 Reliable kernel crash / strong DoS: stale `cec_follower` use during `cec_queue_msg_fh()` can plausibly cause a kernel fault or crash.
* +1 paranoid attacker-influenced lifetime manipulation: the relevant local actor can plausibly change follower mode or close/release the filehandle while CEC receive delivery is active.
Non-awarded or constrained signals:
* +2 strong exploitation leverage not awarded: no controlled reclaim, arbitrary write, callback control, credential corruption, or demonstrated LPE is established.
* Structured stale-object leverage not separately awarded: `cec_follower` is an active object, but the patch does not show controlled replacement or attacker-shaped reclaimed contents.
* Confidentiality and integrity points are not awarded as direct signals: C/I impact is possible in a paranoid CVSS sense for UAF triage, but no concrete disclosure or unauthorized modification primitive is demonstrated.
* Extra Availability point not awarded: the same crash/DoS is already counted once.
* Race penalty not awarded: the evidence shows a race, but not a proven hard one-shot or externally uncontrollable timing window.
* Remote reachability not awarded: HDMI CEC is not normal IP networking, and the vulnerable operation requires local CEC userspace interaction or device-node access.
Repeated release/refcount check:
* First release can end lifetime: possible, because the commit explicitly mentions release racing with use of the exclusive follower pointer.
* Exact same reference survives: yes, `adap->cec_follower` can be read and used without serialization.
* Second operation is a real lifetime/refcount mutation: not established from this patch.
* +1 Concrete repeated release/refcount: not awarded.
* Paranoid repeated-refcount leverage +1: not awarded.
* FLOOR 5: applied only in paranoid scoring due to active structured stale use plus repeatable lifetime manipulation, not due to a repeated refcount operation.
## 3. Reachability analysis
The evaluated attacker is a local user or local process with access to the CEC device node, typically `/dev/cecX`, or an equivalent local CEC userspace context. This is below host-admin-equivalent privilege, so `HostAdminRequired=NO`.
The complete trigger is not namespace-scoped. CEC device access is device-node and media-subsystem based, not a network namespace capability path, so `NamespaceScopedPrivilege=NO`.
The bug is not reachable over normal TCP/IP networking. An HDMI-connected CEC peer may help generate incoming CEC messages, but the security-relevant race requires local interaction with the CEC follower state or filehandle lifetime. Prior privileged setup may exist in practice if device permissions are not granted to users, but supplied context supports a local user with CEC device access rather than host root.
Special hardware is required in the sense that a CEC adapter or exposed CEC device is needed, but this is not treated as rare physical-only hardware severe enough for a scoring penalty. Race repeatability is plausible because userspace can repeatedly change follower mode or release filehandles while messages are delivered.
## 4. Primitive and severity interpretation
The exact primitive is an unlocked access to an exclusive follower pointer in the HDMI CEC receive path. The pointer may change or be released while `cec_receive_notify()` performs the no-follower check and later delivers a message through `cec_queue_msg_fh()`.
This is an active structured object lifetime bug rather than fixed stale storage. The stale object is a CEC filehandle/follower object used for message delivery. The vulnerable use is broader than a pure destructor-only cleanup dereference because it happens in the receive path while delivering messages.
No concrete repeated release/refcount operation is established. This is category A ordinary stale object use, not category B second lifetime/refcount mutation through the same dangling reference.
PrimitiveClass is `CLASS_B`: a real active-object lifetime/UAF primitive exists, but controlled reclaim, arbitrary read/write, callback hijack, or demonstrated privilege escalation is not established.
Demonstrated consequence is local DoS through kernel crash or fault. Speculative consequence is stronger memory-corruption impact if stale object reuse becomes controllable, but the patch does not establish that. ImportantCandidate is `YES` because the bug is a real active-object lifetime race with unresolved exploitation leverage beyond a simple validation bug. AutoDowngradeAllowed is `NO` because automatically closing it as ordinary Moderate could discard a meaningful unresolved UAF primitive.
Likely operational severity is Actionable Moderate, with Strong Important review needed if downstream analysis finds controlled reclaim, attacker-shaped replacement, or deeper stale field consumption in `cec_queue_msg_fh()`.
## 5. One-sentence report phrase
A race in the HDMI CEC receive path allows `cec_receive_notify()` to use the exclusive follower pointer without adapter locking while follower mode changes or filehandle release occur, creating a possible stale pointer or UAF that can cause local kernel DoS and requires manual review for unresolved memory corruption leverage.
## 6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a real active-object lifetime race with possible UAF during CEC follower release and message delivery, and the post-lifetime behavior in the queueing path needs human review before safe downgrade.
## 7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: media: cec: Serialize exclusive follower delivery
Commit: efc829b4b937f98c4467bffe9243f756b7cfbf7f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=efc829b4b937f98c4467bffe9243f756b7cfbf7f
Commit description:
cec_receive_notify() reads the exclusive follower pointer without the
adapter lock. Serialize the no-follower check and message delivery
against mode changes and release.
Fixes: 9881fe0 ("[media] cec: add HDMI CEC framework (adapter)")
Cc: [email protected]
Signed-off-by: Ruoyu Wang <[email protected]>
Signed-off-by: Hans Verkuil <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/media/cec/core/cec-adap.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=efc829b4b937f98c4467bffe9243f756b7cfbf7f
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89897 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89897
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:L/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: 4
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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