[CVE-2026-53151][MODERATE REGULAR] rxrpc: Fix the ACK parser to extract the SACK table for parsing

[CVE-2026-53151][MODERATE REGULAR] rxrpc: Fix the ACK parser to extract the SACK table for parsing

[AL-KERNEL]

CVE: CVE-2026-53151
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: rxrpc: Fix the ACK parser to extract the SACK table for parsing
Commit: 566c4c1244de50fbff1f89ff93c9d7b0fc256db4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=566c4c1244de50fbff1f89ff93c9d7b0fc256db4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53151
Analysis date: Thu, 25 Jun 2026 05:38:37 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended / not auto-close
Manual review required: YES

Summary:
AF_RXRPC ACK parsing may incorrectly read SACK data from a fragmented or non-linear skb after assuming skb_condense made the buffer linear, allowing a remote RXRPC peer in reachable deployments to cause a kernel crash or limited protocol-state corruption.

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

The original Linux kernel CVE announcement for CVE-2026-53151 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53151

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 REGULAR
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-2026-53151	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-125;*CWE-20;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'AF_RXRPC ACK parsing can incorrectly read the SACK table because rxrpc_input_soft_acks assumed that skb_condense made the received skb data linear and directly accessible. A remote peer may trigger the issue with a deliberately fragmented UDP packet that reaches the ACK processing path, causing an out-of-bounds read candidate or incorrect parsing of ACK state. For the CVSS the PR:N is used because no local user account is required when AF_RXRPC traffic from the attacker is accepted by the target. AC:H is used because reliable triggering depends on packet shape and RXRPC call context rather than a simple generic UDP packet. Impact is at least denial of service via kernel crash or protocol state corruption. For the paranoid score, limited confidentiality or integrity impact is kept because the bug class is an invalid read in a network parser, but there is no clear write primitive in the patch context. AF_RXRPC is normally relevant only on systems using AFS or RXRPC based services, so real exposure depends on whether such traffic is reachable from untrusted networks.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended / not auto-close (with actual score 3)	YES	SKB PACKET  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended / not auto-close**

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. AF_RXRPC ACK parsing can be reached by network traffic if RXRPC or AFS traffic is accepted by the target.
* Memory corruption, weak/indirect corruption candidate: +1. The patch fixes unsafe direct access to SACK data in skb->data when the skb may remain non-linear or fragmented. This is best treated as an invalid/OOB read candidate, not a write primitive.
* Reliable kernel crash / strong DoS: +1. Incorrect buffer access in kernel packet parsing can plausibly lead to Oops or crash.
* Hard or special trigger conditions: -1. The commit says the fragmented UDP packet would probably need to be deliberately pre-generated, and the packet must reach the RXRPC ACK/SACK path.
* Rare/difficult-to-reach subsystem: -1. AF_RXRPC is normally relevant to AFS/RXRPC deployments, not a broad default Internet-facing protocol.

Paranoid additional signals:

* Confidentiality impact plausible: +1. Limited only for triage sensitivity because an invalid read in a network parser may consume unintended kernel buffer bytes, although no disclosure path is demonstrated.
* Integrity impact plausible: +1. Incorrectly parsed SACK state may corrupt RXRPC ACK/retransmission state, but no kernel write primitive or privilege escalation path is shown.

Not awarded:

* No strong memory corruption bonus. The patch does not show OOB write, UAF, double-free, type confusion, refcount takeover, or arbitrary write.
* No strong LPE bonus. There is no demonstrated reclaim, callback dispatch, writable stale object, or controlled overwrite.
* No common networking core path bonus. RXRPC is a real network protocol path, but not comparable to TCP/IP core, netfilter packet path, or generic UDP receive logic.

## 3. Reachability analysis

A remote peer can potentially trigger the issue by sending crafted RXRPC ACK traffic using a deliberately fragmented UDP packet, if AF_RXRPC traffic is accepted by the target. No local account is required in that network-reachable deployment, so PR would be treated as none for network CVSS-style reasoning. Realistic exposure is narrower than generic UDP because RXRPC is mainly used with AFS/RXRPC services and is not commonly exposed on public Internet hosts. Namespaces do not materially lower the trigger requirement for the remote case, but local RXRPC traffic generation could also be possible from an unprivileged local process if the protocol is available.

## 4. Severity interpretation

This behaves more like a borderline/actionable Moderate network parser bug than a clear Important vulnerability. The theoretical concern is an invalid/OOB read from non-linear skb data in a remote packet parsing path. The realistic evidence supports crash or protocol-state corruption more strongly than confidentiality disclosure, integrity compromise, or privilege escalation. Because it is network reachable and involves unsafe skb buffer access, it should not be auto-closed without review.

## 5. One-sentence report phrase

AF_RXRPC ACK parsing may incorrectly read SACK data from a fragmented or non-linear skb after assuming skb_condense made the buffer linear, allowing a remote RXRPC peer in reachable deployments to cause a kernel crash or limited protocol-state corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network reachable in affected RXRPC/AFS deployments and involves an invalid/OOB read candidate in kernel packet parsing, even though the patch does not support a strong memory-corruption or LPE primitive.

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

Patch: rxrpc: Fix the ACK parser to extract the SACK table for parsing
Commit: 566c4c1244de50fbff1f89ff93c9d7b0fc256db4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=566c4c1244de50fbff1f89ff93c9d7b0fc256db4

Commit description:

Fix modification of the received skbuff in rxrpc_input_soft_acks() and a
potential incorrect access of the buffer in a fragmented UDP packet (the
packet would probably have to be deliberately pre-generated as fragmented)
when AF_RXRPC tries to extract the contents of the SACK table by copying
out the contents of the SACK table into a buffer before attempting to parse

AF_RXRPC assumes that it can just call skb_condense() and then validly
access the SACK table from skb->data and that it will be a flat buffer -
but skb_condense() can silently fail to do anything under some
circumstances.

Note that whilst rxrpc_input_soft_acks() should be able to parse extended
ACKs, the rest of AF_RXRPC doesn't currently support that.

Further, there's then no need to call skb_condense() in rxrpc_input_ack(),
so don't.

Fixes: d57a3a1 ("rxrpc: Save last ACK's SACK table rather than marking txbufs")
Reported-by: Michael Bommarito <michael.bommarito@gmail.com>
Link: https://lore.kernel.org/r/20260513180907.2061972-1-michael.bommarito@gmail.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
cc: stable@kernel.org
Link: https://patch.msgid.link/105362.1780573560@warthog.procyon.org.uk
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/rxrpc/input.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=566c4c1244de50fbff1f89ff93c9d7b0fc256db4

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

The original Linux kernel CVE announcement for CVE-2026-53151 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53151

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/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: 2
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).