[CVE-2026-90386][LOW] i3c: dw: avoid shift-out-of-bounds when DAA assigns no devices [ Upstream

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

CVE: CVE-2026-90386
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: i3c: dw: avoid shift-out-of-bounds when DAA assigns no devices [ Upstream
Commit: 5650b013e6bfc14c8b30be727f4a715b01860a08
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5650b013e6bfc14c8b30be727f4a715b01860a08
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90386
Analysis date: Fri, 18 Sep 2026 06:36:43 -0400
ActionableScore: 2
ActionableScore lower bound: 1
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-2026-90386 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90386

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-2026-90386	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90386 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-1335;CWE-190;CWE-682;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.6';DESCR 'A shift-out-of-bounds condition can occur in the DesignWare I3C master DAA path when ENTDAA assigns no devices on an empty I3C bus. In that case cmd.rx_len matches master.maxdevs and the computed GENMASK upper index becomes negative, which can trigger UBSAN during boot or device enumeration. For the CVSS the PR:N is used because the physical bus condition itself does not require an authenticated local user on the target system. The issue is not network reachable and is tied to local hardware or platform state. Impact is mainly denial of service through warning or boot disruption on systems configured to treat UBSAN or warnings as fatal.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	WRITE KASAN OOB HARDWARE LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	YES	NO	checked

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

ActionableScore=2
ActionableScoreLower=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1. No floor or cap applied.
* Raw paranoid score: 2
* Paranoid score: 2. No floor or cap applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive conservative signals:

* +1. Invalid arithmetic / invalid-state behavior is established. `GENMASK(master->maxdevs - cmd->rx_len - 1, 0)` can receive an effective upper index of `-1` when no I3C devices are assigned, triggering UBSAN shift-out-of-bounds.

Positive paranoid signals:

* +1. Same invalid arithmetic / UBSAN condition.
* +1. Availability concern is plausible only in configurations where UBSAN or warning handling is fatal, or where the warning disrupts boot or device enumeration on affected hardware.

Non-awarded signals:

* Real UAF not awarded. No stale pointer, freed object, refcount misuse, RCU lifetime issue, or double release is shown.
* Controlled memory corruption not awarded. The patch shows an invalid shift/mask calculation, not an OOB write, arbitrary write, controlled overwrite, or type confusion.
* Confidentiality and integrity not awarded. There is no disclosure path and no unauthorized modification of protected state.
* Remote reachability not awarded. The trigger is tied to local I3C hardware/platform state, not packets or a remotely exposed protocol.
* Local unprivileged trigger not awarded. No evidence shows an ordinary local user can cause the complete vulnerable DAA path.
* Reliable kernel crash not awarded conservatively. The supplied evidence shows UBSAN warning; fatal panic depends on configuration.
* Manual-review escalation not awarded. This is a DoS-only invalid-state bug without a meaningful exploitation primitive.

3. Reachability analysis

The observed trigger occurs during I3C Dynamic Address Assignment on an empty bus, where `ENTDAA` assigns no devices and `cmd->rx_len == master->maxdevs`. The issue is hardware/platform-state dependent and was observed at boot on a Gigabyte/AMD server.

There is no evidence that an ordinary local user can instantiate or control the complete I3C bus enumeration path. There is also no evidence that host administrator privileges are required for the condition itself, because the supplied trigger is a platform empty-bus condition rather than a user-invoked privileged operation. Therefore `HostAdminRequired=UNKNOWN`.

The issue is not namespace-scoped. Network namespaces, user namespaces, and delegated capabilities are not relevant to this I3C master driver path.

Prior setup is hardware/configuration dependent: the system must have the affected DesignWare I3C master driver path and an empty or no-device-assigned I3C bus. No network exposure exists. No race or special timing requirement is established.

4. Primitive and severity interpretation

The exact primitive is an invalid shift/mask computation caused by a negative logical index passed into `GENMASK()`. This is best classified as `CLASS_C`: invalid-state / warning / possible DoS behavior without an independently useful corruption primitive.

This is not active structured-object UAF and not fixed stale storage. No cleanup/destructor stale use exists. No concrete repeated release/refcount operation exists. No first lifetime operation can free an object, and no second lifetime mutation is present.

Demonstrated consequence is UBSAN shift-out-of-bounds during boot or device enumeration. The practical worst case is local availability impact if the platform treats UBSAN or warnings as fatal, causing boot disruption or kernel panic. Stronger consequences such as privilege escalation, data disclosure, arbitrary write, or memory corruption are not supported by the patch.

`ImportantCandidate=NO` because the established primitive is arithmetic invalid-state behavior only. `AutoDowngradeAllowed=YES` because there is no unresolved kernel corruption, lifetime, ownership, authorization, or security-boundary primitive.

Likely operational severity is Low-like / Ordinary Moderate.

5. One-sentence report phrase

A shift-out-of-bounds condition in the DesignWare I3C DAA path can be triggered by an empty I3C bus during device enumeration, causing a UBSAN warning and possible boot or availability disruption only on affected hardware/configurations.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: no memory corruption, UAF, authorization bypass, information disclosure, privilege escalation, network exposure, or meaningful attacker-controlled primitive is established.

7. Call-site confidence

Call-site confidence: high

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

Patch: i3c: dw: avoid shift-out-of-bounds when DAA assigns no devices [ Upstream
Commit: 5650b013e6bfc14c8b30be727f4a715b01860a08
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5650b013e6bfc14c8b30be727f4a715b01860a08

Changed files:
  drivers/i3c/master/dw-i3c-master.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=5650b013e6bfc14c8b30be727f4a715b01860a08

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

The original Linux kernel CVE announcement for CVE-2026-90386 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90386

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:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  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: 1
  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).