[CVE-2026-68278][IMPORTANT] drm/dp/mst: fix buffer overflows in sideband chunk accumulation

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

CVE: CVE-2026-68278
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: drm/dp/mst: fix buffer overflows in sideband chunk accumulation
Commit: 53937a2787d29c7a460e984dc4f20ff6ac91dc65
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=53937a2787d29c7a460e984dc4f20ff6ac91dc65
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68278
Analysis date: Mon, 10 Aug 2026 10:34:50 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious DisplayPort MST monitor, hub, or dock can send malformed sideband reply chunks that trigger OOB reads and OOB writes in drm_dp_sideband_append_payload(), causing kernel memory corruption and at least a host DoS, with possible privilege escalation in the worst case.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68278 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68278

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-2026-68278	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-125;CWE-191;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H';BEST CVSS score: '6.8';DESCR 'A kernel memory corruption issue exists in the DP MST sideband reply parser because device controlled msg_len values can drive invalid chunk accumulation. A malicious DP MST monitor, hub, or dock can send malformed sideband reply chunks that cause an out of bounds read from chunk(48), an out of bounds write past chunk(48), or an out of bounds write past msg(256). For the CVSS the PR:N is used because no host account or kernel privilege is required once the attacker can provide or control the physical DP MST device. The issue is not IP network reachable and requires access to a physical DisplayPort MST connection or a malicious device already attached to the target. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	OOB HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	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:

* Firmware-mediated external influence: +1. The malformed data is supplied by a DP MST device over the physical DisplayPort sideband channel.
* Memory corruption, Strong corruption primitive: +2. The patch prevents concrete OOB writes past chunk(48) and msg(256), plus an OOB read caused by u8 underflow.
* Reliable kernel crash / strong DoS: +1. Kernel memory corruption in a DRM parser can realistically crash the host.
* Confidentiality impact plausible: +1. OOB read and memory corruption make limited disclosure plausible, although no direct read-back primitive is shown.
* Integrity impact plausible: +1. Device-controlled malformed chunks can drive OOB writes into adjacent kernel object fields.
* Physical-only or rare hardware-only condition: -2. Triggering requires a malicious or compromised physical DP MST monitor, hub, dock, or equivalent device.

Conservative total: 5

Paranoid additional signals:

* Privilege escalation plausible: +2 instead of weak LPE concern. This is not a NULL deref or validation-only bug. The patch fixes concrete kernel OOB writes with device-influenced data, so kernel code execution or privilege escalation is technically defensible in a malicious peripheral model.
* Broad/default/common subsystem exposure: +1. DisplayPort MST support is common on laptops, docks, monitors, and workstation setups, although exposure still requires physical device presence.

Paranoid total: 7

## 3. Reachability analysis

The attacker needs control over a DP MST device that can send forged sideband reply messages. This is not IP network reachable and should not be treated as a remote network vulnerability. No local Linux account is required once the malicious device is attached, so PR is effectively none in the device attack model. User interaction depends on scenario. Connecting a malicious dock or monitor is UI:R, while an already attached compromised device or attacker-accessible physical port can be modeled as UI:N for paranoid scoring.

Namespaces and containers do not materially reduce the trigger requirement because the vulnerable parser runs in the host kernel DRM stack and is driven by physical display hardware. The path is configuration and hardware dependent, but DisplayPort MST is common enough that this should not be auto-closed as rare-only.

Call-site confidence: high. The vulnerable function and bounds checks are directly present in the patch.

## 4. Severity interpretation

This behaves more like an Important candidate than an ordinary Moderate because the patch fixes concrete OOB read and OOB write conditions in kernel memory. Realistic exploitation is constrained by physical-device requirements, which keeps the conservative score at Actionable Moderate. The paranoid score is higher because the corruption is device-controlled and occurs in a trusted kernel parser, making manual analysis necessary.

The theoretical risk includes kernel memory corruption and possible privilege escalation or code execution. The realistic demonstrated impact from the patch is malformed DP MST sideband data causing memory corruption and likely kernel crash.

## 5. One-sentence report phrase

A malicious DisplayPort MST monitor, hub, or dock can send malformed sideband reply chunks that trigger OOB reads and OOB writes in drm_dp_sideband_append_payload(), causing kernel memory corruption and at least a host DoS, with possible privilege escalation in the worst case.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: concrete kernel OOB writes are present, input is attacker-controlled through a physical peripheral protocol, and the affected parser is in a host kernel driver path. This should not be auto-closed despite the physical-access requirement.

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

Patch: drm/dp/mst: fix buffer overflows in sideband chunk accumulation
Commit: 53937a2787d29c7a460e984dc4f20ff6ac91dc65
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=53937a2787d29c7a460e984dc4f20ff6ac91dc65

Commit description:

drm_dp_sideband_append_payload() has three related bugs when processing
device-provided sideband reply data:

1. Zero-length curchunk_len underflow: msg_len is a 6-bit field taken
   directly from the DP sideband header. If a device sends msg_len=0,
   curchunk_len is set to zero. The condition (curchunk_idx >= curchunk_len)
   is immediately true, and curchunk_len-1 wraps to 255 (u8 underflow).
   drm_dp_msg_data_crc4() reads 255 bytes from chunk[48], then memcpy()
   writes 255 bytes into msg[], both far out of bounds.

2. chunk[48] overflow: curchunk_len can reach 63 (6-bit field). chunk[] is
   only 48 bytes. Multi-iteration payload assembly appends 16-byte blocks
   until curchunk_idx reaches curchunk_len, writing up to 15 bytes past
   the end of chunk[] into msg[].

3. msg[256] overflow: each chunk contributes (curchunk_len-1) bytes to
   msg[]. No check ensures curlen + (curchunk_len-1) stays within msg[256],
   so the memcpy can spill into adjacent struct fields.

All three are reachable from any DP MST device that can forge sideband
reply messages on a physical connection.

Fixes: ad7f8a1 ("drm/helper: add Displayport multi-stream helper (v0.6)")
Cc: <stable@vger.kernel.org> # v3.17+
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260410041901.2438960-1-ashutoshdesai993@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/display/drm_dp_mst_topology.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=53937a2787d29c7a460e984dc4f20ff6ac91dc65

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

The original Linux kernel CVE announcement for CVE-2026-68278 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68278

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:L/PR:N/UI:R/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.8

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).