[CVE-2026-89942][MODERATE REGULAR] iio: buffer: Fix potential use-after-free in anonymous buffer release

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

CVE: CVE-2026-89942
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: iio: buffer: Fix potential use-after-free in anonymous buffer release
Commit: ab53077511cde811bf3ee219426c522329c41226
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ab53077511cde811bf3ee219426c522329c41226
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89942
Analysis date: Wed, 16 Sep 2026 11:46:20 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89942 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89942

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 REGULAR
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-2026-89942	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89942 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-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'An anonymous IIO buffer release path can hit a use-after-free because iio_device_put() may drop the last device reference while buffer->dmabufs_mutex is still protected by an automatic guard. If the device was removed through unbind or hotplug and the anonymous buffer handle holds the last reference, the IIO device and buffer can be freed before the guard unlocks the mutex. This can cause a use-after-free on mutex unlock and at minimum a local kernel crash. For the CVSS the PR:L is used for the paranoid score because a local user with access to the IIO buffer interface may be able to hold the anonymous buffer handle while removal is triggered by hotplug, a management service, or delegated device control. The issue is not network reachable. Impact is at least local denial of service and in worst case may allow confidentiality or integrity impact due to kernel use-after-free memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)  SKIP CVE-2026-89942 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	LOCK INIT UAF HARDWARE MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

Applied conservative floor: 3 -> 4 because a real kernel UAF is established and host-admin-equivalent privilege is not positively established. No additional paranoid escalation is applied because controlled reclaim, repeated refcount mutation, arbitrary write, disclosure, or privilege escalation is not demonstrated.

2. Signal breakdown

Positive signals:

* +2 Real UAF / lifetime violation. `iio_device_put()` may drop the last reference and free the IIO device and buffer while `buffer->dmabufs_mutex` is still guarded.
* +1 Reliable kernel crash / strong DoS. The later automatic mutex unlock can operate on freed storage and plausibly cause a kernel fault or memory corruption.
* +1 paranoid only considered for post-free mutex mutation. It is not added as an independent score because it is the same stale operation already counted as the UAF and crash consequence.

Negative signals:

* No -2 host-admin penalty. Unbind is host-admin-controlled, but hot-unplug or delegated management is also mentioned, so complete privilege boundary is unresolved rather than positively host-admin-only.
* No -1 hard race penalty. The issue is an ordered lifetime/scope bug in release handling, not a narrow timing race.
* No hardware rarity penalty. IIO is device-specific, but the patch does not establish rare hardware as a necessary security condition.

Important non-awarded signals:

* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact not awarded because no disclosure path, controlled replacement, or security-sensitive write primitive is established.
* Local unprivileged trigger not awarded because a local user may hold the buffer fd, but the complete removal plus release trigger below host admin is not established.
* Structured stale-object leverage not awarded because the stale use is limited to release cleanup and mutex unlock.
* Concrete repeated release/refcount signal not awarded. The second stale operation is mutex unlock, not a second put/free/refcount mutation on the same dangling reference.
* Extra availability point not awarded because the same crash is already counted.

Repeated release/refcount checklist:

* First release can end lifetime: yes, `iio_device_put()` may drop the final device reference.
* Exact same reference survives: the guarded mutex address remains scheduled for unlock by the guard mechanism.
* Second operation is a real lifetime/refcount mutation: no, it is mutex unlock, not object lifetime release.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no, because no concrete second lifetime/refcount mutation and no meaningful reuse leverage is established.

3. Reachability analysis

The direct vulnerable operation occurs when a process releases an anonymous IIO buffer file descriptor after the underlying IIO device has already been removed. A local process with access to the IIO buffer interface may influence the lifetime of the open handle and can trigger the release path by closing the fd.

The device removal prerequisite is less certain. The commit explicitly mentions unbind and hot-unplug. Unbind normally requires host-admin-equivalent privilege, while hot-unplug may be physical or externally initiated and not necessarily controlled by the same local user. The supplied evidence does not prove that an ordinary user can perform the complete trigger sequence alone. It also does not prove that host admin is always required. Therefore HostAdminRequired is UNKNOWN.

No namespace-scoped privilege model is established. This is not a netns or userns-scoped capability path. NamespaceScopedPrivilege is NO.

Prior setup requires an IIO device and anonymous buffer interface to exist. This is device-dependent but not enough to apply a rare-hardware penalty from the supplied evidence.

The bug is not meaningfully race-dependent. The relevant condition is that the device was removed and the anonymous buffer handle later releases the last reference while the mutex guard is still active.

4. Primitive and severity interpretation

The exact primitive is a real use-after-free in the IIO anonymous buffer release path. The buffer mutex is still under an automatic guard when `iio_device_put()` may free the device and buffer containing that mutex. The subsequent guard cleanup can unlock a mutex located in freed memory.

This is an active structured kernel object, but the stale use is cleanup/release-path limited. The demonstrated consequence is kernel crash or local DoS. A broader exploitation chain is speculative because the patch does not establish controlled reclaim, attacker-shaped replacement contents, callback use, arbitrary read/write, disclosure, or a concrete second lifetime/refcount mutation.

PrimitiveClass is CLASS_B because a real kernel lifetime violation exists. It is not CLASS_A because no strong exploitation primitive is demonstrated. It is not CLASS_C because the patch directly establishes a UAF rather than merely an invalid state or WARN.

ImportantCandidate is NO because the available evidence fits a constrained cleanup UAF with demonstrated crash impact and no meaningful unresolved exploitation leverage beyond the stale mutex operation. ManualReview is RECOMMENDED because it is still a real kernel UAF in a release path. AutoDowngradeAllowed is YES because no concrete repeated refcount/lifetime mutation, security-boundary bypass, controlled corruption, or LPE path remains unresolved.

Likely operational severity is borderline Moderate. It should be reviewed for affected product exposure and device access policy, but the patch alone does not justify Important-class treatment.

5. One-sentence report phrase

An anonymous IIO buffer release can unlock a mutex after `iio_device_put()` frees the last device and buffer reference, causing a local use-after-free and likely kernel crash when device removal and buffer fd release overlap.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

A real kernel UAF is established, but the stale operation is cleanup-path mutex unlock and there is no demonstrated controlled reclaim, arbitrary write, information disclosure, privilege escalation, or repeated lifetime/refcount mutation.

7. Call-site confidence

Call-site confidence: high

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

Patch: iio: buffer: Fix potential use-after-free in anonymous buffer release
Commit: ab53077511cde811bf3ee219426c522329c41226
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ab53077511cde811bf3ee219426c522329c41226

Commit description:

An anonymous buffer handle holds a reference to the underlying IIO device.
The reference is dropped in the buffer handle's release function. If the
device has been removed, either through unbind or hot-unplug, the buffer
handle might hold the last reference.

The release function takes the mutex for the buffer using a guard, which
means the unlock happens after all the code in the function, including
`iio_device_put()`. If the anonymous buffer holds the last reference this
might free both the IIO device and the buffer, which contains the mutex,
leading to use-after-free when the mutex is unlocked.

Fix this by using a scoped guard just around the buffer dmabuf list access,
making sure the mutex is unlocked before releasing the IIO device.

Version 10 of the patch that introduced this issue used this exact scheme
of first unlocking and then dropping the reference [1]. During review it
was suggested to use a guard instead, and version 11 made that change [2].

Reported-by: codex:gpt-5.6
Fixes: 3e26d9f ("iio: core: Add new DMABUF interface infrastructure")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/linux-iio/20240605110845.86740-4-paul@crapouillou.net #[1]
Link: https://lore.kernel.org/linux-iio/20240618100302.72886-4-paul@crapouillou.net #[2]
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
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=ab53077511cde811bf3ee219426c522329c41226

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

The original Linux kernel CVE announcement for CVE-2026-89942 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89942

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:N/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: unknown
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).