[CVE-2026-53136][IMPORTANT] drm/amd/display: Clamp VBIOS HDMI retimer register count to array size

[CVE-2026-53136][IMPORTANT] drm/amd/display: Clamp VBIOS HDMI retimer register count to array size

[AL-KERNEL]

CVE: CVE-2026-53136
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: drm/amd/display: Clamp VBIOS HDMI retimer register count to array size
Commit: 029571d51140650783be4fb98fe7cb4754752086
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=029571d51140650783be4fb98fe7cb4754752086
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53136
Analysis date: Fri, 31 Jul 2026 16:39:22 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A malformed AMD VBIOS can set HDMI retimer register counts larger than the fixed destination arrays, causing an out-of-bounds heap write during amdgpu driver probe with potential kernel crash or privilege escalation depending on firmware control.

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

The original Linux kernel CVE announcement for CVE-2026-53136 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53136

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
======================================================================

CVE-2026-53136	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-787;CWE-129;*CWE-20;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H';BEST CVSS score: '7';DESCR 'An out-of-bounds heap write can occur in the AMD display VBIOS parser because HdmiRegNum and Hdmi6GRegNum are used as copy loop bounds for fixed size HDMI retimer register arrays. A malformed VBIOS can provide counts up to 255 while the destination arrays hold only 9 normal register entries and 3 6G register entries, causing kernel heap memory corruption during amdgpu driver probe. For the CVSS the PR:L is used in the paranoid score because some environments may delegate GPU device control, driver binding, or firmware update paths to a less than fully privileged local user or service. The issue is not network reachable and normally requires local influence over VBIOS contents or the device firmware path. Impact may include kernel crash and potential privilege escalation due to an OOB heap write, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7) 	MAYBE	WRITE OOB TIMER HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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

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

* Firmware-mediated external influence: +1
  The malformed input comes from VBIOS integrated info tables, not from ordinary network or syscall input.

* Memory corruption, strong corruption primitive: +2
  The commit explicitly states that unchecked HdmiRegNum and Hdmi6GRegNum can cause an out-of-bounds heap write into fixed-size arrays during amdgpu driver probe.

* Weak LPE concern in conservative scoring: +1
  Kernel heap OOB write can plausibly affect adjacent kernel objects, but no concrete reclaim, target control, or exploitation path is demonstrated.

* Stronger LPE plausibility in paranoid scoring: +2
  If an attacker can influence VBIOS or firmware loading in a reduced-privilege device-management environment, the OOB heap write could be security-relevant beyond DoS.

* Reliable kernel crash / strong DoS: +1
  Heap corruption during driver probe can realistically crash the kernel or destabilize the display driver.

* Privileged kernel/device-management memory corruption path: +1
  The corruption occurs in a trusted DRM/AMDGPU firmware parsing path during device initialization.

* Broad/default/common subsystem exposure: +1
  amdgpu is a common default driver on AMD GPU systems, and the affected parser runs automatically during probe on relevant hardware.

* Requires admin/root/device-firmware control in typical deployments: -2 conservative
  Realistic triggering usually requires control over VBIOS contents, firmware path, driver bind/probe, physical device state, or a supply-chain style firmware condition.

* Reduced-privilege delegated-device scenario: -1 paranoid
  Some environments may delegate GPU management, driver rebinding, firmware update paths, or device assignment to less-than-full-host-root actors.

## 3. Reachability analysis

This is not remotely network-triggerable. The realistic trigger is a malformed AMD VBIOS or firmware-provided integrated info table parsed during amdgpu driver probe.

In normal deployments, modifying VBIOS or controlling the firmware path usually requires administrative privileges, physical access, compromised firmware supply chain, or privileged device-management access. Containers and namespaces normally do not lower this requirement unless GPU management, firmware loading, or PCI device assignment is delegated to a reduced-privilege service or container root.

The path is default-reachable on affected AMD GPU systems because the driver parses VBIOS during probe. Exploitability depends heavily on whether the attacker can influence the firmware/VBIOS data supplied to the kernel.

Call-site confidence: high. The patch directly shows the untrusted count fields used as loop bounds for writes into fixed-size arrays.

## 4. Severity interpretation

This is not an ordinary Moderate correctness issue. The bug has an explicit OOB heap write primitive in kernel driver initialization code.

Realistically, the main limiting factor is reachability: most attackers cannot modify VBIOS or firmware inputs without elevated privileges or a device supply-chain position. However, because the primitive is kernel heap corruption and not merely a warning, leak, or resource exhaustion, it should not be auto-closed as low-risk.

Conservative interpretation: Actionable Moderate due to firmware-mediated OOB heap write with difficult triggering.
Paranoid interpretation: Strong Important candidate if reduced-privilege firmware or GPU management control is plausible.

## 5. One-sentence report phrase

A malformed AMD VBIOS can set HDMI retimer register counts larger than the fixed destination arrays, causing an out-of-bounds heap write during amdgpu driver probe with potential kernel crash or privilege escalation depending on firmware control.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch explicitly fixes an OOB heap write in kernel memory, and although triggering usually requires firmware or privileged device-control influence, the corruption primitive is strong enough to require manual triage rather than automatic closure.

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

Patch: drm/amd/display: Clamp VBIOS HDMI retimer register count to array size
Commit: 029571d51140650783be4fb98fe7cb4754752086
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=029571d51140650783be4fb98fe7cb4754752086

Commit description:

[Why & How]
The VBIOS integrated info tables (v1_11 and v2_1) contain HdmiRegNum and
Hdmi6GRegNum fields that are used as loop bounds when copying retimer I2C
register settings into fixed-size arrays (dp*_ext_hdmi_reg_settings[9]
and dp*_ext_hdmi_6g_reg_settings[3]). These u8 fields are not validated
before use, so a malformed VBIOS can specify values up to 255, causing an
out-of-bounds heap write during driver probe.

Clamp each register count to the destination array size using min_t()
before the copy loops, in both get_integrated_info_v11() and
get_integrated_info_v2_1().

Assisted-by: GitHub Copilot:claude-opus-4.6
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 5a7f0ef )
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/amd/display/dc/bios/bios_parser2.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=029571d51140650783be4fb98fe7cb4754752086

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

The original Linux kernel CVE announcement for CVE-2026-53136 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53136

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:H/I:H/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).