[CVE-2026-74690][MODERATE 7.0] s390/ism: Fix UAF of sba and ieq during ism_dev_exit() [ Upstream

[CVE-2026-74690][MODERATE 7.0] s390/ism: Fix UAF of sba and ieq during ism_dev_exit() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74690
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/ism: Fix UAF of sba and ieq during ism_dev_exit() [ Upstream
Commit: fc3021284050ecb3bba8a7851340cedcb5037928
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fc3021284050ecb3bba8a7851340cedcb5037928
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74690
Analysis date: Sat, 22 Aug 2026 13:41:44 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in s390 ISM device teardown can let an active interrupt handler access freed `sba` or `ieq` structures after `ism_dev_exit()`, causing a race-based UAF with realistic local DoS impact and limited but nonzero memory-corruption review concern.

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

The original Linux kernel CVE announcement for CVE-2026-74690 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74690

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-2026-74690	MODERATE	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-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in the s390 ISM driver can allow an interrupt handler to run in parallel with ism_dev_exit while sba and ieq data structures are being released. This creates a use after free condition in the interrupt path because an already active handler may still access those objects after they are freed. For the CVSS the PR:L in the paranoid vector represents an attacker or service account that can trigger driver unbind, reset, module unload, or device teardown through delegated administration rather than full host root. The issue is not directly network reachable and requires local control over the affected device lifecycle or a closely related management path. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to kernel memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	INIT UAF NETWORK HARDWARE LINUS MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Triggered signals:

* Generic memory corruption, weak race UAF primitive: +1
  The commit explicitly describes UAF of `sba` and `ieq` because an interrupt handler can run concurrently with `ism_dev_exit()` and access freed structures.

* Real lifetime corruption: +1
  This is a real object lifetime ordering bug. The fix separates unregistering from freeing and moves actual free after `free_irq()` drains active handlers.

* Reliable kernel crash / strong DoS: +1
  An interrupt handler dereferencing freed DMA coherent driver structures can realistically lead to kernel crash or corrupted driver state.

* Privileged kernel/device-management memory corruption path: +1
  The affected path is device teardown in a trusted s390 ISM driver, involving IRQ handling and DMA coherent memory.

Negative or limiting signals:

* Requires admin/root/device-management privileges in typical deployments: -2 conservative
  Reliable triggering normally requires driver unbind, device removal, module unload, reset, or equivalent device lifecycle control.

* Hard or unreliable race / special timing required: -1 conservative
  The bug depends on an interrupt handler being active in parallel with teardown.

* Rare hardware/subsystem exposure: -1 conservative
  s390 ISM is platform and hardware specific, not a broad default Linux exposure.

Paranoid interpretation:

* The paranoid score reduces the privilege penalty because delegated device management, service accounts, or reduced root in managed environments may be able to trigger teardown without full host-root equivalence.
* The paranoid score keeps the issue actionable because this is an explicit UAF in an interrupt path, not merely a warning or validation failure.
* No strong LPE bonus is awarded because the patch does not show attacker-controlled reclaim, object replacement, callback control, type confusion, or arbitrary write.

## 3. Reachability analysis

The bug is locally reachable through device lifecycle operations such as ISM device teardown, driver unbind, reset, or module unload. In normal deployments this is privileged and likely requires root or equivalent device-management authority. It is not directly network reachable and there is no evidence that ordinary packet traffic alone can trigger the vulnerable path.

Namespaces and containers may matter only if device management is delegated. A reduced-capability service account or containerized management component with access to unbind or reset the device would make PR closer to a practical PR:L scenario, but ordinary unprivileged local users should not be able to trigger it directly.

The path is not broad/default for all Linux systems because it is specific to s390 ISM hardware. On affected s390 systems using ISM, however, the code path is real and the interrupt lifetime ordering bug is concrete.

## 4. Severity interpretation

This behaves more like an Actionable Moderate issue than an ordinary Moderate because it is an explicit race-based UAF in an IRQ path. Realistic exploitation evidence mainly supports local DoS through kernel crash or device state corruption. Privilege escalation is theoretically possible in the broad sense of kernel UAF bugs, but the supplied patch does not show attacker-controlled reclaim, controlled replacement, write-after-free, callback dispatch, or refcount takeover.

Therefore the conservative score stays low due to privileged triggering, race timing, and rare hardware exposure. The paranoid score reaches 5 because kernel lifetime corruption in an interrupt handler should not be auto-closed without review.

## 5. One-sentence report phrase

A race in s390 ISM device teardown can let an active interrupt handler access freed `sba` or `ieq` structures after `ism_dev_exit()`, causing a race-based UAF with realistic local DoS impact and limited but nonzero memory-corruption review concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit UAF plus IRQ handler concurrency plus DMA coherent driver structures. Even though realistic triggering is privileged and hardware-specific, this is a concrete lifetime corruption bug and should not be auto-closed solely as a low-priority teardown crash.

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

Patch: s390/ism: Fix UAF of sba and ieq during ism_dev_exit() [ Upstream
Commit: fc3021284050ecb3bba8a7851340cedcb5037928
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fc3021284050ecb3bba8a7851340cedcb5037928

Changed files:
  drivers/s390/net/ism_drv.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=fc3021284050ecb3bba8a7851340cedcb5037928

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

The original Linux kernel CVE announcement for CVE-2026-74690 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74690

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: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: 3
  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: NO
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).