[CVE-2026-74430][MODERATE 7.0] rxrpc: Fix ACKALL packet handling

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

CVE: CVE-2026-74430
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: rxrpc: Fix ACKALL packet handling
Commit: 8db6a2c95e36938076b37466ce36774526a5c8c3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8db6a2c95e36938076b37466ce36774526a5c8c3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74430
Analysis date: Sun, 16 Aug 2026 07:17:12 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A forged RXRPC ACKALL packet can be accepted in an invalid call state before transmit buffers exist, causing rxrpc_rotate_tx_window() to dereference a NULL tx_queue and crash the kernel remotely.

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

The original Linux kernel CVE announcement for CVE-2026-74430 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74430

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-2026-74430	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-20;CWE-754;BEST CVSS score: '7.5';DESCR 'An RXRPC ACKALL handling flaw allows an out of state ACKALL packet to be accepted for a new service call before any reply data has been queued. In RXRPC_CALL_SERVER_RECV_REQUEST, call tx_top can be zero and call tx_queue can be NULL, so processing ACKALL can reach rxrpc_rotate_tx_window and trigger a NULL pointer dereference. For the CVSS the PR:N because a remote peer only needs network reachability to an RXRPC endpoint and does not need local access or authenticated kernel privileges. The issue is network reachable over RXRPC protocol traffic, but practical exposure depends on whether RXRPC or AFS style services are enabled and reachable from untrusted networks. Impact is denial of service via kernel crash. No confidentiality or integrity impact is assigned because the supported primitive is a NULL pointer dereference rather than UAF, OOB access, or arbitrary write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74430 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: b341a0263b1b804d329f864c2dc24815364510ec YES  NO NO unknown 	MAYBE	DANGER LINUS KPANIC PACKET  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=5

## 1. ActionableScore

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

This is not an Important-class memory corruption candidate because the supported primitive is a NULL pointer dereference, not UAF, OOB write, type confusion, arbitrary write, or reclaimable lifetime corruption. It is still actionable because it is a network-triggerable kernel crash in a protocol receive path.

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2
  A forged RXRPC ACKALL packet can be sent by a remote peer to a reachable RXRPC endpoint.

* Reliable kernel crash / strong DoS: +1
  The commit explicitly states that rxrpc_rotate_tx_window() can dereference a NULL txqueue and trigger a NULL pointer dereference.

* Availability impact realistic: +1
  The described failure is a kernel NULL dereference, so impact is host availability loss rather than only a local process failure.

* Networking protocol receive path: +1
  The vulnerable path is RXRPC input packet handling, specifically ACKALL processing.

Not awarded:

* Generic memory corruption: +0
  This is a NULL pointer dereference, not a demonstrated UAF, OOB write, double free, or type confusion.

* Privilege escalation plausible: +0
  No reclaim, overwrite, callback control, stale object reuse, or arbitrary write primitive is shown.

* Confidentiality / Integrity: +0
  The patch supports DoS only.

## 3. Reachability analysis

A remote peer can trigger the bug if it can send RXRPC traffic to an affected endpoint and cause a forged or out-of-state ACKALL packet to be processed for a call that has no queued transmit buffer. No local user account or kernel privileges are required on the victim, so PR:N is appropriate for CVSS-style reasoning.

Exposure depends on whether RXRPC/AFS-style services are enabled and reachable. RXRPC is not as broadly exposed as TCP/IP core paths, but when deployed it is a real network protocol input path. Namespaces do not materially reduce the remote-trigger nature if the endpoint is reachable from the attacker.

Call-site confidence: high, because the affected handler and the direct NULL dereference path are described in the commit and visible in the patch context.

## 4. Severity interpretation

This behaves like an actionable Moderate remote DoS rather than an ordinary low-priority Moderate. The realistic impact is a kernel crash from a crafted protocol packet.

The theoretical memory-corruption potential is low. The patch and commit identify a NULL tx_queue dereference, but do not show attacker-controlled reuse, stale object lifetime, writable freed memory, object confusion, or code execution potential. Therefore the score should not be raised into Strong Important solely based on kernel context.

## 5. One-sentence report phrase

A forged RXRPC ACKALL packet can be accepted in an invalid call state before transmit buffers exist, causing rxrpc_rotate_tx_window() to dereference a NULL tx_queue and crash the kernel remotely.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is network reachable and can cause a remote kernel crash, even though the demonstrated primitive is DoS-only and does not currently support confidentiality, integrity, or privilege-escalation impact.

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

Patch: rxrpc: Fix ACKALL packet handling
Commit: 8db6a2c95e36938076b37466ce36774526a5c8c3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8db6a2c95e36938076b37466ce36774526a5c8c3

Commit description:

rxrpc_input_ackall() accepts ACKALL packets without checking whether the
call is in a state that can legitimately have outstanding transmit buffers.
A forged ACKALL can therefore reach a new service call in
RXRPC_CALL_SERVER_RECV_REQUEST before any reply packets have been queued.

In that state call->tx_top is zero and call->tx_queue is NULL, so
rxrpc_rotate_tx_window() dereferences a NULL txqueue and triggers a
null-pointer dereference.

Fix the handling of ACKALL packets by the following means:

 (1) Add two new call states: RXRPC_CALL_CLIENT_PRE_SEND which indicates
     that the client call is connected, but nothing has been transmitted as
     yet; and RXRPC_CALL_CLIENT_AWAIT_ACK, which indicates that everything
     has been transmitted at least once, but we're now waiting for the
     stuff remaining in the Tx buffer to be ACK'd (retransmissions may
     still happen).

     The RXRPC_CALL_CLIENT_PRE_SEND state is set when the call is assigned
     a channel and transitions to RXRPC_CALL_CLIENT_SEND_REQUEST when the
     first packet is transmitted.

     RXRPC_CALL_CLIENT_AWAIT_REPLY is then narrowed in scope to indicate
     that all Tx packets have been ACK'd and we're now waiting for the
     reply to be received.

 (2) As per Wyatt Feng's original patch[1], the ACKALL handler then checks
     that the call state is one in which there might be stuff in the Tx
     buffer to ACK, but now this includes AWAIT_ACK rather than
     AWAIT_REPLY.  ACKALL packets are ignored if received in the wrong
     state.

     Note that unlike Wyatt Feng's patch, it's no longer necessary to check
     to see if the Tx buffer exists as this the state set now covers this.

 (3) Make the ACKALL handler use call->tx_transmitted rather than
     call->tx_top as the former is explicitly the highest packet seq number
     transmitted, whereas the latter has a looser definition.

Thanks to Jeffrey Altman for a description of the history of the ACKALL
packet[1].

Fixes: b341a02 ("rxrpc: Implement progressive transmission queue struct")
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Wyatt Feng <bronzed_45_vested@icloud.com>
Co-developed-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Ren Wei <n05ec@lzu.edu.cn>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20260616155749.2125907-2-dhowells@redhat.com/ [1]
Link: https://lore.kernel.org/r/c0fd4fec-1576-4070-b31e-a37d5506f5ed@auristor.com/ [2]
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Link: https://patch.msgid.link/20260624163819.3017002-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/rxrpc/ar-internal.h
  net/rxrpc/call_event.c
  net/rxrpc/call_object.c
  net/rxrpc/conn_client.c
  net/rxrpc/input.c
  net/rxrpc/sendmsg.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=8db6a2c95e36938076b37466ce36774526a5c8c3

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

The original Linux kernel CVE announcement for CVE-2026-74430 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74430

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: unknown
  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.
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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).