[CVE-2026-74434][IMPORTANT] rxrpc: Don't move a peeked OOB message onto the pending queue

[CVE-2026-74434][IMPORTANT] rxrpc: Don't move a peeked OOB message onto the pending queue

[AL-KERNEL]

CVE: CVE-2026-74434
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: rxrpc: Don't move a peeked OOB message onto the pending queue
Commit: 9ada3931beb37068fcb725b34b0398457009f343
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9ada3931beb37068fcb725b34b0398457009f343
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74434
Analysis date: Sat, 15 Aug 2026 23:22:26 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An rxrpc MSG_PEEK handling bug can leave one skb reachable from both recvmsg_oobq and pending_oobq, corrupting skb linkage and causing double free and connection use after free during socket cleanup.

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

The original Linux kernel CVE announcement for CVE-2026-74434 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74434

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-2026-74434	IMPORTANT	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-415;*CWE-787;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'rxrpc OOB recv handling can corrupt memory because MSG_PEEK leaves an skb on recvmsg_oobq while the same skb may also be inserted into pending_oobq. The skb rbnode shares storage with list next and prev pointers, so inserting it into the pending tree can overwrite list linkage while the skb is still on the receive queue. When the socket is closed, both queues may be drained and the same skb and its connection reference can be released twice, leading to bad pointer writes, double free, and use after free. For the CVSS the PR:L value is used because reliable triggering requires a local process that can operate an rxrpc socket and call recvmsg with MSG_PEEK, even though a network peer may help create the OOB challenge state. The issue is not treated as directly network reachable without evidence that a remote peer alone can force the local MSG_PEEK behavior. Impact is at least local denial of service via kernel crash and in the paranoid case may allow confidentiality and integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-74434 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 5800b1cf3fd8ccab752a101865be1e76dac33142 YES  NO NO unknown 	MAYBE	OOB DANGER INIT UAF SKB LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process that can operate an rxrpc socket and call recvmsg(MSG_PEEK) can participate in triggering the buggy path.
* Privilege escalation plausible, weak: +1. The patch describes double free, bad pointer writes, and connection refcount underflow, but does not demonstrate attacker controlled reclaim or a reliable overwrite primitive.
* Memory corruption, strong corruption primitive: +2. The commit explicitly describes corrupted skb list linkage, double free, and use after free.
* Real lifetime corruption: +1. The same skb and the connection reference can be released twice, causing a lifetime and refcount violation.
* Reliable kernel crash / strong DoS: +1. __skb_unlink() can follow corrupted next/prev pointers and write to a bad address during socket close.
* Rare or narrower subsystem exposure: -1. AF_RXRPC is not a central default TCP/IP receive path and practical triggering requires a specific rxrpc OOB challenge state.

Paranoid difference:

* Strong LPE plausibility: +2 instead of weak +1. Double free plus connection refcount underflow and corrupted skb linkage are serious enough to justify Important candidate handling even without a public exploit primitive.
* Paranoid score therefore reaches 7.

## 3. Reachability analysis

The bug is not treated as directly remote reachable because the remote peer alone is not shown to force the local MSG_PEEK operation. The realistic trigger requires a local process with access to an rxrpc socket, plus an OOB challenge state that can involve protocol interaction with a peer.

Namespaces may matter if AF_RXRPC sockets are available to unprivileged users inside containers or delegated environments. In a restricted product configuration where AF_RXRPC is unavailable, disabled, or blocked by policy, practical exposure is lower. If ordinary unprivileged users can create and use rxrpc sockets, the local trigger assumption is reasonable.

Call-site confidence: high. The patch and commit message describe the exact queue movement, skb storage aliasing, close-time drain behavior, double free, and connection UAF mechanism.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup. The issue has explicit kernel memory corruption, double free, and use-after-free semantics.

Realistically, the most directly supported impact is local kernel crash / DoS. The privilege escalation path is not demonstrated, but the primitive is strong enough that manual review is warranted, especially because skb lifetime corruption and refcount underflow have historically produced exploitable kernel bugs.

## 5. One-sentence report phrase

An rxrpc MSG_PEEK handling bug can leave one skb reachable from both recvmsg_oobq and pending_oobq, corrupting skb linkage and causing double free and connection use after free during socket cleanup.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: explicit memory corruption, double free, and UAF are present, and the paranoid score reaches Important candidate territory even though direct remote triggering and reliable privilege escalation are not demonstrated.

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

Patch: rxrpc: Don't move a peeked OOB message onto the pending queue
Commit: 9ada3931beb37068fcb725b34b0398457009f343
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9ada3931beb37068fcb725b34b0398457009f343

Commit description:

rxrpc_recvmsg_oob() takes a received oob message off recvmsg_oobq and,
if a response is needed, moves it onto the pending_oobq tree. However,
only the unlink from recvmsg_oobq is guarded by MSG_PEEK; the move onto
pending_oobq always runs.

As a result, reading a challenge with MSG_PEEK leaves the skb on
recvmsg_oobq while also adding it to pending_oobq. Since struct
sk_buff's rbnode shares storage with its next and prev pointers,
rb_insert_color() overwrites the list linkage, and the skb, which holds
a single reference, becomes reachable from both queues at once.

When the socket is closed both queues are drained in turn. While
draining recvmsg_oobq, __skb_unlink() follows the next and prev
pointers that rbnode has overwritten and writes to a bad address. Also,
as the skb holds a single reference but is freed from each queue, both
the skb and the connection reference it holds are released twice. This
leads to memory corruption and to a use-after-free caused by the
connection refcount underflow.

MSG_PEEK does not consume the message from the queue, so only unlink it
from recvmsg_oobq and then move it onto pending_oobq or free it when
the message is actually consumed.

Fixes: 5800b1c ("rxrpc: Allow CHALLENGEs to the passed to the app for a RESPONSE")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260609140911.838677-3-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=9ada3931beb37068fcb725b34b0398457009f343

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

The original Linux kernel CVE announcement for CVE-2026-74434 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74434

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:L/I:L/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: 5
  Paranoid ActionableScore: 7

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).