[CVE-2026-43284][IMPORTANT] xfrm: esp: avoid in-place decrypt on shared skb frags

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

CVE: CVE-2026-43284
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: xfrm: esp: avoid in-place decrypt on shared skb frags
Commit: 50ed1e7873100f77abad20fd31c51029bc49cd03
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=50ed1e7873100f77abad20fd31c51029bc49cd03
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43284
Analysis date: Tue, 09 Jun 2026 18:28:43 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
ESP input can decrypt in place into UDP skb fragments backed by shared pipe pages created via MSG_SPLICE_PAGES, causing a shared-page ownership and COW bypass with plausible confidentiality, integrity, and availability impact.

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

The original Linux kernel CVE announcement for CVE-2026-43284 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43284

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-2026-43284	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-668;*CWE-664;CWE-669;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An in-place ESP decrypt can occur on skb fragments backed by shared pipe pages because UDP MSG_SPLICE_PAGES did not mark those frags with SKBFL_SHARED_FRAG while TCP already did. A local user may be able to create an ESP-in-UDP skb from shared pipe backed pages and reach the ESP input no-COW fast path, causing plaintext writes into memory that is not privately owned by the skb. For the CVSS the PR:L is used because reliable triggering requires local code execution and access to the relevant socket and xfrm path, not full host administrator privileges in an already configured environment. The issue is not directly network reachable because a remote peer cannot normally force the victim kernel to use local shared pipe pages for skb frags. Impact is at least denial of service or data corruption and in the paranoid case may include confidentiality or integrity impact due to in-place writes to shared backing pages.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	REMOTE DANGER NETWORK SKB LINUS IPV6 KPANIC PACKET 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: **7**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**
  A local process may be able to create UDP skb fragments from pipe pages via `MSG_SPLICE_PAGES`, assuming the relevant XFRM or ESP-in-UDP path is configured.

* **Dirty-Pipe-like ownership/COW violation: +2**
  ESP input may decrypt in place into skb paged fragments backed by shared pipe pages. This is an ownership and copy-on-write isolation failure rather than a simple crash-only bug.

* **Confidentiality impact plausible: +1**
  Decrypted plaintext may be written into externally owned shared pages, so data that should remain isolated may become visible through the original backing object.

* **Integrity impact plausible: +1**
  The kernel writes decrypted data into memory not privately owned by the skb, creating a plausible cross-object or shared-page data corruption primitive.

* **Availability impact realistic: +1**
  Shared-page corruption or unexpected in-place modification can plausibly lead to data corruption, protocol failure, or kernel instability.

* **Networking skb/page-frag exposure: +1**
  The affected path is in XFRM/ESP and IPv4/IPv6 UDP skb construction, involving common skb fragment ownership semantics. The specific trigger is configuration-dependent, but this is not a purely obscure driver-only path.

Paranoid additional signal:

* **Weak LPE concern: +1**
  The primitive is not an ordinary constrained overwrite. It involves kernel writes into externally owned shared pages, which historically deserves manual review for privilege escalation or policy-bypass potential, even though a concrete LPE chain is not demonstrated by the patch.

## 3. Reachability analysis

The bug is primarily **local**, not directly remote. A remote peer normally cannot force the victim kernel to build an skb from local shared pipe pages. Reliable triggering requires local code execution plus a usable UDP `MSG_SPLICE_PAGES` path and an ESP-in-UDP or XFRM configuration that reaches ESP input.

If the system already has the relevant IPsec or XFRM state configured, the attacker may not need full administrator privileges. If the attacker must create XFRM state, that usually requires `CAP_NET_ADMIN`. Namespaces can matter because delegated network administration inside containers or netns may lower the practical barrier in some deployments.

The path is not default-enabled for all systems because it depends on IPsec/XFRM usage, but XFRM, UDP, splice, and skb paged-fragment handling are important kernel networking mechanisms and should not be treated as rare hardware-only exposure.

## 4. Severity interpretation

This behaves more like an **Important candidate** than an ordinary Moderate issue. The key reason is the shared-page/COW ownership violation: ESP input can decrypt in place over pages that are not privately owned by the skb.

Realistic exploitation is still configuration-dependent and no arbitrary write or complete privilege escalation chain is demonstrated in the patch. However, the primitive is stronger than a NULL dereference or resource leak because it can modify externally owned shared pages. That makes it an actionable case requiring manual review.

## 5. One-sentence report phrase

ESP input can decrypt in place into UDP skb fragments backed by shared pipe pages created via MSG_SPLICE_PAGES, causing a shared-page ownership and COW bypass with plausible confidentiality, integrity, and availability impact.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This is a shared-page corruption and COW-bypass candidate in the networking/XFRM path. The main open question is whether the externally backed pages can be steered toward sensitive pipe-backed, file-backed, or otherwise security-relevant memory in realistic deployments.

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

Patch: xfrm: esp: avoid in-place decrypt on shared skb frags
Commit: 50ed1e7873100f77abad20fd31c51029bc49cd03
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=50ed1e7873100f77abad20fd31c51029bc49cd03

Commit description:

MSG_SPLICE_PAGES can attach pages from a pipe directly to an skb. TCP
marks such skbs with SKBFL_SHARED_FRAG after skb_splice_from_iter(),
so later paths that may modify packet data can first make a private
copy. The IPv4/IPv6 datagram append paths did not set this flag when
splicing pages into UDP skbs.

That leaves an ESP-in-UDP packet made from shared pipe pages looking
like an ordinary uncloned nonlinear skb. ESP input then takes the no-COW
fast path for uncloned skbs without a frag_list and decrypts in place
over data that is not owned privately by the skb.

Mark IPv4/IPv6 datagram splice frags with SKBFL_SHARED_FRAG, matching
TCP. Also make ESP input fall back to skb_cow_data() when the flag is
present, so ESP does not decrypt externally backed frags in place.
Private nonlinear skb frags still use the existing fast path.

This intentionally does not change ESP output. In esp_output_head(),
the path that appends the ESP trailer to existing skb tailroom without
calling skb_cow_data() is not reachable for nonlinear skbs:
skb_tailroom() returns zero when skb->data_len is nonzero, while ESP
tailen is positive. Thus ESP output will either use the separate
destination-frag path or fall back to skb_cow_data().

Fixes: cac2661 ("esp4: Avoid skb_cow_data whenever possible")
Fixes: 03e2a30 ("esp6: Avoid skb_cow_data whenever possible")
Fixes: 7da0dde ("ip, udp: Support MSG_SPLICE_PAGES")
Fixes: 6d8192b ("ip6, udp6: Support MSG_SPLICE_PAGES")
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Reported-by: Kuan-Ting Chen <h3xrabbit@gmail.com>
Tested-by: Hyunwoo Kim <imv4bel@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Kuan-Ting Chen <h3xrabbit@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/esp4.c
  net/ipv4/ip_output.c
  net/ipv6/esp6.c
  net/ipv6/ip6_output.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=50ed1e7873100f77abad20fd31c51029bc49cd03

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

The original Linux kernel CVE announcement for CVE-2026-43284 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43284

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: 7
  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).