[CVE-2026-89816][LOW] drm: Fix drm_crtc_commit leak if signaled when PAGE_FLIP_EVENT is used

[CVE-2026-89816][LOW] drm: Fix drm_crtc_commit leak if signaled when PAGE_FLIP_EVENT is used

[AL-KERNEL]

CVE: CVE-2026-89816
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm: Fix drm_crtc_commit leak if signaled when PAGE_FLIP_EVENT is used
Commit: c5517b71101f61a45144e3cf77a19626355272de
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c5517b71101f61a45144e3cf77a19626355272de
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89816
Analysis date: Wed, 16 Sep 2026 09:34:28 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A local DRM atomic user with page flip event capability can leak drm_crtc_commit references by interrupting a fence wait, potentially causing gradual resource exhaustion and denial of service without evidence of memory corruption or privilege escalation.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89816 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89816

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-2026-89816	LOW	https://www.kernelcve.org/list/?q=CVE-2026-89816 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:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A DRM CRTC commit reference leak can occur when PAGE_FLIP_EVENT is used and the atomic ioctl is interrupted while waiting on fences. A local process that can use the DRM atomic interface can repeatedly trigger interrupted non-blocking commits and leak drm_crtc_commit references, causing gradual kernel memory/resource exhaustion. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires local access to the DRM device and the ability to issue atomic page flip operations, but not full system administrator privileges in typical desktop or graphics workloads. The issue is not network reachable. Impact is denial of service through resource exhaustion and not privilege escalation because the patch indicates a reference leak rather than memory corruption.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	LOCK DANGER SIMPLEFIX HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

## 1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap changed the score.

## 2. Signal breakdown

Conservative positive signals:

* +1 Concrete persistent kernel resource/reference leak. The patch directly establishes that a `drm_crtc_commit` reference is not put when `complete_signaling()` cancels and clears an event allocated by `prepare_signaling()`.

Paranoid additional signals:

* +1 Resource exhaustion concern. The leak is reproducible and can plausibly accumulate through repeated interrupted atomic commits using `DRM_MODE_PAGE_FLIP_EVENT` and a fence wait, so gradual kernel resource exhaustion is defensible.

Non-awarded signals:

* Real UAF not awarded. The bug is a missing `put`, not stale post-free use.
* Concrete repeated release/refcount not awarded. The issue is missing release, not a second release through a dangling reference.
* Reliable kernel crash not awarded. The patch establishes a leak, not an immediate Oops or panic.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact not awarded. No disclosure, corruption, or privilege escalation primitive is established.
* Remote reachability not established.
* Extra availability point not awarded beyond the paranoid resource exhaustion concern.

Negative signals:

* No host-admin penalty applied. Host-admin-equivalent privilege is not positively established.
* No race/timing penalty applied. The trigger uses interruption during a wait, but the issue is an ordered leak path rather than a hard uncontrolled race.
* No rare-hardware penalty applied. The issue reproduced on amdgpu and vkms, and the code is generic DRM atomic UAPI.

## 3. Reachability analysis

The trigger is local. A process must be able to use the DRM atomic interface with `DRM_MODE_PAGE_FLIP_EVENT`, arrange a blocking fence wait such as through `sw_sync`, and send a signal to interrupt the ioctl path.

For DRM/KMS, the complete operation likely requires DRM master or an equivalent display-control context, such as a compositor or delegated graphics service. The supplied evidence does not prove that an arbitrary ordinary logged-in user can perform the complete vulnerable operation on all systems. It also does not prove that host administrator privilege is required. Therefore `HostAdminRequired=UNKNOWN`.

This is not namespace-scoped. No evidence shows that user or network namespaces provide the complete required DRM/KMS operation. No network trigger is present. No prior special hardware requirement is established beyond having a DRM device or vkms-like setup capable of the affected atomic path.

## 4. Primitive and severity interpretation

The exact primitive is a persistent kernel reference leak of `drm_crtc_commit`. The event is canceled and `crtc_state->event` is set to `NULL`, which prevents the normal later path from dropping the commit reference in `__drm_atomic_helper_crtc_destroy_state()`.

This is not an active structured UAF and not fixed stale storage. It is a resource/lifetime cleanup bug where a reference remains held. There is no concrete second lifetime mutation through a dangling reference.

Primitive class is `CLASS_C` because the demonstrated behavior is resource leak and potential DoS without an independently useful corruption primitive. Demonstrated consequence is gradual resource exhaustion. Speculative stronger consequences such as LPE are not supported by the patch.

ImportantCandidate is `NO` because no memory corruption, authorization bypass, COW/ownership violation, controlled reclaim, or repeated dangling refcount mutation is established. AutoDowngradeAllowed is `YES` because the issue is a local resource leak with no unresolved Important-grade primitive.

Likely operational severity is Low to ordinary Moderate. It may matter on shared graphics systems if an authorized display client can repeatedly trigger the leak, but it does not require urgent Important-class handling based on supplied evidence.

## 5. One-sentence report phrase

A local DRM atomic user with page flip event capability can leak drm_crtc_commit references by interrupting a fence wait, potentially causing gradual resource exhaustion and denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a concrete local reference leak with possible resource exhaustion, but no UAF, no double release, no arbitrary read/write, no authorization bypass, and no privilege escalation evidence.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: drm: Fix drm_crtc_commit leak if signaled when PAGE_FLIP_EVENT is used
Commit: c5517b71101f61a45144e3cf77a19626355272de
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c5517b71101f61a45144e3cf77a19626355272de

Commit description:

Commit 1c6ceee ("drm/atomic: Fix memleak on ERESTARTSYS during
non-blocking commits") fixed a very similar issue when the event was
allocated by drm_atomic_helper_setup_commit() itself.

However, if the event is allocated in prepare_signaling(), it will also be
set to NULL in complete_signaling(), which prevents drm_crtc_commit from
being put in __drm_atomic_helper_crtc_destroy_state().

Dropping the reference when the event is set to NULL at
complete_signaling() fixes the leak.

The leak can be reproduced by sending a signal to the thread using
DRM_MODE_PAGE_FLIP_EVENT and using a sw_sync fence to cause the atomic
ioctl to block at drm_atomic_helper_wait_for_fences(). It happened both
with amdgpu and vkms.

Fixes: 24835e4 ("drm: reference count event->completion")
Cc: stable@vger.kernel.org
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Link: https://patch.msgid.link/20260727-drm_crtc_atomic_commit_leak-v1-1-23d9948a9d7c@igalia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/drm_atomic_uapi.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=c5517b71101f61a45144e3cf77a19626355272de

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

The original Linux kernel CVE announcement for CVE-2026-89816 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89816

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:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  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: 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).