[CVE-2026-31431][IMPORTANT] crypto: algif_aead - Revert to operating out-of-place [ Upstream

[CVE-2026-31431][IMPORTANT] crypto: algif_aead - Revert to operating out-of-place [ Upstream

[AL-KERNEL]

CVE: CVE-2026-31431
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: crypto: algif_aead - Revert to operating out-of-place [ Upstream
Commit: fafe0fa2995a0f7073c1c358d7d3145bcc9aedd8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fafe0fa2995a0f7073c1c358d7d3145bcc9aedd8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31431
Analysis date: Wed, 20 May 2026 22:22:27 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
AF_ALG AEAD in-place processing reused and chained TX and RX scatterlists across different mappings, creating a local unprivileged crash risk and a plausible page lifetime or UAF-style memory corruption concern that warrants Important-class manual review.

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

The original Linux kernel CVE announcement for CVE-2026-31431 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31431

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: IMPORTANT
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
======================================================================

CVE-2026-31431	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-664;*CWE-416;CWE-119;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.8';DESCR 'The AF_ALG AEAD path added an in place operation mode that reuses and chains TX and RX scatterlists even though source and destination buffers come from different mappings. A local user that can use AF_ALG AEAD sockets may be able to trigger incorrect scatterlist page lifetime or offset handling during decrypt or encrypt processing, leading at least to a kernel crash and a plausible use after free style memory corruption risk. For the CVSS the PR:L is used because the attacker needs local code execution and access to the AF_ALG socket interface, but no administrative privileges are normally required. The issue is not network reachable. Impact is at least local denial of service and in the paranoid interpretation may include confidentiality and integrity impact if the stale or incorrectly referenced pages can be reused in a controlled way.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	DANGER INIT NOMEMCHK LINUS KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	guess

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

ActionableScore=8
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. AF_ALG sockets are local user reachable in typical systems unless restricted by policy.
* Local unprivileged high-control kernel data-plane API: +1. AF_ALG lets users shape crypto requests, buffers, scatterlists, lengths, AAD, and recv/send sequencing.
* Memory corruption, weak/indirect corruption candidate: +1. The reverted code used complex TX/RX scatterlist reuse, partial offset reassignment, and SGL chaining across different mappings.
* Real lifetime corruption concern: +1. The removed dst_offset path involved page reference transfer and later crypto request use, making stale or mismatched page lifetime plausible.
* Reliable kernel crash / strong DoS: +1. Incorrect SGL or page lifetime handling in crypto requests can realistically produce kernel crash or BUG conditions.

Paranoid additional signals:

* Generic memory corruption, strong candidate: +2 instead of weak +1. The patch removes page reassignment and chained scatterlist lifetime complexity, which is consistent with a possible UAF or page reference lifetime bug.
* Privilege escalation plausible, weak to moderate: +1. No arbitrary write is demonstrated, but AF_ALG gives high control over buffer layout and request timing, so kernel memory corruption deserves LPE-oriented manual review.
* Confidentiality impact plausible: +1. If stale or incorrectly referenced pages are reused, limited disclosure cannot be excluded.
* Integrity impact plausible: +1. If crypto output or SGL writes land on unintended pages, limited memory or data integrity impact is plausible.

Negative signals:

* Not remote reachable: 0. This is a local AF_ALG socket path, not a network protocol parser.
* No admin requirement: 0. Typical AF_ALG use does not require CAP_SYS_ADMIN.
* No hard race penalty: 0. The concern is request construction and SGL lifetime logic, not a narrow timing-only race.
* No constrained primitive penalty in paranoid score. The primitive is not proven, but the patch removes cross-mapping SGL reuse rather than only a harmless metadata mismatch.

## 3. Reachability analysis

A local user or local process that can create AF_ALG AEAD sockets can reach the affected path through sendmsg and recvmsg operations with controlled AEAD input, AAD length, output length, and buffer layout. This is not network reachable and does not require packet input.

Namespaces usually do not make the bug remote, but containers may matter because AF_ALG can be available inside containerized workloads depending on kernel configuration and seccomp or LSM policy. If AF_ALG socket creation is allowed to untrusted container users, the practical PR remains low.

The path is not as universal as TCP or VFS, but AF_ALG is a standard kernel userspace crypto interface and is a high-control local data-plane API. The realistic exploitation condition is local access plus availability of AEAD algorithms through AF_ALG.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup. The patch removes a large and complex in-place AEAD path that reused and chained TX and RX scatterlists even though the source and destination come from different mappings. That is a meaningful memory lifetime and ownership risk area.

The conservative interpretation is Actionable Moderate: at least local DoS is plausible, and the affected interface is reachable by local unprivileged users on many systems. The paranoid interpretation is Strong Important candidate because the removed logic touches page references, scatterlist offsets, and crypto request lifetime, which can plausibly become UAF-style memory corruption even though the patch does not demonstrate a reliable arbitrary write or LPE primitive.

Theoretical memory corruption potential is significant enough for manual review. Realistic exploitation evidence is incomplete from the patch alone, so this should not be treated as confirmed privilege escalation without further analysis.

## 5. One-sentence report phrase

AF_ALG AEAD in-place processing reused and chained TX and RX scatterlists across different mappings, creating a local unprivileged crash risk and a plausible page lifetime or UAF-style memory corruption concern that warrants Important-class manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the affected path is a local unprivileged high-control AF_ALG API, the patch removes complex scatterlist and page-reference reassignment logic, and the issue was reported by a security researcher. Even though exploitation beyond DoS is not proven by the diff, the memory lifetime pattern is strong enough to require manual kernel security review.

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

Patch: crypto: algif_aead - Revert to operating out-of-place [ Upstream
Commit: fafe0fa2995a0f7073c1c358d7d3145bcc9aedd8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fafe0fa2995a0f7073c1c358d7d3145bcc9aedd8

Changed files:
  crypto/af_alg.c
  crypto/algif_aead.c
  crypto/internal/aead.h
  crypto/scatterwalk.h
  crypto/if_alg.h
  crypto/skcipher.h
  crypto/algif_skcipher.c
  include/crypto/if_alg.h

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=fafe0fa2995a0f7073c1c358d7d3145bcc9aedd8

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

The original Linux kernel CVE announcement for CVE-2026-31431 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31431

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).