[CVE-2026-90035][LOW] drm/amd/display: fix division by zero in get_estimated_bw()

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

CVE: CVE-2026-90035
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/amd/display: fix division by zero in get_estimated_bw()
Commit: 4c5b61b9df6ab33b55f228e1b6579cefad11a9f7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4c5b61b9df6ab33b55f228e1b6579cefad11a9f7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90035
Analysis date: Wed, 16 Sep 2026 08:38:11 -0400
ActionableScore: 2
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-2026-90035 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90035

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-2026-90035	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90035 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-369;CWE-703;CWE-754;BEST CVSS score: '4.6';DESCR 'A divide by zero can occur in the AMD display USB4/DPIA bandwidth handling path because get_estimated_bw() divides by bw_granularity before that value is guaranteed to be initialized. A connected USB4/DPIA tunneling device can trigger the fault by reporting DP_TUNNELING_ESTIMATED_BW_CHANGED before any DP_TUNNELING_BW_ALLOC_CAP_CHANGED event has populated the bandwidth granularity. For the CVSS the PR:N is used because no OS level privileges are required once the attacker controls the connected device behavior. The issue is not network reachable and requires physical or hardware adjacent access to the display tunneling path. Impact is denial of service via a kernel divide by zero in the interrupt handling path. No memory corruption primitive or privilege escalation path is indicated by the code change.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)  SKIP CVE-2026-90035 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX HARDWARE 	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied.

2. Signal breakdown

Positive signals:

* +1 Firmware or device mediated external influence. The trigger is a connected USB4/DPIA tunneling device reporting DP_TUNNELING_ESTIMATED_BW_CHANGED before DP_TUNNELING_BW_ALLOC_CAP_CHANGED initializes bw_granularity.
* +1 Reliable kernel crash / strong DoS. The vulnerable path can divide by zero in the IRQ handling path, which is a realistic kernel fault / DoS condition.

Negative or non-awarded signals:

* +0 Remote reachable not awarded. This is not a network packet path and requires a connected display tunneling device or equivalent hardware-adjacent influence.
* +0 UAF or lifetime corruption not awarded. The patch shows a missing zero guard before division, not a stale object or post-free access.
* +0 Controlled corruption not awarded. No OOB write, arbitrary write, controlled reclaim, refcount mutation, or object replacement is established.
* +0 Confidentiality and integrity not awarded. The established consequence is divide-by-zero DoS only.
* +0 Extra availability not awarded. The same crash is already counted once as strong DoS.
* No race/timing penalty was applied. The issue is an ordering/state validation bug, not a hard timing race.
* No physical-only penalty was separately applied because device mediation is already reflected by using only +1 instead of direct remote reachability.

3. Reachability analysis

The complete trigger requires control over, or malicious behavior from, a connected USB4/DPIA tunneling device or equivalent display tunneling path. No host administrator privilege is required once the attacker controls that device behavior, so HostAdminRequired=NO.

This is not namespace-scoped and does not involve CAP_NET_ADMIN or another namespaced capability. It is also not directly reachable over the network. The relevant prerequisite is hardware-adjacent or physical access to the display tunneling path.

For DRM/KMS/display roles, the supplied evidence does not show an ordinary logged-in user, DRM master, compositor, or display service directly issuing the vulnerable state transition. The concrete trigger is device-reported DPCD bandwidth status ordering.

4. Primitive and severity interpretation

The exact primitive is a divide-by-zero in get_estimated_bw() caused by using link->dpia_bw_alloc_config.bw_granularity before it is guaranteed to be initialized. This is CLASS_C crash/DoS behavior.

There is no active structured stale object, no fixed stale storage UAF, and no concrete repeated release/refcount operation. The demonstrated consequence is a kernel fault via divide-by-zero in an interrupt-related path. A privilege escalation or memory corruption consequence is speculative and not supported by the patch.

ImportantCandidate=NO because the bug class is validation/logic leading to DoS only, with no unresolved corruption or security-boundary primitive. AutoDowngradeAllowed=YES because no UAF, ownership violation, authorization bypass, controlled corruption, or meaningful post-lifetime leverage remains unresolved.

Likely operational severity is ordinary Moderate / Low-like for kernel CVE triage. It may still be relevant for systems using AMD display USB4/DPIA tunneling, but it should not be treated as an Important-class kernel vulnerability based on the supplied evidence.

5. One-sentence report phrase

A connected USB4/DPIA tunneling device can trigger a divide by zero in AMD display bandwidth handling by reporting estimated bandwidth before bandwidth granularity is initialized, causing a kernel DoS without evidence of memory corruption or privilege escalation.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch establishes a straightforward divide-by-zero DoS with hardware-adjacent reachability and no UAF, memory corruption, authorization bypass, or plausible Important-grade primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: drm/amd/display: fix division by zero in get_estimated_bw()
Commit: 4c5b61b9df6ab33b55f228e1b6579cefad11a9f7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4c5b61b9df6ab33b55f228e1b6579cefad11a9f7

Commit description:

get_estimated_bw() divides by link->dpia_bw_alloc_config.bw_granularity,
which is zeroed by reset_bw_alloc_struct() and only populated once
DP_TUNNELING_BW_ALLOC_CAP_CHANGED has been handled.

link_dp_dpia_handle_bw_alloc_status(), the DPCD interrupt handler,
calls get_estimated_bw() whenever DP_TUNNELING_ESTIMATED_BW_CHANGED
is set, independently of whether DP_TUNNELING_BW_ALLOC_CAP_CHANGED
has ever fired for that link. A connected USB4/DPIA tunneling device
that reports an estimated-bandwidth change before ever reporting a
capability change drives a division by zero in this IRQ path.

link_dpia_send_bw_alloc_request() already guards the same
bw_granularity division; add the identical guard here rather than
introducing a new pattern.

Fixes: 8e5cfe5 ("drm/amd/display: upstream link_dp_dpia_bw.c")
Reviewed-by: Alex Hung <alex.hung@amd.com>
Assisted-by: gkh_clanker_t1000
Signed-off-by: Hari Mishal <harimishal1@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit f2a961457c33dc34223aad5c9e8971de34a4eed3)
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/amd/display/dc/link/protocols/link_dp_dpia_bw.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=4c5b61b9df6ab33b55f228e1b6579cefad11a9f7

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

The original Linux kernel CVE announcement for CVE-2026-90035 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90035

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: Not available
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.6

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: 2

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).