[CVE-2026-74665][MODERATE 7.0] net: fix skb length accounting after generic XDP frag adjustment

[CVE-2026-74665][MODERATE 7.0] net: fix skb length accounting after generic XDP frag adjustment

[AL-KERNEL]

CVE: CVE-2026-74665
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: fix skb length accounting after generic XDP frag adjustment
Commit: 72468efcc089782fa047afddd10856b11a14ef5b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=72468efcc089782fa047afddd10856b11a14ef5b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74665
Analysis date: Sat, 22 Aug 2026 16:22:36 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Generic XDP fragment shrink can desynchronize `skb->len` and `skb->data_len`, causing the UDP receive path to copy bytes past the real packet tail from `skb_shared_info` to userspace, including a kernel pointer, with practical reachability depending on privileged or delegated XDP/BPF access.

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

The original Linux kernel CVE announcement for CVE-2026-74665 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74665

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-2026-74665	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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:L/A:N';CWE-125;*CWE-200;CWE-682;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '5.3';DESCR 'Generic XDP with non-linear skb fragments can leave skb len and skb data_len inconsistent after an XDP program shrinks the fragment area. The stale length makes skb_headlen larger than the real linear data area, so the UDP receive path may copy bytes past the actual packet tail into userspace. The observed leak starts at skb_shared_info and includes nr_frags, xdp_frags_size, and a kernel pointer from skb_shinfo(skb) frags(0). For the CVSS the PR:L is selected for the paranoid score because a reduced-capability user or container root with delegated network and BPF permissions may be able to attach generic XDP and receive affected traffic. The issue is not directly Internet reachable by packet traffic alone unless a vulnerable generic XDP setup is already present and attacker controlled traffic reaches it. Impact is mainly information disclosure and payload corruption, with possible KASLR bypass from leaked kernel pointers rather than a proven privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)  SKIP CVE-2026-74665 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE BPF SKB LINUS PACKET  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Weak read-only OOB / invalid length accounting candidate: +1
  `skb->len` and `skb->data_len` become inconsistent, causing `skb_headlen()` to cover bytes past the real packet tail.

* Memory layout invariant restoration: +1
  The patch restores consistency between interdependent skb length fields, specifically `skb->len`, `skb->data_len`, and fragment size accounting.

* Confidentiality impact plausible: +1
  The reproducer shows bytes copied from `skb_shared_info` to userspace, including `nr_frags`, `xdp_frags_size`, and a kernel pointer.

* Integrity impact plausible: +1
  Real packet data is displaced and truncated, so delivered payload integrity can be affected, although this is not a kernel write primitive.

* Common networking / packet-processing path: +1
  The bug is in `net/core/dev.c` in the generic XDP skb processing path.

* Requires admin/root/CAP_* in typical deployments: -2
  Loading or attaching the required XDP/eBPF program normally requires elevated privileges such as CAP_BPF, CAP_NET_ADMIN, CAP_SYS_ADMIN, or root-controlled BPF configuration.

Paranoid adjustment:

* The -2 privilege penalty is not applied in the paranoid score because reduced-capability container root, delegated network administration, or service accounts with BPF/XDP permissions may be meaningful attackers without full host-root equivalence.

Not awarded:

* No remote reachable bonus. Packet traffic alone is not sufficient unless a vulnerable generic XDP program is already installed.
* No strong memory corruption bonus. The demonstrated primitive is an OOB read / information leak, not UAF, OOB write, double free, or controlled overwrite.
* No LPE bonus. Kernel pointer disclosure may assist exploitation, but the patch does not show a direct privilege escalation primitive.
* No reliable kernel crash bonus. The demonstrated effect is payload corruption and disclosure, not panic.

## 3. Reachability analysis

The realistic trigger requires a generic XDP setup using non-linear skb fragments and an XDP program that shrinks the fragment area, for example through `bpf_xdp_adjust_tail`. In typical hardened product configurations, unprivileged BPF is disabled and attaching XDP requires privileged capabilities, so the conservative interpretation treats this as a privileged local issue.

Namespaces and containers can change the practical risk. If a container root or delegated service account can attach generic XDP or otherwise control the affected BPF/XDP path, the issue becomes more actionable because the attacker may not have full host-root privileges.

The bug is not directly Internet-reachable by malformed packets alone. Remote traffic can participate in the trigger only after the vulnerable XDP program and network setup are already present.

Call-site confidence: high. The commit message describes the concrete UDP receive path through `__skb_datagram_iter()` and identifies the exact leaked structure contents.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary low-risk Moderate. The main demonstrated primitive is a kernel information leak to userspace caused by stale skb length accounting, including disclosure of a kernel pointer from `skb_shared_info`.

The theoretical concern is that kernel pointer disclosure may assist later exploitation, for example KASLR bypass, but the patch does not support treating this as direct LPE or strong memory corruption. The realistic impact is privileged or delegated-local information disclosure plus packet payload corruption.

## 5. One-sentence report phrase

Generic XDP fragment shrink can desynchronize `skb->len` and `skb->data_len`, causing the UDP receive path to copy bytes past the real packet tail from `skb_shared_info` to userspace, including a kernel pointer, with practical reachability depending on privileged or delegated XDP/BPF access.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: the issue demonstrates a concrete kernel out-of-bounds read and kernel pointer disclosure in a networking data path, even though direct privilege escalation is not proven.

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

Patch: net: fix skb length accounting after generic XDP frag adjustment
Commit: 72468efcc089782fa047afddd10856b11a14ef5b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=72468efcc089782fa047afddd10856b11a14ef5b

Commit description:

Generic XDP exposes non-linear skb fragments through an xdp_buff. If an
XDP program adjusts the fragment area, bpf_prog_run_generic_xdp() copies
xdp_frags_size back to skb->data_len but leaves skb->len containing the
old fragment contribution.

After a fragment shrink, this makes skb_headlen() larger than the actual
linear area. In the reproduced UDP receive path, __skb_datagram_iter()
copied 1024 bytes past the actual linear tail to userspace, starting at
struct skb_shared_info. The copied bytes included the affected skb's
nr_frags, xdp_frags_size and a kernel pointer from
skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same
amount and truncated at the end.

Subtract the old data_len before replacing it and add the new data_len
afterwards, keeping skb->len and skb->data_len synchronized.

A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by
1024 bytes from its fragment area. Before the fix, all 10 runs produced
corrupted payloads. After the fix, all 10 runs matched the expected
payload exactly.

Fixes: e6d5dbd ("xdp: add multi-buff support for xdp running in generic mode")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/
Reviewed-by: Mohsin Bashir <hmohsin@meta.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Link: https://patch.msgid.link/20260804054040.613675-2-sun.jian.kdev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/dev.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=72468efcc089782fa047afddd10856b11a14ef5b

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

The original Linux kernel CVE announcement for CVE-2026-74665 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74665

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:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:L/A:N
  The Best / paranoid CVSS score: 5.3

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: NO
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).