[CVE-2026-97900][LOW] drm/drm_exec: fix up contended obj when num_objects is 0

[CVE-2026-97900][LOW] drm/drm_exec: fix up contended obj when num_objects is 0

[AL-KERNEL]

CVE: CVE-2026-97900
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/drm_exec: fix up contended obj when num_objects is 0
Commit: 831cd124ce99c3e61266bed30519d52f7c26abed
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=831cd124ce99c3e61266bed30519d52f7c26abed
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97900
Analysis date: Sat, 26 Sep 2026 16:13:47 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97900 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97900

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-97900	LOW	https://www.kernelcve.org/list/?q=CVE-2026-97900 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-835;CWE-667;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A logic error in drm_exec_prepare_array can cause an infinite retry loop when an empty object array is processed while a previously contended DRM object remains pending. A local process using a DRM driver with this call pattern such as amdgpu userq signal or wait operations may trigger contention and cause the kernel to spin in drm_exec_retry_on_contention. This can consume CPU time, stall the affected ioctl path, and potentially cause a soft lockup or local denial of service. For the CVSS the PR:L because a local process requires access to the DRM device interface and relevant buffer objects. The issue is not directly network reachable. No confidentiality or integrity impact is supported because the patch indicates an unresolved locking state and infinite loop rather than a memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	LOCK HARDWARE 	YES	NO	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

The patch establishes a persistent kernel retry loop and local availability impact, but no memory corruption, lifetime violation, authorization bypass, disclosure, or integrity primitive.

2. Signal breakdown

* +1 Reliable local DoS. The patch explicitly states that drm_exec_retry_on_contention can spin forever after an unresolved contended object is retained across an empty drm_exec_prepare_array call.
* No Local unprivileged trigger point. The supplied context identifies amdgpu userq ioctls as an example, but does not establish that the complete BO creation, sharing, contention, and ioctl sequence is accessible to an ordinary user in the evaluated product.
* No remote reachability point. This is a local DRM ioctl and buffer-object locking path.
* No UAF, OOB, double free, refcount mutation, or stale-object primitive is established.
* No C or I impact is established. The failure is an unresolved lock-contention state causing an infinite loop.
* No extra Availability point. CPU spinning and stalled ioctl behavior describe the same DoS consequence.
* No hard-race penalty. The issue requires lock contention, but the bug is an ordered retry-state logic failure rather than a narrow externally uncontrollable timing race.
* No hardware or niche-subsystem penalty. AMDGPU hardware and DRM are not inherently rare enough to justify a negative point, and the supplied context does not prove a special hardware prerequisite.

3. Reachability analysis

The direct trigger is a local DRM driver path. A driver must use multiple drm_exec_prepare_array calls within one drm_exec_until_all_locked iteration, with an empty array preceding a non-empty array that encounters buffer-object lock contention. The commit identifies amdgpu userq signal and wait ioctls as an example.

An ordinary logged-in user may have access to a DRM render node on many systems, while DRM master, compositor, display server, delegated graphics service, and host administrator may have broader DRM access. However, the supplied patch does not establish that an ordinary user can perform the complete relevant userq ioctl and BO contention sequence on all affected configurations.

HostAdminRequired is therefore UNKNOWN rather than YES or NO. No namespace-scoped capability path is established. No network trigger, boot parameter, sysctl, prior privileged setup, or special device configuration is demonstrated by the supplied evidence. The contention condition may be reproducible by concurrent users of a shared BO, but the exact user-access and BO-sharing requirements require product-specific confirmation.

4. Primitive and severity interpretation

The exact primitive is an infinite retry loop caused by failure to process exec contended state when drm_exec_prepare_array receives num_objects equal to zero. The empty-array path incorrectly returns success without invoking drm_exec_lock_contended. A later drm_exec_retry_on_contention immediately restarts the loop, preventing progress to the non-empty call that would resolve the lock contention.

This is not active structured stale-object use, fixed stale storage, UAF, refcount corruption, double release, arbitrary write, object replacement, or semantic security-boundary reinterpretation. The demonstrated consequence is CPU spinning, a stalled ioctl path, and possible local soft lockup or local DoS. Privilege escalation, confidentiality impact, integrity impact, and host-wide kernel compromise are speculative and unsupported.

PrimitiveClass is CLASS_C because the demonstrated behavior is invalid-state and DoS behavior without an independently useful kernel-corruption primitive. ImportantCandidate is NO because there is no unresolved lifetime, ownership, authorization, or controlled-corruption mechanism. AutoDowngradeAllowed is YES because the patch supports a constrained availability-only condition.

5. One-sentence report phrase

A logic error in drm_exec_prepare_array can leave a contended DRM object unresolved when an empty object array is processed, causing drm_exec_retry_on_contention to spin indefinitely and resulting in local CPU consumption and denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm whether affected amdgpu userq ioctls and the required shared BO contention scenario are reachable through render-node permissions by ordinary local users in the evaluated distribution and deployment.

7. Call-site confidence

Call-site confidence: medium

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

Patch: drm/drm_exec: fix up contended obj when num_objects is 0
Commit: 831cd124ce99c3e61266bed30519d52f7c26abed
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=831cd124ce99c3e61266bed30519d52f7c26abed

Commit description:

drm_exec_prepare_array() silently returns success without calling
drm_exec_lock_contended() when num_objects is zero. This breaks the
invariant upheld by drm_exec_lock_obj(), where every entry point into
the locking sequence must first attempt to lock any previously
contended object before proceeding.

Drivers that chain multiple drm_exec_prepare_array() calls per
drm_exec_until_all_locked() iteration (e.g. amdgpu's userq signal/wait
ioctls, which prepare separate read and write BO arrays) can pass an
empty array for one of the two calls. If contention is hit while
preparing the non-empty array, exec->contended is set and the loop
retries; on retry, the empty-array call preceding it is a no-op that
never clears exec->contended, so drm_exec_retry_on_contention()
immediately jumps back to the top of the loop without ever reaching
the call that would resolve the contention. This spins forever.

Fix it by having drm_exec_prepare_array() call drm_exec_lock_contended()
directly when num_objects is zero, so a pending contended object dont
loop infinitely.

Fixes: 0959321 ("drm: execution context for GEM buffers v7")
CC: stable@vger.kernel.org # v6.6+
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Link: https://lore.kernel.org/r/20260908091729.2749399-1-sunil.khatri@amd.com
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/drm_exec.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=831cd124ce99c3e61266bed30519d52f7c26abed

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

The original Linux kernel CVE announcement for CVE-2026-97900 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97900

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: unknown
  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).