[CVE-2026-63863][MODERATE REGULAR] drm/gpusvm: Fix unbalanced unlock in drm_gpusvm_scan_mm() [ Upstream

[CVE-2026-63863][MODERATE REGULAR] drm/gpusvm: Fix unbalanced unlock in drm_gpusvm_scan_mm() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63863
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: drm/gpusvm: Fix unbalanced unlock in drm_gpusvm_scan_mm() [ Upstream
Commit: 8efaa47a871662a8c21b819cec60786f7ef17ab4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8efaa47a871662a8c21b819cec60786f7ef17ab4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63863
Analysis date: Sun, 19 Jul 2026 11:10:53 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline Moderate, manual review recommended
Manual review required: YES

Summary:
An unbalanced unlock in `drm_gpusvm_scan_mm()` can release `gpusvm->notifier_lock` on a path where it was not acquired, causing a local bad unlock balance warning and possible synchronization disruption in GPU SVM memory scanning.

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

The original Linux kernel CVE announcement for CVE-2026-63863 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63863

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
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CVE-2026-63863	MODERATE	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:L/I:L/A:H';CWE-832;CWE-667;CWE-703;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.8';DESCR 'An unbalanced unlock in drm_gpusvm_scan_mm() can call drm_gpusvm_notifier_unlock() on paths where the gpusvm notifier_lock was not acquired. The observed symptom is a bad unlock balance warning, but an unlock of a non held synchronization primitive in GPU SVM memory scanning can also corrupt lock state or weaken notifier serialization depending on kernel config and lock type. For the CVSS the PR:L is used for the paranoid score because a local process with access to the DRM GPU SVM interface or render node may be able to reach this path without full administrative privileges. The issue is not network reachable and requires local GPU driver interaction. Impact is at least local denial of service via warning or lock state disruption, and worst case may allow limited confidentiality or integrity impact due to broken synchronization around memory migration state.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate, manual review recommended (with actual score 3)  SKIP CVE-2026-63863 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: f1d08a5864822684773105c60528e2abb577ca6c YES  NO NO unknown 	MAYBE	INIT WARNONLY SIMPLEFIX HARDWARE  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline Moderate, manual review recommended**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process with access to the DRM GPU interface or render node may plausibly reach GPU SVM scan paths without full administrative privileges.

* Availability impact realistic: +1
  The observed failure is a kernel warning for bad unlock balance. This is not a proven panic by itself, but it is a real kernel synchronization error and may become DoS relevant with panic_on_warn or unstable lock state.

Paranoid additional signals:

* Weak or indirect corruption candidate: +1
  The bug releases `gpusvm->notifier_lock` on a path where it was not acquired. This is not a demonstrated UAF or overwrite, but an unbalanced unlock in GPU SVM memory migration scanning can weaken serialization and create inconsistent synchronization state.

* Driver memory management synchronization risk: +1
  The affected code is in GPU SVM and HMM related memory scanning, where notifier locking protects migration state. This is enough for manual review, but not enough to classify it as strong memory corruption.

Not applied:

* No remote reachable signal.
* No strong LPE signal.
* No strong generic memory corruption signal.
* No Dirty Pipe like or page cache corruption signal.
* No high control GPU API bonus, because the patch does not show attacker controlled memory layout, callback control, refcount abuse, DMA mapping control, or page ownership corruption.

## 3. Reachability analysis

The likely trigger is local interaction with a DRM GPU SVM capable driver, probably through GPU userspace workloads using HMM or memory migration paths. In typical desktop or compute deployments, render node access may be available to non root users, so PR:L is the practical paranoid assumption. There is no network trigger.

The path is not universal across all Linux systems. It depends on GPU driver support for `drm_gpusvm` and relevant SVM or HMM functionality. This reduces broad exposure, but does not make it unreachable when the hardware and driver are present.

Call-site confidence: medium. The patch shows the direct broken control flow inside `drm_gpusvm_scan_mm()`, but not all external callers or userspace trigger paths.

## 4. Severity interpretation

This behaves like a borderline Moderate issue, not an Important class vulnerability based on the available patch. The concrete demonstrated impact is a bad unlock balance warning. The theoretical concern is broken synchronization around GPU SVM memory migration state, but the patch does not demonstrate UAF, stale object reuse, controlled overwrite, callback dispatch, type confusion, or privilege escalation.

The conservative score is low because the demonstrated failure is warning or local DoS like behavior. The paranoid score reaches manual review range because lock imbalance in memory management code can have non obvious follow on effects.

## 5. One-sentence report phrase

An unbalanced unlock in `drm_gpusvm_scan_mm()` can release `gpusvm->notifier_lock` on a path where it was not acquired, causing a local bad unlock balance warning and possible synchronization disruption in GPU SVM memory scanning.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a GPU SVM memory-management synchronization bug. It should not be treated as proven LPE from the patch alone, but it also should not be auto-closed without checking reachable callers and whether lock state corruption can affect memory migration or notifier safety.

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

Patch: drm/gpusvm: Fix unbalanced unlock in drm_gpusvm_scan_mm() [ Upstream
Commit: 8efaa47a871662a8c21b819cec60786f7ef17ab4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8efaa47a871662a8c21b819cec60786f7ef17ab4

Changed files:
  drivers/gpu/drm/drm_gpusvm.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=8efaa47a871662a8c21b819cec60786f7ef17ab4

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

The original Linux kernel CVE announcement for CVE-2026-63863 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63863

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:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 2
  Paranoid ActionableScore: 4

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).