[CVE-2026-63830][IMPORTANT] net: skmsg: preserve sg.copy across SG transforms

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

CVE: CVE-2026-63830
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net: skmsg: preserve sg.copy across SG transforms
Commit: 9bb86d8184b37503816150c4a6ad3c17dfdbe827
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9bb86d8184b37503816150c4a6ad3c17dfdbe827
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63830
Analysis date: Sun, 19 Jul 2026 08:51:48 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A sk_msg ownership-state bug can desynchronize sg.copy from sg.data entries during BPF and TLS scatterlist transforms, allowing writable BPF access to externally backed page-cache pages and creating a high-risk page-cache corruption primitive when SK_MSG BPF control is available.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63830 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63830

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-2026-63830	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-668;CWE-281;CWE-284;CWE-693;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:H/A:H';BEST CVSS score: '7';DESCR 'sk_msg can lose the sg.copy ownership state while moving or splitting scatterlist entries, which can expose externally backed page cache pages as writable BPF ctx data. A local attacker with access to BPF sk_msg control paths may be able to make BPF stores modify the original page cache rather than a private sk_msg page, creating a potential page cache write primitive. For the CVSS the PR:L is used for the paranoid score because delegated BPF capabilities, container service accounts, or unprivileged BPF configurations can make the trigger reachable without full administrator control. The issue is not directly network reachable by packet traffic alone, but network data may participate once a vulnerable SK_MSG BPF setup exists. Impact is at least local integrity compromise and denial of service, and in the worst case may allow confidentiality or privilege escalation impact due to page cache corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	INIT BPF LINUS TEST  KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	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

Conservative signals:

* **Dirty-Pipe-like ownership/COW violation: +2**
  `sg.copy` protects externally backed pages from writable BPF exposure. Losing this bit can expose splice-backed page-cache pages as writable `ctx->data`.

* **Page-cache corruption or privileged file-backed data corruption: +2**
  The commit explicitly says BPF stores can modify the original page cache, not a private copy.

* **Memory layout invariant restoration: +1**
  The patch restores consistency between `sg.data[]` and the matching `sg.copy` ownership bitmap across transfer, shift, split, and compaction paths.

* **Integrity impact plausible: +1**
  Writable access to externally owned page-cache pages is a practical integrity violation.

* **Requires admin/root/CAP_* in typical deployments: -2**
  SK_MSG BPF setup normally requires privileged BPF and networking capabilities in hardened/default product configurations.

* **Common kernel networking/BPF data path exposure: +1**
  This is in `net/core/skmsg`, BPF SK_MSG helpers, and TLS sk_msg handling. It is not ordinary driver-only code.

Paranoid additional interpretation:

* **Privilege escalation plausible: +2**
  Page-cache corruption is a historically high-risk primitive because it can bypass normal file ownership, read-only mapping, or COW expectations. If reachable from a reduced-capability service, delegated BPF environment, container root, or unprivileged BPF-enabled system, LPE becomes plausible.

* **Confidentiality impact plausible: +1**
  The main primitive is write-oriented, but page-cache ownership violations can affect sensitive file-backed state and cross-boundary assumptions.

* **Availability impact realistic: +1**
  Corrupting shared/file-backed pages can cause persistent data corruption or system/service disruption beyond a simple crash.

* **Hard/specific trigger conditions: -1**
  Exploitation requires a specific SK_MSG/BPF setup, externally backed pages such as splice-backed page-cache pages, and a transform path that desynchronizes `sg.copy`.

## 3. Reachability analysis

The bug is **local**, not directly remote by packet traffic alone. A remote peer may participate in data flow once a vulnerable SK_MSG/TLS/BPF setup exists, but the attacker normally needs local ability to load or control SK_MSG BPF programs and prepare splice-backed or externally backed pages.

In typical hardened deployments, BPF program loading and attachment require `CAP_BPF`, `CAP_NET_ADMIN`, `CAP_SYS_ADMIN`, or root-controlled configuration. Therefore the conservative score applies the privilege penalty.

Namespaces and containers matter. If a container root, service account, delegated BPF manager, or product-specific configuration allows BPF/sk_msg control without full host-root equivalence, this should not be treated as ordinary PR:H. In that interpretation the issue becomes a strong manual-review candidate because the primitive is page-cache/shared-page corruption.

Path default exposure is limited by BPF privilege policy, but the affected code is in core networking/BPF/TLS sk_msg infrastructure, not rare hardware.

Call-site confidence: **high**. The patch shows multiple concrete call paths: `bpf_msg_pull_data`, `bpf_msg_push_data`, `bpf_msg_pop_data`, `sk_msg_shift_left/right`, `sk_msg_xfer`, and `tls_split_open_record`.

## 4. Severity interpretation

This does **not** behave like an ordinary Moderate crash bug. The patch describes a concrete ownership-state loss that can make externally backed page-cache memory writable through BPF `ctx->data`.

Realistic exploitation is gated by BPF privileges and setup complexity, so the conservative result is **Actionable Moderate** rather than automatic Important. However, the underlying primitive is page-cache/shared-page corruption, which is much more security-relevant than a NULL dereference or resource leak. Under delegated BPF or reduced-capability-root assumptions, it is a **Strong Important candidate**.

Theoretical worst case includes privilege escalation through page-cache corruption. Realistic evidence supports at least integrity impact and mandatory manual review.

## 5. One-sentence report phrase

A sk_msg ownership-state bug can desynchronize sg.copy from sg.data entries during BPF and TLS scatterlist transforms, allowing writable BPF access to externally backed page-cache pages and creating a high-risk page-cache corruption primitive when SK_MSG BPF control is available.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.**

Reason: the patch explicitly describes writable exposure of externally backed page-cache pages, which is a Dirty-Pipe-like ownership/COW bypass class. Even though conservative reachability is gated by privileged BPF setup, this is not safe to auto-close because delegated BPF or reduced-capability environments can materially change exploitability.

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UPSTREAM PATCH SUMMARY
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Patch: net: skmsg: preserve sg.copy across SG transforms
Commit: 9bb86d8184b37503816150c4a6ad3c17dfdbe827
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9bb86d8184b37503816150c4a6ad3c17dfdbe827

Commit description:

The sk_msg sg.copy bitmap is part of the scatterlist entry ownership
state. A set bit tells sk_msg_compute_data_pointers() not to expose the
entry through writable BPF ctx->data. This protects entries backed by
pages that are not private to the sk_msg, such as splice-backed file
page-cache pages.

Several sk_msg transform paths move, copy, split, or compact
msg->sg.data[] entries without moving the matching sg.copy bit. This can
make an externally backed entry arrive at a new slot with a clear copy
bit. A later SK_MSG verdict can then expose sg_virt(sge) as writable
ctx->data and BPF stores can modify the original page cache.

Keep sg.copy synchronized with sg.data[] whenever entries are
transferred, shifted, split, or copied into a new sk_msg. Clear the bit
when an entry is replaced by a newly allocated private page or freed.
This covers the BPF pull/push/pop helpers, sk_msg_shift_left/right(),
sk_msg_xfer(), and tls_split_open_record(), including the partial tail
entry created during TLS open-record splitting.

Fixes: d3b18ad ("tls: add bpf support to sk_msg handling")
Cc: stable@vger.kernel.org
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Reported-by: Keenan Dong <keenanat2000@gmail.com>
Signed-off-by: Yiming Qian <yimingqian591@gmail.com>
Link: https://patch.msgid.link/20260610062137.49075-1-yimingqian591@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/linux/skmsg.h
  net/core/filter.c
  net/core/skmsg.c
  net/tls/tls_sw.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=9bb86d8184b37503816150c4a6ad3c17dfdbe827

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

The original Linux kernel CVE announcement for CVE-2026-63830 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63830

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:H/UI:N/S:U/C:L/I:H/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).