[CVE-2026-89940][MODERATE 7.0] iio: buffer: Tie IIO dma fence lock lifetime to the fence

[CVE-2026-89940][MODERATE 7.0] iio: buffer: Tie IIO dma fence lock lifetime to the fence

[AL-KERNEL]

CVE: CVE-2026-89940
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: iio: buffer: Tie IIO dma fence lock lifetime to the fence
Commit: 6865d79fca17a80fbd60c12550ca9a5e0e20e0eb
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6865d79fca17a80fbd60c12550ca9a5e0e20e0eb
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89940
Analysis date: Wed, 16 Sep 2026 13:24:25 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
An IIO DMABUF dma_fence may outlive its iio_dmabuf_priv owner while still using a spinlock stored inside that freed object, creating a local kernel use-after-free that can cause DoS and requires manual review for possible post-free synchronization-state corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-89940 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89940

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-2026-89940	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89940 CHECK WITH IMPACT FROM ORIG NN 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:H/I:H/A:H';*CWE-416;*CWE-672;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An IIO DMABUF dma_fence can outlive its iio_dmabuf_priv object while the fence still points to a spinlock stored inside that object. Later dma_fence operations may take the stale lock pointer after iio_dmabuf_priv has been freed, which creates a use-after-free in kernel synchronization state. For the CVSS the PR:L is used because a local user or process needs access to the IIO buffer DMABUF interface and a suitable dmabuf workflow, but full administrative privileges are not inherently required if the device node is delegated. The issue is not network reachable and requires local interaction with the device interface. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, the UAF could also have confidentiality or integrity impact because the freed lock storage may be reused and later accessed by dma_fence code, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) 	MAYBE	LOCK INIT UAF DMAorINTERRUPT LINUS KPANIC MEMORY 	-	-	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: 3
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

Conservative arithmetic: +2 real UAF / lifetime violation, +1 realistic kernel crash or lock corruption DoS. Raw 3 is raised to 4 by REAL UAF FLOOR 4 because a real stale post-lifetime object use is directly established and host-admin-equivalent privilege is not positively established.

Paranoid arithmetic: +2 real UAF / lifetime violation, +1 realistic kernel crash or lock corruption DoS, +1 attacker-influenced lifetime workflow for enqueue/release/fence lifetime under a local IIO DMABUF user model, +1 unresolved post-free synchronization-state mutation because dma_fence may operate on a lock whose storage already belongs to freed or reused memory. No additional floor or cap changes the paranoid score.

## 2. Signal breakdown

Positive signals:

* +2 Real UAF / lifetime violation: the commit explicitly states that `iio_dma_fence` can outlive `iio_dmabuf_priv`, while the fence still uses a spinlock embedded in `iio_dmabuf_priv`.
* +1 Reliable kernel crash / strong DoS: later dma_fence operations may take or manipulate a stale spinlock pointer after the backing object has been freed, which can realistically produce a kernel fault, lock corruption, or hang.
* +1 Paranoid attacker-influenced lifetime workflow: under the model where a local user has access to the IIO buffer DMABUF interface, the user can plausibly create/enqueue DMABUF-backed fences and influence release ordering.
* +1 Paranoid unresolved post-free synchronization-state mutation: the stale operation is not merely a passive read. Spinlock operations mutate synchronization state at an address whose original object lifetime may already have ended.

Non-awarded or withheld signals:

* Strong LPE / strong exploitation leverage +2 not awarded: controlled reclaim, attacker-shaped replacement object, function-pointer/callback control, arbitrary read/write, or demonstrated LPE is not established.
* Structured stale-object exploitation leverage +1 not separately awarded: the main stale operation is use of a freed spinlock storage by dma_fence, not demonstrated consumption of attacker-controlled structured fields after replacement.
* Concrete repeated release/refcount +1 not awarded: this is not a first put followed by a second put/release on the exact same dangling reference.
* Extra Availability +1 not awarded: the same crash/hang consequence is already counted once.
* Confidentiality and Integrity impact not awarded as separate ActionableScore signals: C/I effects are plausible enough for manual review but not concretely established by the patch.
* Remote reachability +2 not awarded: this is a local device/API path, not a network-triggered path.
* Host-admin penalty not applied: host-admin-equivalent privilege is not positively established.
* Namespace-scoped privilege bonus not applied: no evidence shows that the complete operation is namespace-scoped.

Repeated-release/refcount checklist:

* First release can end lifetime: not applicable to a repeated-release pattern.
* Exact same reference survives to a second release: not established.
* Second operation is a real lifetime/refcount mutation: not established.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied for repeated release: no.

## 3. Reachability analysis

The likely attacker is a local process able to access the IIO buffer DMABUF interface and submit or enqueue a DMABUF-backed buffer that creates an `iio_dma_fence`. The supplied patch does not prove that an ordinary unprivileged user can always access the relevant device node or complete the full workflow on all systems.

Host-admin-equivalent privilege is also not positively established. IIO device access may be delegated to non-root users or services depending on udev rules, device permissions, container/device assignment, or product configuration. Therefore `HostAdminRequired=UNKNOWN` is more accurate than YES or NO.

This is not network reachable. No namespace-scoped capability path is shown. No prior host setup is proven as a strict prerequisite beyond the existence of an IIO device and the DMABUF interface. The issue is not primarily a timing race from the diff. The lifetime mismatch is structural: the fence lifetime can exceed the `iio_dmabuf_priv` lifetime.

Special hardware exists in the sense that an IIO device and the IIO buffer DMABUF path are needed, but the patch does not establish a rare physical-only prerequisite sufficient for a hardware penalty.

## 4. Primitive and severity interpretation

The exact primitive is a lifetime mismatch: `dma_fence_init()` receives a pointer to `priv->lock`, but the `dma_fence` object may outlive `priv`. After `iio_dmabuf_priv` is released, the fence may still use the stale lock pointer.

This is a real UAF / stale post-lifetime object use. It is closer to an active synchronization-state lifetime bug than to fixed stale DMA storage. The stale object is not just a passive buffer. A spinlock is an active kernel synchronization primitive, and later dma_fence operations may lock or unlock memory that no longer belongs to the original object.

This is not a concrete repeated release/refcount case. There is no visible first lifetime put followed by a second lifetime put on the exact same reference. It is also not CLASS_A because controlled reclaim, arbitrary kernel read/write, callback control, or demonstrated privilege escalation is not shown.

PrimitiveClass is CLASS_B because a real kernel lifetime/corruption primitive exists and exploitation leverage remains unresolved. Demonstrated consequence is at least local DoS through crash, hang, or corrupted synchronization state. Speculative consequence is broader memory corruption if the freed lock storage is reused in an attacker-influenced way, but that is not proven.

ImportantCandidate=YES because this is an explicit UAF in a shared DMA fence synchronization path, and the stale operation can mutate freed synchronization storage. AutoDowngradeAllowed=NO because automatically closing it as ordinary Moderate would discard a concrete unresolved kernel lifetime primitive. Operationally, this is best treated as Actionable Moderate requiring manual review, with possible Important relevance depending on actual device-node reachability and reuse control.

## 5. One-sentence report phrase

An IIO DMABUF dma_fence may outlive its iio_dmabuf_priv owner while still using a spinlock stored inside that freed object, creating a local kernel use-after-free that can cause DoS and requires manual review for possible post-free synchronization-state corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch directly fixes a real UAF/lifetime bug where an active dma_fence synchronization primitive can reference freed object storage, and the practical exploitation impact depends on local IIO DMABUF reachability and whether freed lock storage can be reused in a meaningful way.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: iio: buffer: Tie IIO dma fence lock lifetime to the fence
Commit: 6865d79fca17a80fbd60c12550ca9a5e0e20e0eb
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6865d79fca17a80fbd60c12550ca9a5e0e20e0eb

Commit description:

The `iio_dma_fence` implementation currently uses a lock embedded in the
`iio_dmabuf_priv`. But the `iio_dma_fence` can outlive the
`iio_dmabuf_priv`, which can cause a use-after-free.

Tie the lifetime of the lock to the lifetime of the fence by embedding them
in the same struct.

We can't just hold a reference to the `iio_dmabuf_priv` from the
`iio_dma_fence` since `iio_buffer_dmabuf_release()` might sleep and the
fence release callback is not allowed to sleep.

Note that the `dma_fence` framework now has an internal lock that gets used
when the passing `NULL` for `lock` in `dma_fence_init()`, but in order to
allow this patch to be backportable use an external lock.

Reported-by: codex:gpt-5.6
Fixes: 3e26d9f ("iio: core: Add new DMABUF interface infrastructure")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/iio/industrialio-buffer.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=6865d79fca17a80fbd60c12550ca9a5e0e20e0eb

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

The original Linux kernel CVE announcement for CVE-2026-89940 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89940

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:L/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).