[CVE-2026-74436][MODERATE 7.0] rxrpc: serialize kernel accept preallocation with socket teardown [ Upstream

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

CVE: CVE-2026-74436
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: rxrpc: serialize kernel accept preallocation with socket teardown [ Upstream
Commit: 0337cdba0c477f176c0459bed012109453184573
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0337cdba0c477f176c0459bed012109453184573
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74436
Analysis date: Sat, 15 Aug 2026 03:39:48 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in `rxrpc_kernel_charge_accept()` can allow a preallocation path to use a freed `rxrpc_backlog` after concurrent socket teardown, causing at least kernel crash risk and requiring manual review for possible memory corruption impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74436 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74436

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-2026-74436	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'rxrpc_kernel_charge_accept read rx backlog without socket or backlog synchronization while concurrent teardown could clear rx backlog and free the backlog rings. A preallocation worker could then keep using a freed rxrpc_backlog object while updating backlog head and tail fields and array slots, which makes this a use after free candidate rather than a pure crash only bug. For the CVSS the PR:L is used because reliable triggering likely requires a local or delegated actor able to influence rxrpc service socket teardown or kernel service lifecycle, while network traffic may help hit the race. The issue is not proven to be directly network reachable as a standalone remote attack, but rxrpc is a network protocol and incoming service traffic can be part of the vulnerable timing window. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DANGER INIT UAF LOG IMPROVEONLY LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

* Local unprivileged or delegated trigger: +1
  Reliable triggering likely requires a local or delegated actor that can influence an RxRPC service socket lifecycle or kernel service using RxRPC.

* Memory corruption, strong corruption primitive: +2
  The patch describes use of a freed `struct rxrpc_backlog` after concurrent teardown. The stale object is not only read, it may be updated through backlog head and tail fields and array slots.

* Real lifetime corruption: +1
  This is a real lifetime race between `rxrpc_kernel_charge_accept()` and `rxrpc_discard_prealloc()`, where teardown can free backlog rings while another path still uses the pointer.

* Reliable kernel crash / strong DoS: +1
  Use-after-free in kernel networking state can realistically crash the kernel.

* Hard or unreliable race / special timing required: -1
  The issue depends on a concurrent preallocation worker and socket teardown window.

* Rare or narrower subsystem exposure: -1 for conservative score only
  RxRPC is not a broad default packet path like TCP, UDP, netfilter, or core skb receive processing. Exposure depends on RxRPC or AFS-style service usage.

* Weak LPE concern: +1 for paranoid score only
  The stale object is writable after free, so manual review should consider whether allocator reuse can turn this into a stronger primitive. No controlled reclaim or callback control is demonstrated in the patch.

## 3. Reachability analysis

The bug is not proven to be a standalone remote network attack. RxRPC is a network protocol, and incoming traffic can contribute to backlog/preallocation activity, but the race also requires teardown or disabling of the service backlog. In typical deployments, reliable triggering likely needs local influence over an RxRPC service socket, an in-kernel RxRPC service, or a delegated service lifecycle.

Namespaces and containers may matter if a reduced-privilege service or containerized workload can create or control the relevant RxRPC socket lifecycle. This should not be treated as full host-root-only without checking the product configuration. The path is not a broad default networking path, but it is kernel networking code and the patch clearly describes a freed object being reused.

Call-site confidence: medium. The changed function and direct stale pointer use are visible, but full service lifecycle callers are not included in the supplied patch.

## 4. Severity interpretation

This is stronger than an ordinary Moderate crash because the patch describes a real UAF on `struct rxrpc_backlog` with writes into freed backlog state. However, the realistic exploitation evidence remains limited because the race is timing-dependent and no attacker-controlled reclaim, type confusion, callback dispatch, or arbitrary write primitive is shown.

Conservatively this is borderline/manual-review Moderate. Paranoid handling makes it an Actionable Moderate because kernel lifetime corruption in networking code has enough risk to avoid auto-closure.

## 5. One-sentence report phrase

A race in `rxrpc_kernel_charge_accept()` can allow a preallocation path to use a freed `rxrpc_backlog` after concurrent socket teardown, causing at least kernel crash risk and requiring manual review for possible memory corruption impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch describes a real race-based use-after-free with writable stale object access in kernel networking state. No proven LPE primitive is shown, but the lifetime corruption is concrete enough that this should not be auto-closed.

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

Patch: rxrpc: serialize kernel accept preallocation with socket teardown [ Upstream
Commit: 0337cdba0c477f176c0459bed012109453184573
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0337cdba0c477f176c0459bed012109453184573

Changed files:
  net/rxrpc/call_accept.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=0337cdba0c477f176c0459bed012109453184573

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

The original Linux kernel CVE announcement for CVE-2026-74436 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74436

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).