[CVE-2026-64219][IMPORTANT] drm/amd/display: Validate payload length and link_index in dc_process_dmub_aux_transfer_async

[CVE-2026-64219][IMPORTANT] drm/amd/display: Validate payload length and link_index in dc_process_dmub_aux_transfer_async

[AL-KERNEL]

CVE: CVE-2026-64219
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: drm/amd/display: Validate payload length and link_index in dc_process_dmub_aux_transfer_async
Commit: d6590e3f766e3111dd1beaf88b9384d117acfa6b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d6590e3f766e3111dd1beaf88b9384d117acfa6b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64219
Analysis date: Fri, 24 Jul 2026 12:03:38 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A kernel stack buffer overflow in AMD display DMUB AUX transfer handling can occur when an oversized payload is copied into a 16-byte stack buffer, with an additional unchecked link_index OOB risk, making this a local DRM-path memory corruption issue with DoS impact and plausible LPE concern.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64219 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64219

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: IMPORTANT
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-2026-64219	IMPORTANT	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-121;*CWE-787;CWE-129;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A kernel stack buffer overflow can occur in dc_process_dmub_aux_transfer_async() because payload length was only guarded by ASSERT before copying into the 16-byte dpaux.data buffer. In release builds this assertion may not stop oversized input, so a reachable caller that supplies a payload larger than 16 bytes can corrupt kernel stack memory. The same function also used link_index to access dc links without validating it against link_count, creating an additional out-of-bounds access risk. For the CVSS the PR:L is used because a local user or local process with access to DRM display paths may be able to reach the vulnerable kernel interface, while full administrator privileges are not clearly required. The issue is not directly network reachable. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to kernel stack memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-64219 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: **5**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged or low-privileged trigger: +1**. A local process with access to DRM/KMS display paths may be able to reach AMD display control code. Full host root is not clearly required, but exact call-site reachability is not proven from this patch alone.
* **Memory corruption, strong corruption primitive: +2**. The commit explicitly describes `memcpy` into a 16-byte stack buffer with only an `ASSERT`, which is ineffective in release builds.
* **Reliable kernel crash / strong DoS: +1**. Oversized payload length or invalid `link_index` can plausibly produce stack corruption, OOB access, NULL dereference, or crash.
* **Weak LPE concern: +1**. Kernel stack corruption can theoretically affect control data or adjacent stack state, but this patch does not demonstrate a reliable overwrite target, reclaim primitive, or exploit chain.
* **Constrained or uncertain reachability/target control: -1**. The vulnerable buffer is a fixed local command buffer and the patch does not show the caller path or how much control userspace has over `payload->length`, payload contents, or `link_index`.
* Conservative total: **5**

Paranoid signals:

* **Local low-privileged trigger: +1**. DRM device access is often available to local desktop/session users or delegated containers with `/dev/dri`.
* **Memory corruption, strong corruption primitive: +2**. Stack buffer overflow in kernel code is a high-risk primitive.
* **Privilege escalation plausible: +2**. Not proven, but defensible for paranoid triage because kernel stack corruption can sometimes be exploitable beyond DoS if the input path is reachable and data is attacker-influenced.
* **Reliable kernel crash / strong DoS: +1**. Crash impact is realistic.
* **Privileged kernel/device-management memory corruption path: +1**. The corruption occurs inside trusted AMD display/DC/DMUB command construction code.
* Paranoid total: **7**

## 3. Reachability analysis

The issue is local, not network reachable. The likely trigger would be through AMD DRM/display paths that eventually call `dc_process_dmub_aux_transfer_async()` with attacker-influenced AUX payload length or `link_index`. In normal desktop deployments, DRM/KMS access may be available to a logged-in local user, compositor, display manager, or a process with access to `/dev/dri`, so **PR:L** is more appropriate than **PR:H** for paranoid triage.

Namespace and container impact depends on device passthrough. Containers without `/dev/dri` access are unlikely to reach this path. Containers or sandboxed services with delegated GPU/display access may reduce the effective privilege requirement. The path is hardware and driver dependent, but AMDGPU display/DC is common on systems with AMD GPUs, so this should not be treated as an obscure debug-only path.

Call-site confidence: **medium-low**. The patch clearly shows the vulnerable sink, but the exact userspace-to-kernel call path and degree of attacker control over `payload->length` and `link_index` are not included.

## 4. Severity interpretation

This is not an ordinary Moderate correctness fix. The patch explicitly fixes a kernel stack buffer overflow and an unchecked array index in a privileged display driver path. Realistic exploitation may still be difficult because caller reachability and input control are not shown, so the conservative interpretation is **Actionable Moderate**.

The paranoid interpretation is **Strong Important candidate** because kernel stack corruption can plausibly lead to more than DoS if reachable from a local low-privileged DRM path. Theoretical LPE potential exists, but a practical privilege escalation primitive is not demonstrated by the patch alone.

## 5. One-sentence report phrase

A kernel stack buffer overflow in AMD display DMUB AUX transfer handling can occur when an oversized payload is copied into a 16-byte stack buffer, with an additional unchecked link_index OOB risk, making this a local DRM-path memory corruption issue with DoS impact and plausible LPE concern.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This should not be auto-closed because the patch fixes explicit kernel stack memory corruption. Manual review should confirm whether unprivileged or DRM-session users can control `payload->length`, payload contents, or `link_index` through reachable KMS/AUX/display paths.

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

Patch: drm/amd/display: Validate payload length and link_index in dc_process_dmub_aux_transfer_async
Commit: d6590e3f766e3111dd1beaf88b9384d117acfa6b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d6590e3f766e3111dd1beaf88b9384d117acfa6b

Commit description:

[Why&How]
dc_process_dmub_aux_transfer_async() copies payload->length bytes into a
16-byte stack buffer (dpaux.data[16]) guarded only by an ASSERT(), which
is a no-op in release builds. If a caller ever passes length > 16 this
results in a stack buffer overflow via memcpy.

Additionally, link_index is used to dereference dc->links[] without
bounds checking against dc->link_count, risking an out-of-bounds access.

Replace the ASSERT with a hard runtime check that returns false when
payload->length exceeds the destination buffer size, and add a bounds
check for link_index before it is used.

Assisted-by: GitHub Copilot:Claude claude-4-opus
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit ba4caa9 )
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/amd/display/dc/core/dc.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=d6590e3f766e3111dd1beaf88b9384d117acfa6b

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

The original Linux kernel CVE announcement for CVE-2026-64219 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64219

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: 5
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).