[CVE-2026-90427][MODERATE 7.0] iommu/tegra241-cmdqv: Don't fall back to a freed smmu after devm_krealloc() [ Upstream

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

CVE: CVE-2026-90427
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: iommu/tegra241-cmdqv: Don't fall back to a freed smmu after devm_krealloc() [ Upstream
Commit: 86197679b293f0601c3331d545f99a70a7780aa9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86197679b293f0601c3331d545f99a70a7780aa9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90427
Analysis date: Fri, 18 Sep 2026 00:53:25 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A use-after-free in the Tegra241 CMDQV SMMU probe fallback path can occur after devm_krealloc relocates and frees the original smmu object, causing stale reads and a constrained stale write that may crash the kernel and requires manual review for possible post-free corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-90427 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90427

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-2026-90427	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90427 CHECK WITH IMPACT FROM ORIG NN MODERATE	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-672;CWE-703;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the Tegra241 CMDQV SMMU probe path when devm_krealloc relocates the original smmu object and a later failure makes the caller fall back while still using the freed pointer. The fallback path can read fields from the stale smmu pointer and also update smmu options, so the impact is not limited to a simple warning and includes a constrained write to freed kernel memory. For the CVSS the PR:L is used for the paranoid score because a local user may be able to influence memory pressure and failure timing, while direct and reliable control of device probing usually needs administrative or platform control. The issue is not network reachable and requires a local system with NVIDIA Tegra241 CMDQV support. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	INIT NOMEMCHK ERRORPATH UAF HARDWARE DEBUGFS LINUS KPANIC 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 2
* Conservative score: 4
* Applied conservative floor/cap: Real UAF floor 4 applied because a real stale post-lifetime `smmu` use is established and host-admin-equivalent triggering is not positively established.
* Raw paranoid score: 3
* Paranoid score: 5
* Applied paranoid floor/cap: Floor 5 applied because the stale object is an active structured kernel object and the fallback path performs more than a shallow read: it reads fields, writes `smmu->options`, and calls `put_device()` via stale state.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

## 2. Signal breakdown

Conservative signals:

* +2 Real UAF / active lifetime violation: `devm_krealloc()` may relocate `smmu` and free the original object, while the caller can later use the old pointer on fallback.
* +1 Reliable kernel crash / strong DoS: fallback dereferences a freed `smmu` and can fault or corrupt kernel state.
* +1 Constrained stale write: `smmu->options &= ~ARM_SMMU_OPT_TEGRA241_CMDQV` writes a fixed bit clear through a potentially freed object.
* -1 Platform-specific path: issue is limited to NVIDIA Tegra241 Grace CMDQV SMMU support.
* -1 No attacker-controlled payload or target: the stale write is fixed and there is no demonstrated controlled reclaim, arbitrary write, or attacker-selected replacement object.

Paranoid differences:

* The no-attacker-controlled-target reduction is withheld in the paranoid score because the stale write occurs after a real `devm_krealloc()` relocation/free and later allocation failure, leaving post-free reuse consequences unresolved.
* No +2 strong exploitation leverage awarded: controlled reclaim, arbitrary kernel read/write, callback control, or demonstrated privilege escalation is not established.
* No concrete repeated release/refcount +1 awarded: the patch shows stale dereference/write of `smmu`, but not a second lifetime/refcount mutation on the exact same freed `smmu` reference.
* No extra Availability point awarded: the crash/fault risk is already counted once.
* No C/I-style actionable bonus awarded merely because UAF could theoretically corrupt reclaimed memory.

Repeated-release checklist:

* First release can end lifetime: yes, `devm_krealloc()` may free the original `smmu` if it relocates.
* Exact same reference survives: yes, old caller-local `smmu` remained reachable in the old code.
* Second operation is a real lifetime/refcount mutation on same object: no, stale `smmu` is dereferenced and written, but no direct second `put/free` of the same `smmu` object is shown.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: yes for paranoid only, due active structured stale object plus constrained post-free write/use, not due repeated refcount.

## 3. Reachability analysis

The vulnerable path is in the Tegra241 CMDQV SMMU probe/fallback path. A complete reliable trigger appears to require a system with NVIDIA Tegra241 CMDQV support, successful `devm_krealloc()` relocation of `smmu`, and a later failure after relocation, such as allocation failure for CMDQV structures.

Host-admin-equivalent privilege is not positively established from the supplied patch. However, direct and reliable control over device probing, driver binding, platform configuration, or fault injection would often require administrative or platform-level control. A local non-admin user may be able to influence memory pressure, but the supplied evidence does not prove that an ordinary user can complete the full trigger sequence reliably.

Namespace-scoped privilege is not applicable. This is an IOMMU/platform-device probe path, not a namespace-created object path.

No network reachability is established. No firmware-mediated remote input path is established. This is local/platform/device-initialization exposure.

Race repeatability is not the main issue. This is an ordered error-path/lifetime bug rather than a timing race. No race penalty is applied.

## 4. Primitive and severity interpretation

The exact primitive is a real use-after-free caused by `devm_krealloc()` relocating and freeing the original `struct arm_smmu_device`, followed by fallback code that continues to use the stale pointer.

This is an active structured object UAF, not fixed stale storage. The stale path reads `smmu->impl_dev`, writes `smmu->options`, and calls `put_device()` on a pointer obtained through stale state. That is stronger than a pure NULL dereference or warning, but it is still not a demonstrated Class A primitive because there is no controlled reclaim, arbitrary write, refcount takeover, callback control, or demonstrated LPE.

PrimitiveClass is CLASS_B. Demonstrated consequence is at least local kernel crash or memory corruption risk through stale object use. Speculative consequence is privilege escalation if the stale write/use can be shaped through allocator reuse, but that is not proven by the patch.

ImportantCandidate is YES because a concrete active-object UAF with a constrained stale write leaves meaningful exploitation leverage unresolved. AutoDowngradeAllowed is NO because automatically closing this as ordinary crash-only would discard a real post-lifetime kernel corruption primitive that deserves manual review.

Likely operational severity is Actionable Moderate, with Strong Important candidacy depending on whether manual analysis finds attacker-influenced reclaim or a stronger post-free corruption path.

## 5. One-sentence report phrase

A use-after-free in the Tegra241 CMDQV SMMU probe fallback path can occur after devm_krealloc relocates and frees the original smmu object, causing stale reads and a constrained stale write that may crash the kernel and requires manual review for possible post-free corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: real UAF is directly established, the stale object is active structured kernel state, and the fallback path performs a constrained write through the freed pointer. Controlled exploitation is not demonstrated, but the primitive is not safe to autoclose as ordinary DoS.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: iommu/tegra241-cmdqv: Don't fall back to a freed smmu after devm_krealloc() [ Upstream
Commit: 86197679b293f0601c3331d545f99a70a7780aa9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86197679b293f0601c3331d545f99a70a7780aa9

Changed files:
  drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.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=86197679b293f0601c3331d545f99a70a7780aa9

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

The original Linux kernel CVE announcement for CVE-2026-90427 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90427

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:H/UI:N/S:U/C:N/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).