[CVE-2026-74435][MODERATE 7.0] rxrpc: rxrpc_verify_data ensure rx_dec_buffer alloc

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

CVE: CVE-2026-74435
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: rxrpc: rxrpc_verify_data ensure rx_dec_buffer alloc
Commit: 8bbede0afced346b24e4fbde0c68cf12980ba948
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8bbede0afced346b24e4fbde0c68cf12980ba948
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74435
Analysis date: Sun, 16 Aug 2026 03:51:11 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Not a Strong Important candidate unless later analysis shows memory corruption beyond NULL dereference.
Manual review required: YES

Summary:
An RxRPC receive-path allocation bug can allow a remote peer to trigger a NULL destination in skb_copy_bits(), causing a kernel crash and denial of service when an RxRPC or AFS endpoint is exposed.

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

The original Linux kernel CVE announcement for CVE-2026-74435 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74435

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-2026-74435	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-754;CWE-703;BEST CVSS score: '7.5';DESCR 'An RxRPC receive path NULL pointer dereference can occur because rxrpc_verify_data() may return success without allocating rxrpc_call.rx_dec_buffer when the received skb data length is zero. A remote peer can then drive rxrpc_recvmsg_data() into a later skb_copy_bits() call with a NULL destination, causing a kernel crash and denial of service. For the CVSS the PR:N is used because the vulnerable receive path can be reached by a network peer without local code execution when an RxRPC or AFS endpoint is exposed. The issue is network reachable over RxRPC deployments and does not require user interaction. Impact is denial of service only based on the visible primitive, because this is a NULL pointer dereference rather than a demonstrated UAF, OOB write, or information leak.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74435 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: d2bc90cf6c75cb96d2ce549be6c35efa3099d25b YES  NO NO unknown 	MAYBE	NULLPTR SIMPLEFIX LINUS KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Not a Strong Important candidate unless later analysis shows memory corruption beyond NULL dereference.

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  RxRPC receive handling can be reached by a remote peer when RxRPC or kAFS service exposure exists.

* Reliable kernel crash / strong DoS: +1
  The commit explicitly describes `skb_copy_bits()` being called with a NULL destination, producing a NULL pointer dereference.

* Availability impact realistic: +1
  If the affected RxRPC endpoint is reachable, the outcome is a kernel crash rather than a local warning or recoverable protocol error.

Paranoid additional signal:

* Common networking receive-path scrutiny: +1
  This is not TCP/IP core, but it is still kernel network receive-path protocol parsing. A malformed peer-controlled packet sequence reaches kernel memory-copy logic.

Not awarded:

* No generic strong memory corruption bonus. The visible primitive is NULL dereference, not UAF, OOB write, double free, type confusion, or arbitrary write.
* No privilege escalation bonus. The patch does not support attacker-controlled reuse, overwrite, callback control, or object confusion.
* No confidentiality or integrity impact. No leak or write primitive is shown.
* No hard race penalty. This is a protocol state and allocation logic bug, not a timing-dependent race.

## 3. Reachability analysis

A remote peer can plausibly trigger the vulnerable path if RxRPC or kAFS traffic is accepted by the target. The bug is caused by `rxrpc_verify_data()` returning success while `rx_dec_buffer` remains NULL, after which `rxrpc_recvmsg_data()` may continue into `skb_copy_bits()` with a NULL destination.

No local user account is required in the network-exposed case. Namespaces and containers are not central to the trigger. The main deployment limiter is exposure: RxRPC and AFS are not normally Internet-facing on most systems, but they can be present in enterprise or internal network environments. This makes the issue more actionable than a local-only Moderate, but not automatically Important without a stronger primitive.

Call-site confidence: high. The commit message directly names the caller expectation and the later NULL destination use.

## 4. Severity interpretation

This behaves like an actionable network-triggerable DoS. It should not be treated as a routine low-priority cleanup because the crash is in kernel network receive handling and can be peer-triggered when the protocol is exposed.

Theoretical memory corruption potential is low based on the supplied patch. Realistic evidence supports NULL pointer dereference and kernel crash only. There is no supported path to LPE, data corruption, or information disclosure in the provided material.

## 5. One-sentence report phrase

An RxRPC receive-path allocation bug can allow a remote peer to trigger a NULL destination in skb_copy_bits(), causing a kernel crash and denial of service when an RxRPC or AFS endpoint is exposed.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-reachable kernel crash in protocol receive handling. Even though the demonstrated impact is DoS only, exposed RxRPC or kAFS deployments should be reviewed rather than auto-closed.

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

Patch: rxrpc: rxrpc_verify_data ensure rx_dec_buffer alloc
Commit: 8bbede0afced346b24e4fbde0c68cf12980ba948
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8bbede0afced346b24e4fbde0c68cf12980ba948

Commit description:

rxrpc_recvmsg_data() calls rxrpc_verify_data() whenever the
rxrpc_call.rx_dec_buffer is unallocated and assumes that upon
successful return that rx_dec_buffer must be allocated.
However, rxrpc_verify_data() does not request an allocation if
the rxrpc_skb_priv.len is zero.

In addition, failure to allocate rx_dec_buffer will result in a
call to skb_copy_bits() with a NULL destination which can
trigger a NULL pointer dereference.

To prevent these issues rxrpc_verify_data() is modified to
always attempt to allocate the rxrpc_call.rx_dec_buffer if it
is NULL.

This issue was identified with assistance of a private
sashiko instance.

Fixes: d2bc90c ("rxrpc: Fix DATA decrypt vs splice() by copying data to buffer in recvmsg")
Reported-by: Simon Horman <simon.horman@redhat.com>
Signed-off-by: Jeffrey Altman <jaltman@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jiayuan Chen <jiayuan.chen@linux.dev>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260609140911.838677-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/rxrpc/recvmsg.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=8bbede0afced346b24e4fbde0c68cf12980ba948

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

The original Linux kernel CVE announcement for CVE-2026-74435 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74435

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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).