[CVE-2026-68256][LOW] drm/amd/display: detect_link_and_local_sink: DP alt mode timeout path leaks prev_sink reference

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

CVE: CVE-2026-68256
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/amd/display: detect_link_and_local_sink: DP alt mode timeout path leaks prev_sink reference
Commit: f9922828a4ebd26286fbe0286cc61695e7d9b07b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9922828a4ebd26286fbe0286cc61695e7d9b07b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68256
Analysis date: Mon, 10 Aug 2026 16:39:52 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A missing `dc_sink_release(prev_sink)` on the DP alt mode timeout path in AMD display link detection can leak sink references during repeated failed detection attempts, causing possible resource exhaustion or display stack degradation but no supported UAF or privilege escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-68256 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68256

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-68256	LOW	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:N/I:N/A:H';*CWE-772;*CWE-401;*CWE-664;Other CVSS 'AV:P/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A reference leak in `detect_link_and_local_sink()` occurs because `prev_sink` is retained on function entry but is not released when the DP alt mode timeout path returns false. Repeated DP or USB-C link detection failures could keep stale sink references alive and gradually leak kernel display resources. For the CVSS the PR:L is used for the higher risk case because reliable repeated triggering may require a local user or local display management context able to provoke link detection or hotplug style events. The issue is not network reachable and normally requires local hardware conditions such as a DP alt mode timeout. Impact is denial of service via resource exhaustion or display stack degradation. No UAF or arbitrary memory corruption primitive is supported by the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-68256 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX LEAK HARDWARE 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Triggered signals:

* Real lifetime corruption / refcount lifetime bug: +1
  `prev_sink` is retained with `dc_sink_retain()` but not released on one error path. This is a reference leak, not a UAF.

* Availability impact realistic: +1
  Repeated DP alt mode timeout events could gradually leak display sink resources and degrade the display stack or system resources.

* Broad/default/common subsystem: +1
  AMD display code is broadly deployed on systems using AMD GPUs, although this exact path needs USB-C DP alt mode behavior.

Subtracted signals:

* Physical-only or rare hardware-only condition: -2
  Triggering depends on DP / USB-C alt mode timeout behavior and likely requires local physical hardware interaction or a local display-management context.

Not triggered:

* Remote reachable: +0
  No network path.

* Local unprivileged trigger: +0 conservative
  A normal local user may not be able to reliably force repeated DP alt mode timeout events without hardware interaction or display stack control.

* Generic memory corruption: +0
  The bug leaks a reference. The patch does not indicate UAF, double free, OOB write, arbitrary write, or type confusion.

* Privilege escalation plausible: +0
  No reclaim, stale callback, attacker-controlled object reuse, or write primitive is shown.

* Reliable kernel crash / strong DoS: +0
  The issue is resource leakage over repeated failures, not an immediate reliable crash.

Paranoid delta:

* Firmware-mediated or external hardware influence: +1
  A malicious or faulty USB-C / DP alt mode device or firmware behavior could plausibly help trigger repeated timeout paths, but this is weaker than network reachability.

Call-site confidence: medium. The function body and missing cleanup path are clear, but broader caller frequency and practical trigger reliability are not shown in the supplied patch.

## 3. Reachability analysis

The bug is reachable through AMD display link detection when the path enters `SIGNAL_TYPE_DISPLAY_PORT` and USB-C DP alt mode times out. Realistic triggering usually requires local physical access to display hardware, a problematic USB-C / DP alt mode device, or some local display-management path that can repeatedly cause link detection attempts.

Namespaces and containers do not materially lower privileges here because the affected path is hardware/display-stack driven rather than a normal namespaced syscall or network API. The code is default-enabled only on systems with the relevant AMD display stack and hardware configuration. There is no evidence of remote network reachability.

## 4. Severity interpretation

This behaves like an ordinary Moderate / Low-like resource leak, not an Important-class kernel vulnerability. The theoretical worst case is availability impact from repeated leaked references and display resource exhaustion. Realistic exploitation evidence for privilege escalation, information disclosure, arbitrary memory corruption, or reliable kernel crash is not supported by the patch.

## 5. One-sentence report phrase

A missing `dc_sink_release(prev_sink)` on the DP alt mode timeout path in AMD display link detection can leak sink references during repeated failed detection attempts, causing possible resource exhaustion or display stack degradation but no supported UAF or privilege escalation primitive.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be handled as a low-priority resource leak because the patch shows a missing reference release only, with no UAF, no memory overwrite, no network reachability, and no plausible LPE primitive.

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

Patch: drm/amd/display: detect_link_and_local_sink: DP alt mode timeout path leaks prev_sink reference
Commit: f9922828a4ebd26286fbe0286cc61695e7d9b07b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9922828a4ebd26286fbe0286cc61695e7d9b07b

Commit description:

prev_sink is unconditionally retained via dc_sink_retain at function
  entry, but the DP alt mode timeout path inside SIGNAL_TYPE_DISPLAY_PORT
  returns false without releasing prev_sink. All other return paths in the
  function correctly call dc_sink_release(prev_sink), making this the only
  missing cleanup.

Fixes: 5461888 ("drm/amd/display: break down dc_link.c")
Signed-off-by: WenTao Liang <vulab@iscas.ac.cn>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Link: https://patch.msgid.link/20260626124555.36910-1-vulab@iscas.ac.cn
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 45510cf )
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/amd/display/dc/link/link_detection.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=f9922828a4ebd26286fbe0286cc61695e7d9b07b

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

The original Linux kernel CVE announcement for CVE-2026-68256 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68256

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:P/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 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).