[CVE-2026-43500][IMPORTANT] rxrpc: Also unshare DATA/RESPONSE packets when paged frags are present

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

CVE: CVE-2026-43500
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: rxrpc: Also unshare DATA/RESPONSE packets when paged frags are present
Commit: 3eae0f4f9f7206a4801efa5e0235c25bbd5a412c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3eae0f4f9f7206a4801efa5e0235c25bbd5a412c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43500
Analysis date: Wed, 20 May 2026 21:43:45 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
RxRPC may decrypt DATA or RESPONSE packets in place into externally shared skb paged fragments, allowing a local splice based path to corrupt shared pages and potentially causing denial of service or higher impact if the shared-page write can be shaped into page-cache or privilege-relevant corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-43500 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43500

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
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CVE-2026-43500	IMPORTANT	CHECK	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-664;CWE-668;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'rxrpc could perform in place decryption on an skb that is not cloned but still contains externally shared paged fragments. In that state skb_to_sgvec can bind external frag pages into the crypto scatterlist and the decrypt operation can write into pages that are shared with another owner. A local user may be able to trigger this through a splice based UDP loopback path or another source of shared skb fragments that reaches an rxrpc DATA or RESPONSE security operation. For the CVSS the PR:L is used because reliable triggering requires a local user or local process context rather than pure unauthenticated remote packet delivery. The issue is not treated as generally Internet reachable because normal NIC RX and GRO frags are expected to be kernel private, but local or same host network paths can be relevant. Impact is at least denial of service and data corruption. In the paranoid case it may allow high confidentiality and integrity impact if the shared fragment write can be shaped into page cache or memory corruption useful for privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	SIMPLEFIX SKB LINUS PACKET  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	YES	NO	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1. The described splice into UDP loopback path can plausibly be driven by a local user or local process.
* Shared-page / Dirty-Pipe-like ownership violation: +2. The kernel may decrypt in place into externally owned paged fragments without first unsharing them.
* Memory corruption, strong corruption primitive: +2. The primitive is not just a NULL dereference. It is a write into shared skb fragment pages through the crypto SGL.
* Availability impact realistic: +1. Corrupting shared pages or unexpected in-place crypto writes can plausibly crash or destabilize the kernel path.
* Integrity impact plausible: +1. Externally owned shared pages may be modified across an ownership boundary.
* Confidentiality impact plausible in paranoid interpretation: +1. If the shared-page corruption can be shaped into page-cache or cross-object corruption, secondary disclosure impact is possible.
* Privilege escalation plausible in paranoid interpretation: +2. This is not proven, but shared-page or page-cache corruption is a known high-risk primitive class.
* Hard or constrained triggering conditions: -1. Reliable exploitation likely requires rxrpc security processing plus a suitable shared-frag skb source such as splice-loopback.

High-risk primitive counting is capped conceptually to avoid over-stacking the same underlying ownership/COW bypass.

## 3. Reachability analysis

The most realistic trigger is local. A local user may be able to create externally shared skb fragments through splice into UDP and route them into an rxrpc path that performs security processing and in-place decryption. This supports PR:L rather than PR:N.

The issue is not treated as broadly Internet reachable. Normal NIC RX, GRO, and page_pool frags are expected to be kernel-private according to the commit context, and the patch preserves that fast path. Remote triggering would require more specific packet construction and deployment conditions and is not sufficiently supported for a main remote score.

Namespaces may matter if an unprivileged container can access the relevant sockets, rxrpc endpoint, or loopback path. This should be checked per distribution and container policy. RxRPC itself is less common than core TCP/IP or netfilter, but the primitive involves skb paged fragments and splice/shared-page ownership, so it should not be auto-closed as a niche protocol-only issue.

## 4. Severity interpretation

This behaves above an ordinary Moderate because the bug is an ownership and shared-page write issue, not merely a warning, leak, or NULL dereference. The conservative interpretation is Actionable Moderate because exploitation appears local and constrained, but the primitive can corrupt data outside the expected skb ownership boundary.

The paranoid interpretation is a Strong Important candidate because in-place crypto writes into externally owned paged fragments resemble Dirty-Pipe-like page ownership violations. Practical privilege escalation is not demonstrated by the patch alone, but the bug class is serious enough to require manual review for page-cache corruption, cross-object corruption, or LPE feasibility.

## 5. One-sentence report phrase

RxRPC may decrypt DATA or RESPONSE packets in place into externally shared skb paged fragments, allowing a local splice based path to corrupt shared pages and potentially causing denial of service or higher impact if the shared-page write can be shaped into page-cache or privilege-relevant corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a shared-page in-place write primitive with possible Dirty-Pipe-like ownership bypass characteristics. The main open question is whether the splice-loopback or other shared-frag source can be shaped into reliable page-cache corruption or local privilege escalation.

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UPSTREAM PATCH SUMMARY
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Patch: rxrpc: Also unshare DATA/RESPONSE packets when paged frags are present
Commit: 3eae0f4f9f7206a4801efa5e0235c25bbd5a412c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3eae0f4f9f7206a4801efa5e0235c25bbd5a412c

Commit description:

The DATA-packet handler in rxrpc_input_call_event() and the RESPONSE
handler in rxrpc_verify_response() copy the skb to a linear one before
calling into the security ops only when skb_cloned() is true.  An skb
that is not cloned but still carries externally-owned paged fragments
(e.g. SKBFL_SHARED_FRAG set by splice() into a UDP socket via
__ip_append_data, or a chained skb_has_frag_list()) falls through to
the in-place decryption path, which binds the frag pages directly into
the AEAD/skcipher SGL via skb_to_sgvec().

Extend the gate to also unshare when skb_has_frag_list() or
skb_has_shared_frag() is true.  This catches the splice-loopback vector
and other externally-shared frag sources while preserving the
zero-copy fast path for skbs whose frags are kernel-private (e.g. NIC
page_pool RX, GRO).  The OOM/trace handling already in place is reused.

Fixes: d0d5c0c ("rxrpc: Use skb_unshare() rather than skb_cow_data()")
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/rxrpc/call_event.c
  net/rxrpc/conn_event.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=3eae0f4f9f7206a4801efa5e0235c25bbd5a412c

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

The original Linux kernel CVE announcement for CVE-2026-43500 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43500

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: 6
  Paranoid ActionableScore: 8

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).