[CVE-2026-63912][MODERATE 7.0] xfrm: esp: restore combined single-frag length gate

[CVE-2026-63912][MODERATE 7.0] xfrm: esp: restore combined single-frag length gate

[AL-KERNEL]

CVE: CVE-2026-63912
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfrm: esp: restore combined single-frag length gate
Commit: 566295735530ee513326049b0540f32ec050bf2e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=566295735530ee513326049b0540f32ec050bf2e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63912
Analysis date: Sun, 19 Jul 2026 20:13:46 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
The ESP output fast path may build a destination scatterlist whose combined post-trailer length exceeds the single page frag allocation, allowing network-influenced IPsec traffic to cause a kernel crash and potentially memory corruption.

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

The original Linux kernel CVE announcement for CVE-2026-63912 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63912

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-2026-63912	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-131;*CWE-787;CWE-119;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'The ESP output fast path can use an invalid single page frag decision because skb data length and ESP trailer length were checked separately while the later tail path uses the combined post trailer length. If the aligned combined length exceeds PAGE_SIZE, the destination scatterlist may cover more data than the allocated single page frag can safely back. This can result in a kernel crash and may represent a memory corruption risk in the outbound IPsec ESP path. For the CVSS the PR:N is used because an attacker does not need a local account if they can inject traffic that the target IPsec gateway encrypts on the affected path. The issue is not treated as general public Internet AV:N because exploitation normally requires access to a network path or host context that is covered by the target xfrm policy. Impact is at least denial of service and in the paranoid interpretation may include confidentiality and integrity impact due to possible out of bounds memory access.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	REMOTE DANGER SIMPLEFIX NETWORK SKB IPV6 KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative score signals:

* Remote reachable / network-triggerable: +2. The affected path is outbound ESP/XFRM processing and can be influenced by traffic that matches an IPsec policy on a gateway or endpoint.
* Constrained kernel buffer overwrite / weak indirect corruption candidate: +1. The destination scatterlist may span more data than the single allocated page frag can safely back, but the patch does not show attacker-controlled target selection or arbitrary overwrite.
* Memory layout invariant restoration: +1. The fix restores consistency between skb->data_len, tailen, alignment, PAGE_SIZE, and page-frag allocation.
* Reliable kernel crash / strong DoS: +1. A bad scatterlist/page-frag size relationship in ESP crypto output can realistically crash the kernel.
* Common packet-processing path: +1. XFRM/ESP is a kernel networking packet path, although policy-gated.
* Hard or special triggering conditions: -1. Triggering requires IPsec/XFRM configuration, the ESP out-of-place fast path, and specific packet/trailer sizing.
* Corruption primitive constrained / no attacker-controlled target: -1. The visible primitive is a size/layout mismatch, not a demonstrated arbitrary write.

Paranoid additional interpretation:

* Generic memory corruption, strong candidate: +2 instead of weak +1. If the crypto scatterlist writes past the actual single page frag backing, this may become real memory corruption.
* Confidentiality impact plausible: +1. Possible out-of-bounds memory interaction in a network crypto path may expose kernel memory in worst case, though not demonstrated.
* Integrity impact plausible: +1. Possible out-of-bounds write/corruption is enough for paranoid manual-review handling.

## 3. Reachability analysis

An attacker may be able to trigger the bug by sending traffic that the affected host or IPsec gateway encrypts through the ESP output path. This is not a general Internet-wide unauthenticated parser bug, because packets must hit a configured XFRM/IPsec policy and the vulnerable fast path. No local account is necessarily required if the attacker can inject traffic from a network segment covered by the policy, so PR:N is defensible for the network-gateway scenario. Containers and namespaces matter only if they can generate traffic through the host XFRM policy or configure their own policy. The path is not universally default-enabled, but XFRM/IPsec is a core kernel networking facility and should not be treated as an obscure subsystem.

Call-site confidence: medium. The patch gives enough ESP head/tail behavior to infer the allocation/layout mismatch, but not the full downstream crypto scatterlist write path.

## 4. Severity interpretation

This is not an ordinary low-risk bounds check. The conservative view is an Actionable Moderate issue because a network-influenced ESP output path can create a page-frag/scatterlist length mismatch and likely crash the kernel. The paranoid view reaches Strong Important candidate because the mismatch may become memory corruption in a packet-processing crypto path. Realistic exploitation beyond DoS is not demonstrated, so arbitrary code execution or privilege escalation should not be assumed without manual source review.

## 5. One-sentence report phrase

The ESP output fast path may build a destination scatterlist whose combined post-trailer length exceeds the single page frag allocation, allowing network-influenced IPsec traffic to cause a kernel crash and potentially memory corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: possible memory corruption in a network-influenced XFRM/ESP packet output path, with conservative DoS impact and defensible paranoid C/I impact.

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

Patch: xfrm: esp: restore combined single-frag length gate
Commit: 566295735530ee513326049b0540f32ec050bf2e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=566295735530ee513326049b0540f32ec050bf2e

Commit description:

The ESP out-of-place fast path appends the trailer in esp_output_head()
before esp_output_tail() allocates the destination page frag. The
head-side gate currently checks skb->data_len and tailen separately, but
the tail code allocates a single destination frag from the combined
post-trailer skb->data_len.

Reject the page-frag fast path when the combined aligned length exceeds a
page. Otherwise skb_page_frag_refill() may fall back to a single page while
the destination sg still spans the combined skb->data_len.

Restore this combined-length page gate for both IPv4 and IPv6.

Fixes: 5bd8baa ("esp: limit skb_page_frag_refill use to a single page")
Cc: stable@vger.kernel.org
Signed-off-by: Lin Ma <malin89@huawei.com>
Signed-off-by: Chenyuan Mi <michenyuan@huawei.com>
Signed-off-by: Jingguo Tan <tanjingguo@huawei.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
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/ipv6/esp6.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=566295735530ee513326049b0540f32ec050bf2e

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

The original Linux kernel CVE announcement for CVE-2026-63912 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63912

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/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: 4
  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: 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.
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).