[CVE-2026-68277][MODERATE REGULAR] drm/dp/mst: fix OOB reads on 2-byte fields in sideband reply parsers

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

CVE: CVE-2026-68277
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: drm/dp/mst: fix OOB reads on 2-byte fields in sideband reply parsers
Commit: bdf0508b1e6785d4a8982c637e97e68d60b47d7b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bdf0508b1e6785d4a8982c637e97e68d60b47d7b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68277
Analysis date: Mon, 10 Aug 2026 15:30:43 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A malformed DisplayPort MST sideband reply from a malicious or compromised monitor, dock, or MST hub can trigger a one byte OOB read in DRM MST reply parsing, potentially causing incorrect parser decisions or display disruption, but no direct write primitive or privilege escalation path is shown.

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

The original Linux kernel CVE announcement for CVE-2026-68277 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68277

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: MODERATE REGULAR
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-2026-68277	MODERATE	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:R/S:U/C:L/I:L/A:H';CWE-125;*CWE-20;CWE-193;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L';BEST CVSS score: '5.4';DESCR 'An out-of-bounds read in the DisplayPort MST sideband reply parsers can occur when a 2 byte field is parsed near the end of the received message. A malformed reply can make the parser read one byte beyond the message payload into adjacent fields of the same kernel structure, which may corrupt parser decisions or cause display MST failures. For the CVSS the PR:N because no local OS account is required once a malicious or compromised DP MST device can provide sideband replies to the host. The issue is not network reachable and normally requires physical access or user assisted connection of a malicious monitor, dock, or MST hub. Impact is mainly denial of service or display disruption. For the paranoid score, limited confidentiality and integrity impact are kept because the bug is an externally influenced OOB read, but there is no direct write primitive shown by the patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	OOB HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Firmware or device mediated external influence: +1
  A malformed DisplayPort MST sideband reply can be supplied by a malicious or compromised monitor, dock, MST hub, or similar DP device.

* Availability impact realistic: +1
  The malformed reply can cause incorrect MST parsing and plausible display or MST topology disruption.

* Broad or common subsystem exposure: +1
  DRM DisplayPort MST support is a broadly deployed kernel graphics path, although the affected path requires DP MST hardware and sideband replies.

* Physical only or rare hardware condition: -2
  The issue is not network reachable and normally requires physical access or user assisted connection of a malicious DP MST device.

Paranoid additional signals:

* Confidentiality impact plausible: +1
  The primitive is a one byte OOB read from adjacent fields in the same kernel structure. No clear user visible leak is shown, but limited read influence is technically present.

* Integrity impact plausible: +1
  The adjacent byte can influence parsed bandwidth or payload values, which may affect MST parser decisions. This is limited integrity concern, not arbitrary memory corruption.

Not awarded:

* No remote reachable score. This is not a network protocol path.
* No local unprivileged trigger score. A local user cannot trigger it by normal syscalls alone without the external DP MST device condition.
* No generic strong memory corruption score. The patch shows a bounded one byte OOB read, not OOB write, UAF, double free, type confusion, or arbitrary write.
* No privilege escalation score. There is no demonstrated reclaim, overwrite, callback control, refcount abuse, or object confusion primitive.
* No strong DoS score. A kernel crash is not demonstrated by the patch, only malformed parsing and potential display disruption.

## 3. Reachability analysis

The realistic trigger is a malformed DisplayPort MST sideband reply from an attached monitor, dock, hub, or compromised display path. No local OS account is required once such a device can provide replies, so CVSS PR:N is reasonable for the physical attack model. Network namespaces and containers do not materially lower or raise reachability because the parser is driven by kernel display hardware communication, not by a namespaced userspace API. The path is present on systems with DRM DisplayPort MST support, but exploitation requires relevant hardware and a malicious or faulty DP MST device, so practical exposure is narrower than ordinary local kernel APIs or network parsers.

Call-site confidence: medium. The vulnerable reads and fixed bounds checks are directly visible, but the full downstream impact of corrupted parsed MST fields is not fully shown in the provided context.

## 4. Severity interpretation

This behaves more like a Low to Borderline Moderate issue than an Important-class kernel vulnerability. The theoretical concern is an externally influenced OOB read in kernel parsing code, which deserves manual review because it affects a hardware-facing parser. The realistic evidence supports limited availability or display disruption impact, with only weak confidentiality and integrity concern in the paranoid interpretation. There is no evidence of a write primitive, lifetime corruption, privilege escalation, or reliable system-wide crash.

## 5. One-sentence report phrase

A malformed DisplayPort MST sideband reply from a malicious or compromised monitor, dock, or MST hub can trigger a one byte OOB read in DRM MST reply parsing, potentially causing incorrect parser decisions or display disruption, but no direct write primitive or privilege escalation path is shown.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED.
YES REQUIRES MANUAL CHECK because this is an externally influenced kernel OOB read in a hardware protocol parser, but it is not a Strong Important candidate based on the provided patch.

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

Patch: drm/dp/mst: fix OOB reads on 2-byte fields in sideband reply parsers
Commit: bdf0508b1e6785d4a8982c637e97e68d60b47d7b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bdf0508b1e6785d4a8982c637e97e68d60b47d7b

Commit description:

Three sideband reply parsers read 16-bit fields as:

  val = (raw->msg[idx] << 8) | (raw->msg[idx+1]);

and check bounds only after the fact. When idx == raw->curlen,
raw->msg[idx+1] reads one byte past the received message data into
the following struct fields (curchunk_len, curchunk_idx, curlen).

Affected functions:
 - drm_dp_sideband_parse_enum_path_resources_ack()
   full_payload_bw_number and avail_payload_bw_number fields
 - drm_dp_sideband_parse_allocate_payload_ack()
   allocated_pbn field
 - drm_dp_sideband_parse_query_payload_ack()
   allocated_pbn field

Fix by using a single combined check (idx + 2 > curlen) before each
2-byte read. Since the check is strictly tighter than idx > curlen,
no separate step is needed.

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>
[added fixes tag]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260510203128.2884846-1-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=bdf0508b1e6785d4a8982c637e97e68d60b47d7b

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

The original Linux kernel CVE announcement for CVE-2026-68277 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68277

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.4

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: 3

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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).