[CVE-2026-63885][MODERATE 7.0] drm/gem: fix race between change_handle and handle_delete

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

CVE: CVE-2026-63885
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: drm/gem: fix race between change_handle and handle_delete
Commit: 0dfa42cfe4dbe114533480503934f43e33c1e83d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0dfa42cfe4dbe114533480503934f43e33c1e83d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63885
Analysis date: Sun, 19 Jul 2026 18:33:50 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race between DRM GEM handle replacement and handle deletion can free a GEM object while a new IDR handle still references it, creating a local race-dependent UAF that can crash the kernel and may warrant further review for stronger memory-corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63885 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63885

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 7.0
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-2026-63885	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:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in drm_gem_change_handle_ioctl can leave the old GEM handle visible while the new handle is already being installed. A concurrent GEM_CLOSE or handle delete on the old handle can drop the handle count to zero and free the GEM object while the new IDR entry still references that object. This creates a use-after-free on a kernel GEM object and may lead to a kernel crash or potentially stronger impact depending on allocator reuse and driver behavior. For the CVSS the PR:L is used because a local user or process with access to a DRM file descriptor or render node can trigger the racing ioctls without full administrator privileges. The issue is not network reachable. Impact is at least a local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-63885 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 5e28b7b94408897e41c63477aabc9e1db439bc8c YES  NO NO unknown 	MAYBE	SIMPLEFIX RACE ERRORPATH HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process with access to a DRM fd or render node can race `drm_gem_change_handle_ioctl()` against `GEM_CLOSE` / handle delete. Full root privileges are not required in typical desktop or GPU-enabled environments.

* Memory corruption, weak primitive: +1
  This is a real UAF condition, but the patch does not show attacker-controlled reclaim, type confusion, callback dispatch, arbitrary write, or controlled object replacement. Per race-UAF downgrade guidance, this is weaker than a strong corruption primitive.

* Real lifetime corruption: +1
  The old handle can be deleted concurrently, dropping `handle_count` to zero and freeing the GEM object while the new IDR entry still references it.

* Reliable kernel crash / strong DoS: +1
  A stale GEM object reference can plausibly lead to kernel crash when the new handle is later used.

* Broad/common local subsystem exposure: +1
  DRM GEM ioctls and render nodes are common on systems with GPUs, especially desktops, workstations, and GPU-enabled compute nodes.

Subtracted signal:

* Hard or unreliable race / special timing required: -1
  Exploitation requires hitting a window between lock release and reacquisition during handle replacement.

Paranoid adjustment:

* Paranoid score is 5 rather than 4 because the affected API gives local users meaningful control over DRM object lifetime and handle operations, and the bug leaves a freed kernel object reachable through a new IDR handle. However, no stronger LPE primitive is demonstrated, so the score is capped below Important.

## 3. Reachability analysis

The bug is locally reachable through DRM GEM handle-management ioctls. The likely attacker is a local user or process that can open a DRM render node or otherwise obtain a DRM file descriptor. This is commonly possible for users in graphical sessions or users in the `render` or `video` group, depending on distribution policy.

The issue is not network reachable. Containers may matter if DRM devices are passed through into a container or if a workload is granted access to `/dev/dri/renderD*`. In such cases, the attacker may be container-local rather than host-root, so treating this as PR:L is reasonable for practical triage.

The path is not universally exposed on headless systems without accessible DRM devices, but it is common enough that this should not be auto-closed as a niche driver bug.

Call-site confidence: high. The commit explicitly describes the race with `drm_gem_handle_delete`, the `handle_count` drop to zero, object free, and stale new IDR reference.

## 4. Severity interpretation

This behaves above ordinary Moderate because it is a real kernel object lifetime bug with a stale reference after free. Realistic demonstrated impact is local DoS through kernel crash. Theoretical stronger impact exists because GEM object UAFs can sometimes become exploitable depending on allocator reuse and driver behavior, but the patch does not show controlled reclaim, arbitrary write, type confusion, callback control, or a proven privilege-escalation path.

Therefore, the conservative interpretation is borderline/manual-review Moderate. The paranoid interpretation is Actionable Moderate and should not be auto-closed.

## 5. One-sentence report phrase

A race between DRM GEM handle replacement and handle deletion can free a GEM object while a new IDR handle still references it, creating a local race-dependent UAF that can crash the kernel and may warrant further review for stronger memory-corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real UAF in a commonly exposed local GPU/DRM interface, with PR:L reachability in many deployments. The current evidence supports DoS most strongly, but the stale kernel object reference is sufficient to require manual security review.

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

Patch: drm/gem: fix race between change_handle and handle_delete
Commit: 0dfa42cfe4dbe114533480503934f43e33c1e83d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0dfa42cfe4dbe114533480503934f43e33c1e83d

Commit description:

drm_gem_change_handle_ioctl leaves the old handle live in the IDR
during the window between spin_unlock(table_lock) and the final
spin_lock(table_lock). A concurrent drm_gem_handle_delete on the old
handle succeeds in this window, decrements handle_count to 0, and frees
the GEM object while the new handle's IDR entry still references it.

NULL the old handle's IDR entry before dropping table_lock so that any
concurrent GEM_CLOSE on the old handle sees NULL and returns -EINVAL.
Restore the old entry on the prime-bookkeeping error path.

Fixes: 5e28b7b ("drm: Set old handle to NULL before prime swap in change_handle")
Signed-off-by: Zhenghang Xiao <kipreyyy@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patch.msgid.link/20260526085313.26791-1-kipreyyy@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/drm_gem.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=0dfa42cfe4dbe114533480503934f43e33c1e83d

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

The original Linux kernel CVE announcement for CVE-2026-63885 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63885

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:L/UI:N/S:U/C:L/I:L/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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).