* [CVE-2026-89941][MODERATE 7.0] iio: buffer: Make IIO DMA fence release RCU-safe
@ 2026-09-16 14:39 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 14:39 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89941
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: iio: buffer: Make IIO DMA fence release RCU-safe
Commit: 311595dc0b5621f74d8eb4dc38ef4efcdfe7e769
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=311595dc0b5621f74d8eb4dc38ef4efcdfe7e769
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89941
Analysis date: Wed, 16 Sep 2026 10:39:12 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89941 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89941
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89941 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89941 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-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'An RCU lifetime bug in the IIO/DMABUF buffer path can free an iio_dma_fence with kfree() even though dma_fence rules require RCU-safe freeing when a custom release callback is used. Another CPU or RCU reader may still observe the dma_fence after release, which creates a use-after-free risk in kernel memory. For the CVSS the PR:L is used for the paranoid score because an attacker would need local access to an IIO device or a process context that can exercise the IIO DMABUF fence path, but full administrator privileges are not necessarily required in systems that delegate device access. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact or privilege escalation due to a kernel use-after-free.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) SKIP CVE-2026-89941 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE INIT UAF DMAorINTERRUPT LINUS KPANIC MEMORY - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 4. No floor or cap changed the score.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap changed the score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF / lifetime violation. The patch states that iio_dma_fence used a custom dma_fence release callback with kfree(), while dma_fence requires RCU-safe freeing for custom releases. That directly establishes a possible post-lifetime use by RCU readers.
* +1 Reliable kernel crash / strong DoS. A freed dma_fence observed by another CPU or RCU reader can realistically lead to kernel UAF symptoms, including crash.
* +1 RCU lifetime invariant violation. The fix removes the custom release path so the object is freed through dma_fence_free(), which uses kfree_rcu(). This shows the missing grace-period lifetime protection is the core defect.
Paranoid additional signal:
* +1 Attacker-influenced repeatable lifetime concern. A local process able to exercise the IIO DMABUF buffer path may be able to repeatedly create and release these fence objects, making the RCU lifetime issue more actionable. This is not treated as controlled reclaim or arbitrary write.
Non-awarded signals:
* Controlled reclaim is not demonstrated.
* Arbitrary kernel read or write is not demonstrated.
* Confidentiality and integrity impact are not awarded as ActionableScore points.
* Remote reachability is not established.
* Concrete repeated release/refcount mutation is not awarded. The bug is unsafe kfree before RCU grace period, not a visible first put followed by a second put on the same dangling reference.
* Structured stale-object leverage is not separately awarded beyond the paranoid lifetime concern because no distinct post-reuse field consumption or attacker-shaped replacement object is shown.
* Extra Availability is not awarded because the same crash/DoS is already counted once.
3. Reachability analysis
The vulnerable path is local through the IIO DMABUF buffer/fence infrastructure. The supplied patch does not prove that host administrator privilege is required, but it also does not fully prove ordinary unprivileged reachability for the complete operation. Therefore HostAdminRequired is UNKNOWN.
A realistic attacker likely needs local access to an IIO device or a service/process context that can enqueue DMABUF buffers and exercise the associated fence lifecycle. This may be below full host administrator privilege on systems that delegate device access, but that deployment detail is not proven by the patch alone.
There is no network trigger. No namespace-scoped capability chain is shown. No special rare hardware requirement is established beyond availability of the relevant IIO DMABUF functionality. The issue is an RCU lifetime ordering problem rather than a hard timing race requiring a narrow one-shot window.
4. Primitive and severity interpretation
The exact primitive is an RCU lifetime violation causing a possible use-after-free of struct iio_dma_fence / struct dma_fence. The object is an active structured kernel synchronization object, not a fixed passive buffer. The stale use is not merely a local cleanup pointer fault, because dma_fence objects are intentionally visible to concurrent users and require RCU-safe freeing.
This is CLASS_B: a real kernel lifetime/corruption primitive requiring exploitation-leverage analysis. It is not CLASS_A because the patch does not establish controlled reclaim, attacker-controlled replacement contents, callback hijack, arbitrary read/write, or demonstrated privilege escalation.
The demonstrated practical consequence is local kernel crash/DoS. The speculative consequence is stronger exploitation from a kernel UAF, but that is unresolved and not proven. ImportantCandidate is YES because an active RCU-visible dma_fence UAF should not be automatically downgraded without manual review. AutoDowngradeAllowed is NO because the concrete RCU lifetime primitive leaves meaningful post-lifetime behavior unresolved.
5. One-sentence report phrase
An RCU lifetime bug in the IIO DMABUF fence path can free an iio_dma_fence with kfree() instead of RCU-safe freeing, allowing concurrent dma_fence users to observe freed kernel memory and causing at least a local DoS with unresolved UAF exploitation risk.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: real kernel UAF is directly implied by the dma_fence RCU lifetime rule, and the affected object is an active synchronization object rather than a simple fixed stale buffer.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: iio: buffer: Make IIO DMA fence release RCU-safe
Commit: 311595dc0b5621f74d8eb4dc38ef4efcdfe7e769
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=311595dc0b5621f74d8eb4dc38ef4efcdfe7e769
Commit description:
The `dma_fence` documentation states that if a custom release
implementation is provided, the `dma_fence` object must be freed in an
RCU-safe way. The current `iio_dma_fence` implementation uses `kfree()`,
which might result in a use-after-free.
Remove the custom `release` implementation. This makes the DMA fence core
fall back to `dma_fence_free()`, which calls `kfree_rcu()` on the fence.
This requires that the fence be the first member of `struct iio_dma_fence`.
Using the default release method for extended DMA fence structures is a
common pattern.
Reported-by: codex:gpt-5.6
Fixes: 3e26d9f ("iio: core: Add new DMABUF interface infrastructure")
Signed-off-by: Lars-Peter Clausen <[email protected]>
Cc: <[email protected]>
Signed-off-by: Jonathan Cameron <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
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=311595dc0b5621f74d8eb4dc38ef4efcdfe7e769
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89941 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89941
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: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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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