[CVE-2026-99030][MODERATE 7.0] net: skbuff: don't leave stale bytes in skb_copy_and_csum_bits()

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

CVE: CVE-2026-99030
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: skbuff: don't leave stale bytes in skb_copy_and_csum_bits()
Commit: 5f19b9cfff79508e8cb09be40d014cb0807f3854
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5f19b9cfff79508e8cb09be40d014cb0807f3854
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99030
Analysis date: Mon, 26 Oct 2026 23:55:38 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
Unreadable skb fragments can cause stale bytes from an output buffer to be transmitted through ICMP or driver TX paths, creating a configuration-dependent kernel-memory disclosure.

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

The original Linux kernel CVE announcement for CVE-2026-99030 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99030

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-2026-99030	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99030 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N';*CWE-200;CWE-908;CWE-226;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.9';DESCR 'skb_copy_and_csum_bits can copy the linear skb data and then return when it encounters unreadable fragments while leaving the remaining destination bytes unchanged. The affected callers may transmit those stale bytes in an ICMP error or a driver TX bounce buffer, creating a concrete kernel memory disclosure path, although specialized unreadable fragment handling is required and the resulting checksum will usually be invalid. For the CVSS the PR:N value is used because a remote peer may supply traffic that reaches the affected network path without an authenticated user context, but successful disclosure depends on the enabled device memory or zero copy configuration and on observing packets that receivers normally drop. Confidentiality is L for the base score and H for the highest still defensible case because repeated packets could expose more sensitive stale memory. Integrity and availability impact are not demonstrated. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.9 and the production ActionableScore returned 4 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 3 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 4)	MAYBE	REMOTE SIMPLEFIX SKB LINUS KPANIC PACKET 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 3; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; remote reachability is plausible from the supplied summary but not established by the patch alone.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +1: Concrete constrained disclosure primitive. Uncopied destination bytes remain stale and are subsequently transmitted.
* +1: Confidentiality impact is directly supported; this is not merely an internal uninitialized read.
* +1: Affected consumers are network output paths—ICMP error quotation and TX bounce-buffer handling.
* Paranoid +2: A remote peer may induce relevant traffic without authentication, according to the supplied summary. The patch does not independently prove creation of unreadable fragments by remote traffic.
* No rarity penalty: unreadable-fragment/device-memory configuration is specialized, but the evidence does not prove it is both rare and difficult to reach.
* No integrity, availability, OOB write, UAF, arbitrary read, or authorization-bypass points.
* No high-confidence C:H claim: leaked contents, repeatability, packet observability, and checksum acceptance are unresolved.

3. Reachability analysis

The complete trigger requires an skb with unreadable fragments followed by one of the identified transmit consumers. Remote traffic influence is credible but configuration-dependent and not demonstrated by the diff. Required local privileges, namespace capability scope, prior device/zero-copy setup, and whether an unprivileged actor can establish the complete path remain unknown. No timing race is shown; checksum mismatch commonly limits successful observation but does not prevent transmission in every case.

4. Primitive and severity reading

The primitive is disclosure of stale caller-buffer contents after a partial skb copy, not memory corruption or an out-of-bounds access. The commit explicitly states that those bytes can go onto the wire. This is CLASS_B because a real kernel-memory disclosure channel exists, while sensitive contents, reliable collection, and broad remote exploitability are unproven. ImportantCandidate is YES and automatic downgrade is disallowed because the transmitted kernel data and unresolved network exposure require human assessment; this is not proof of Important severity.

5. One-sentence report phrase

Unreadable skb fragments can cause stale bytes from an output buffer to be transmitted through ICMP or driver TX paths, creating a configuration-dependent kernel-memory disclosure.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm remote creation of unreadable-fragment skbs, affected configurations, packet observability despite checksum failure, and the sensitivity and repeatability of leaked bytes.

7. Call-site confidence

Call-site confidence: medium

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

Patch: net: skbuff: don't leave stale bytes in skb_copy_and_csum_bits()
Commit: 5f19b9cfff79508e8cb09be40d014cb0807f3854
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5f19b9cfff79508e8cb09be40d014cb0807f3854

Commit description:

When skb_copy_and_csum_bits() reaches unreadable frags it returns 0
after copying only the linear part, and the rest of the caller's buffer
is left as it was.  The callers copy into a buffer that is about to go
out on the wire: an ICMP error quoting the offending packet, or a
driver's TX bounce buffer in skb_copy_and_csum_dev().  Neither buffer
is zeroed beforehand, so whatever was in memory there gets sent.

Zero the part of the buffer we didn't fill.  The checksum usually
won't match the data any more, so the receiver will usually drop the
packet, but either way it no longer carries anything it shouldn't.
Only zero for a positive @len , a negative one from a broken caller must
not turn into a huge memset().

Fixes: 65249fe ("net: add support for skbs with unreadable frags")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20261007-b4-skb-copy-csum-stale-bytes-v1-1-adbbde033fb3@toxicpanda.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/skbuff.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=5f19b9cfff79508e8cb09be40d014cb0807f3854

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

The original Linux kernel CVE announcement for CVE-2026-99030 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99030

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:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 5.9

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: 3
  Paranoid ActionableScore: 5

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).