[CVE-2026-99007][LOW] drm/fb-helper: defer setup when fbdev_probe() fails, not just on -EAGAIN

[CVE-2026-99007][LOW] drm/fb-helper: defer setup when fbdev_probe() fails, not just on -EAGAIN

[AL-KERNEL]

CVE: CVE-2026-99007
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/fb-helper: defer setup when fbdev_probe() fails, not just on -EAGAIN
Commit: 4a2006c675758f0c765b36fec6b49d17e89159cf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4a2006c675758f0c765b36fec6b49d17e89159cf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99007
Analysis date: Mon, 26 Oct 2026 21:14:16 -0400
ActionableScore: 1
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A failed fbdev probe can leave an undeferred NULL-framebuffer modeset that later reaches a WARN during last-close restoration, with panic possible only under WARN-fatal configuration.

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

The original Linux kernel CVE announcement for CVE-2026-99007 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99007

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-2026-99007	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99007 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both 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:N/I:N/A:H';CWE-754;CWE-670;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A DRM fb helper state handling bug can leave probed mode sets with valid modes and connectors but a NULL framebuffer when fbdev_probe fails with an error other than EAGAIN. A later restore path can commit this inconsistent state and reach WARN_ON in __drm_atomic_helper_set_config. The demonstrated primitive is an invariant warning rather than a NULL pointer dereference, UAF, OOB access, information leak, or arbitrary write. For the CVSS the PR:L value reflects that a local process generally needs access to the DRM primary node and must reach the last close restore path. Triggering the preceding fbdev_probe failure is not shown to be attacker controlled, so AC:H is appropriate. Availability is Low in the base case due to failed display restoration and may become High when panic_on_warn converts the warning into a kernel panic. The issue is not remotely reachable. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	INIT WARNONLY HARDWARE LINUS SYZBOT 	-	-	checked

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

ActionableScore=1
ActionableScoreLower=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 0.
* Conservative score: 0; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; `panic_on_warn` can convert the demonstrated WARN into a host crash.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* Base +0 — invalid modeset state reaches `WARN_ON(!set->fb)`, but WARN-only behavior does not qualify for a reliable-crash point.
* Paranoid +1 — reliable kernel crash is plausible specifically on systems configured with `panic_on_warn`.
* No memory-corruption point: no UAF, OOB access, invalid dereference, write, or lifetime violation is shown.
* No authorization point: the patch preserves deferred setup state; it does not restore a capability or access-control check.
* No extra availability point: failed display restoration and the possible panic are consequences of the same invalid commit path.
* NEW52/B5-R does not apply: there is no retry-accumulated heap OOB write or corrupted descriptor subsequently consumed.
* NEW53/B3-PENDING does not apply: no shared-backing write exists.
* NEW54/B4-PENDING does not apply: no DENY-to-ALLOW authorization transition exists.

3. Reachability analysis

A process able to use the DRM primary node can plausibly cause the later `drm_lastclose()` restore on exit. However, the required preceding non-`EAGAIN` `fbdev_probe()` failure is not shown to be attacker-controlled. The complete trigger is therefore unresolved for ordinary users, DRM master, compositor, delegated display service, and host administrator alike. No namespace-scoped capability, special hardware requirement, prior attacker-controlled setup, or timing race is established.

4. Primitive and severity reading

The exact defect is inconsistent active modeset state: modes and connectors are populated while `mode_set->fb` remains NULL, and a later restore attempts to commit it. The downstream consumer detects this with a WARN. This is CLASS_C invalid-state/availability behavior, not active memory corruption. A panic is configuration-dependent; non-crash exploit leverage, confidentiality, integrity, and authorization impact are unsupported. There is no Important-preserving primitive, so automatic downgrade is allowed.

5. One-sentence report phrase

A failed fbdev probe can leave an undeferred NULL-framebuffer modeset that later reaches a WARN during last-close restoration, with panic possible only under WARN-fatal configuration.

6. Manual review advice

NO MANUAL CHECK NEEDED

The supplied call chain establishes only guarded invalid-state consumption and configuration-dependent denial of service.

7. Call-site confidence

Call-site confidence: high

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

Patch: drm/fb-helper: defer setup when fbdev_probe() fails, not just on -EAGAIN
Commit: 4a2006c675758f0c765b36fec6b49d17e89159cf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4a2006c675758f0c765b36fec6b49d17e89159cf

Commit description:

__drm_fb_helper_initial_config_and_unlock() calls
drm_client_modeset_probe() first, which already fills in each
mode_set->mode/num_connectors for every connected output, and only
afterwards calls drm_fb_helper_single_fb_probe() to create
fb_helper->fb and wire it into those mode_sets via
drm_setup_crtcs_fb().

If drm_fb_helper_single_fb_probe() fails with anything other than
-EAGAIN (e.g. dev->driver->fbdev_probe() itself fails), the function
bails out before drm_setup_crtcs_fb() runs and before setting
fb_helper->deferred_setup. The client is left with mode_sets that
have a mode and connectors but mode_set->fb == NULL, and nothing
marks this fb_helper as needing a retry.

__drm_fb_helper_restore_fbdev_mode_unlocked() only skips committing
when fb_helper->deferred_setup is set, so a later .restore() call
(drm_lastclose() on process exit, in this report) commits the stale
mode_set through drm_client_modeset_commit() ->
drm_client_modeset_commit_atomic() -> __drm_atomic_helper_set_config(),
which hits WARN_ON(!set->fb).

Mark fb_helper->deferred_setup on any drm_fb_helper_single_fb_probe()
failure, not just -EAGAIN, so restore keeps skipping the commit until
a later hotplug event successfully re-probes and re-populates
mode_set->fb. The real error code is still returned to the caller
unchanged.

Reported-by: syzbot+ca23c8570669ead78867@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ca23c8570669ead78867
Fixes: ca91a27 ("drm/fb-helper: Support deferred setup")
Cc: stable@vger.kernel.org
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260927075434.72658-1-ngocthang2710.1999@gmail.com
Link: https://patch.msgid.link/20260917154743.1621-1-shechenglong@xfusion.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/drm_fb_helper.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=4a2006c675758f0c765b36fec6b49d17e89159cf

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

The original Linux kernel CVE announcement for CVE-2026-99007 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99007

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 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).