[CVE-2026-68429][LOW] drm/dp_mst: Handle torn-down topology gracefully in drm_dp_mst_topology_queue_probe()

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

CVE: CVE-2026-68429
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/dp_mst: Handle torn-down topology gracefully in drm_dp_mst_topology_queue_probe()
Commit: b1d05cc61dfa6c4bd5e67855bec6a03e955f512d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1d05cc61dfa6c4bd5e67855bec6a03e955f512d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68429
Analysis date: Tue, 11 Aug 2026 20:53:45 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A DisplayPort MST hotplug race could trigger a spurious kernel warning in `drm_dp_mst_topology_queue_probe()`, causing compositor crashes and, only on `panic_on_warn` systems, a potential kernel panic DoS.

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

The original Linux kernel CVE announcement for CVE-2026-68429 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68429

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

CVE-2026-68429	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-362;CWE-703;CWE-755;Other CVSS 'AV:P/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.2';DESCR 'drm/dp_mst_topology_queue_probe can race with DisplayPort MST topology teardown during hotplug or link loss. The topology can be valid when the caller decides to queue a probe but become inactive before mgr lock is acquired, so the old drm_WARN_ON path reports a warning even though this is a valid state transition. The practical impact is a local or physical availability issue, mainly compositor crashes or a kernel panic only on systems configured with panic_on_warn. For the CVSS the PR:N is used because reliable triggering does not require an authenticated local privilege, but it usually requires physical access to a DP MST monitor or dock, or an equivalent local display hotplug condition. The issue is not network reachable. Impact is denial of service only and there is no supported memory corruption or privilege escalation primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)  SKIP CVE-2026-68429 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: dbaeef363ea54f4c18112874b77503c72ba60fec YES  NO NO unknown 	MAYBE	WARNONLY SIMPLEFIX RACE HARDWARE 	-	-	checked

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

ActionableScore=1
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Triggered signals:

* Hard or unreliable race / special timing required: -1
  The condition depends on a hotplug or link-loss race between MST topology teardown and `drm_dp_mst_topology_queue_probe()` acquiring `mgr->lock`.

* Physical-only or rare hardware-only condition: -2
  Practical triggering requires DisplayPort MST hardware behavior, such as monitor or dock connect or disconnect, or link loss.

* Reliable kernel crash / strong DoS: +1 only in paranoid interpretation
  The main observed impact is compositor crash from a spurious `drm_WARN_ON()`. Kernel panic is plausible only on systems configured with `panic_on_warn`, which is not the default assumption.

* Availability impact realistic: +1 only in paranoid interpretation
  Availability impact is realistic for the graphical session, and host-wide only under `panic_on_warn`.

Not triggered:

* No generic memory corruption. The patch does not show UAF, OOB access, double-free, stale pointer use after free, refcount misuse, or writable corruption.
* No privilege escalation plausible.
* No confidentiality or integrity impact.
* No network reachability.
* No security-boundary bypass.

## 3. Reachability analysis

The bug can be triggered by DisplayPort MST hotplug or link-loss timing. A local physical actor who can connect or disconnect MST displays or docks may influence the condition. It is not reachable over the network and does not require local code execution in the usual sense. Namespaces and containers do not materially affect reachability because the trigger is display hardware state, not a namespaced kernel API. The affected code is in DRM DisplayPort MST handling, which is common on desktop or workstation systems using MST, but the race and impact are hardware and configuration dependent.

Call-site confidence: high. The changed function and guarded call are visible in the patch.

## 4. Severity interpretation

This behaves like a Low or ordinary Moderate availability issue, not an Important-class vulnerability. The old code used `drm_WARN_ON()` for a valid race state and then exited before calling `drm_dp_mst_topology_mgr_invalidate_mstb()` with a NULL `mst_primary`. Therefore the patch supports a warning or compositor-crash fix, not a memory corruption primitive. Theoretical host-wide DoS exists only if warnings are configured to panic the kernel.

## 5. One-sentence report phrase

A DisplayPort MST hotplug race could trigger a spurious kernel warning in `drm_dp_mst_topology_queue_probe()`, causing compositor crashes and, only on `panic_on_warn` systems, a potential kernel panic DoS.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be handled as an auto-close or low-priority availability issue because the patch supports DoS-only impact, physical or local-display reachability, and no memory corruption or privilege escalation evidence.

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

Patch: drm/dp_mst: Handle torn-down topology gracefully in drm_dp_mst_topology_queue_probe()
Commit: b1d05cc61dfa6c4bd5e67855bec6a03e955f512d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b1d05cc61dfa6c4bd5e67855bec6a03e955f512d

Commit description:

A hotplug or link-loss event can tear down the MST topology
(setting mgr->mst_state = false and mgr->mst_primary = NULL) concurrently
with a caller invoking drm_dp_mst_topology_queue_probe(). Since the check
is already performed under mgr->lock, the condition is not a programming
error but a valid race -- the topology was valid when the caller decided
to call this function, but was torn down before the lock was acquired.

Replace the drm_WARN_ON() with a graceful early return. This eliminates
spurious kernel warnings and the resulting compositor crashes observed
when connecting/disconnecting DP MST monitors, while keeping the correct
behavior of doing nothing when MST is not active. A drm_dbg_mst() trace
is added so the skipped probe remains observable under MST debug logging.

The existing WARN_ON(mgr->mst_primary) in drm_dp_mst_topology_mgr_set_mst()
already catches the case where the topology is initialized twice, so no
diagnostic coverage is lost.

Fixes: dbaeef3 ("drm/dp_mst: Add a helper to queue a topology probe")
Cc: Imre Deak <imre.deak@intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Jonas Emilsson <jonas.emilsson@gmail.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/all/20260503034533.1023686-1-jonas.emilsson@gmail.com
Acked-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260622140532.526722-1-luciano.coelho@intel.com
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>
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=b1d05cc61dfa6c4bd5e67855bec6a03e955f512d

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

The original Linux kernel CVE announcement for CVE-2026-68429 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68429

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:P/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.2

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: 0
  Paranoid ActionableScore: 1

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).