[CVE-2026-74751][MODERATE REGULAR] riscv: lib: Fix ZBB strnlen reading past count boundary [ Upstream

[CVE-2026-74751][MODERATE REGULAR] riscv: lib: Fix ZBB strnlen reading past count boundary [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74751
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: riscv: lib: Fix ZBB strnlen reading past count boundary [ Upstream
Commit: e697e30f3dd2da3a1df7dc0980546d5b53aea4b6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e697e30f3dd2da3a1df7dc0980546d5b53aea4b6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74751
Analysis date: Wed, 26 Aug 2026 10:43:57 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline Moderate / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-74751 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74751

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74751	MODERATE	CHECK WITH IMPACT FROM ORIG NN NONE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';CWE-125;CWE-126;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'The RISC-V ZBB strnlen implementation can read one machine word past the supplied count because the optimized loop loads the next word before checking the aligned boundary. If the input is aligned, the count is a multiple of the register size, and s plus count lands on a page boundary with the next page unmapped, this can fault in kernel context and cause a denial of service. For the CVSS the PR:L value is used because reliable triggering requires a local user or local process path that can influence a kernel strnlen call and the relevant buffer layout. The issue is not directly network reachable based on the patch context. Impact is primarily kernel crash or oops. A limited confidentiality impact is kept only in the paranoid score because the primitive is an out-of-bounds read, but no direct information disclosure or write primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate / manual review recommended (with actual score 3)  SKIP CVE-2026-74751 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB SKIP HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline Moderate / manual review recommended**::

2. Signal breakdown
* Local unprivileged trigger: +1. A local process may plausibly influence a kernel `strnlen()` call path and buffer boundary layout, but the supplied patch does not show a specific syscall call site.
* Reliable kernel crash / strong DoS: +1. The commit explicitly describes a kernel fault when the word past `count` starts on an unmapped page.
* Broad/default/common subsystem: +1. `strnlen()` is a core architecture library helper for RISC-V systems using the ZBB optimized path.
* Hard or unreliable boundary condition: -1. Triggering requires alignment, `count` being a multiple of the register size, no earlier NUL byte, and `s + count` landing on a page boundary with the next page unmapped.
* Confidentiality impact plausible: +1 in paranoid score only. The primitive is a one-word out-of-bounds read, but no direct disclosure path is shown.
* No integrity or LPE signal. The patch shows no write primitive, UAF, reclaim, type confusion, refcount takeover, or callback control.

3. Reachability analysis
This is local rather than network reachable. Practical triggering requires a RISC-V kernel using the ZBB optimized `strnlen()` implementation and a call path where an attacker can influence the string address, length, alignment, and page mapping boundary. Namespaces do not materially change the issue except by providing more local workloads that may reach kernel string handling paths. The path is architecture and CPU feature dependent, but it is a core helper once that optimized implementation is active. Call-site confidence: medium, because the patch shows the helper behavior clearly but does not include the concrete caller that gives userspace control.

4. Severity interpretation
This behaves like an actionable but lower-end Moderate issue, not an Important-class vulnerability. The realistic impact is kernel fault or crash from an invalid read past the specified bound. The theoretical concern is limited out-of-bounds read behavior in a core string routine, but the patch does not support memory corruption, privilege escalation, or a direct information leak primitive.

5. One-sentence report phrase
The RISC-V ZBB optimized `strnlen()` path can read one word past the supplied count before checking the aligned boundary, allowing a local boundary-controlled input to fault in kernel context and cause DoS under specific alignment and page-mapping conditions.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED. YES REQUIRES MANUAL CHECK because this is a core architecture string helper with an invalid read primitive, even though the demonstrated impact is primarily DoS and the trigger conditions are constrained.

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

Patch: riscv: lib: Fix ZBB strnlen reading past count boundary [ Upstream
Commit: e697e30f3dd2da3a1df7dc0980546d5b53aea4b6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e697e30f3dd2da3a1df7dc0980546d5b53aea4b6

Changed files:
  arch/riscv/lib/strnlen.S

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=e697e30f3dd2da3a1df7dc0980546d5b53aea4b6

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

The original Linux kernel CVE announcement for CVE-2026-74751 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74751

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:L/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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: 2
  Paranoid ActionableScore: 3

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).