[CVE-2026-74612][MODERATE 7.0] veth: fix skb length accounting after XDP frag adjustment

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

CVE: CVE-2026-74612
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: veth: fix skb length accounting after XDP frag adjustment
Commit: 2f2a7f3f8b9f1bffc9b0488aa02b6951b2aec139
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f2a7f3f8b9f1bffc9b0488aa02b6951b2aec139
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74612
Analysis date: Sat, 22 Aug 2026 12:41:27 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A veth XDP fragment adjustment bug can desynchronize skb length accounting, causing an OOB read of skb_shared_info metadata including a kernel pointer into userspace and a possible SKB assertion based DoS when local or delegated namespace privileges allow XDP control.

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

The original Linux kernel CVE announcement for CVE-2026-74612 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74612

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-2026-74612	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:H';CWE-125;*CWE-200;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '6.5';DESCR 'A veth XDP fragment adjustment can leave skb len and skb data_len inconsistent for a non-linear skb. After a fragment shrink, skb_headlen can become larger than the real linear area and __skb_datagram_iter can copy bytes past the actual packet tail into userspace, including skb_shared_info metadata such as nr_frags, xdp_frags_size, and a kernel pointer. For the CVSS the PR:L value is used because reliable triggering requires a local or container network context with the ability to run or control XDP on veth, but this can be less than full host administrative control in delegated networking setups. The issue is not normally public Internet reachable and is most relevant to local namespaces, containers, or virtual networking paths using veth and XDP. Impact is information disclosure with possible kernel pointer leakage, packet data corruption, and denial of service through SKB assertions or kernel warning paths.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	BPF NETWORK SKB LINUS PACKET  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Memory layout invariant restoration: +1
  The patch restores consistency between `skb->len`, `skb->data_len`, XDP `data_end`, fragment size accounting, and skb tail position.

* Confidentiality impact plausible: +1
  The commit explicitly states that `__skb_datagram_iter()` copied bytes past the actual linear tail into userspace, including `skb_shared_info` metadata and a kernel pointer.

* Integrity impact plausible: +1
  The reproduced UDP receive path produced corrupted payloads, with real packet data displaced and truncated.

* Reliable kernel crash / strong DoS: +1
  The old `__skb_put()` path can trigger `SKB_LINEAR_ASSERT()` when frags remain present after `bpf_xdp_pull_data()`.

* Broad/common subsystem exposure: +1
  `veth` is widely used in container and namespace networking.

* Common packet-processing path: +1
  The bug is in the veth receive path after XDP processing and affects skb construction before normal protocol receive handling.

Subtractions:

* Requires CAP_BPF/CAP_NET_ADMIN/root-like setup in typical deployments: -2
  Loading or attaching XDP/eBPF programs to veth normally requires elevated privileges in the evaluated product configuration.

Paranoid adjustment:

* Reduced privilege penalty instead of full admin penalty: +1 relative to conservative
  In delegated container or network namespace setups, the attacker may have CAP_NET_ADMIN or BPF attachment control without full host-root power.

* Availability impact realistic: +1
  The assertion path is reproducible and may become host-wide DoS on configurations where kernel warnings/assertions are fatal or service-impacting.

## 3. Reachability analysis

This is not remotely reachable from the public Internet by packet traffic alone. A realistic trigger requires local control over a veth plus XDP/eBPF processing path, typically through CAP_NET_ADMIN, CAP_BPF, CAP_SYS_ADMIN, or an already privileged container or network namespace setup. Unprivileged local triggering should not be assumed when unprivileged BPF is disabled. However, veth and XDP are common in container networking, so delegated namespace privileges can make this more practical than a full host-root-only bug.

Call-site confidence: high. The commit describes the affected receive path, the concrete UDP receive behavior, the copied metadata, and the assertion reproducer.

## 4. Severity interpretation

This behaves above an ordinary Moderate because it is not just a crash or warning. It has a demonstrated OOB read into userspace with kernel pointer disclosure plus packet corruption and a separate assertion/DoS path. It does not currently justify Important by itself because there is no demonstrated arbitrary write, UAF reclaim, type confusion, or direct privilege escalation primitive. The practical classification is Actionable Moderate, with manual review warranted due to kernel pointer leakage and skb metadata exposure.

## 5. One-sentence report phrase

A veth XDP fragment adjustment bug can desynchronize skb length accounting, causing an OOB read of skb_shared_info metadata including a kernel pointer into userspace and a possible SKB assertion based DoS when local or delegated namespace privileges allow XDP control.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because the patch documents a real userspace-visible kernel memory disclosure from skb metadata, possible kernel pointer leakage, packet corruption, and a reproducible assertion path, even though practical triggering is gated by XDP/BPF privileges.

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

Patch: veth: fix skb length accounting after XDP frag adjustment
Commit: 2f2a7f3f8b9f1bffc9b0488aa02b6951b2aec139
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f2a7f3f8b9f1bffc9b0488aa02b6951b2aec139

Commit description:

veth exposes non-linear skb fragments through an xdp_buff. If an XDP
program adjusts the fragment area, veth_xdp_rcv_skb() 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.

Additionally, bpf_xdp_pull_data() can advance data_end while leaving
frags present. The skb is then still non-linear, so the old
__skb_put(skb, off) triggers SKB_LINEAR_ASSERT().

Use skb_set_tail_pointer() and update skb->len explicitly instead,
following bpf_prog_run_generic_xdp(). Unlike __skb_put(),
skb_set_tail_pointer() does not require a linear skb.

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. A forced-tailroom reproducer also exercises
bpf_xdp_pull_data() with frags still present; the old code triggers
SKB_LINEAR_ASSERT(), while this fix passes 10/10 runs.

Fixes: 718a18a ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
Cc: stable@vger.kernel.org
Reported-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Link: https://lore.kernel.org/bpf/80687d9c-9c27-494c-b3f2-efd0230b1895@gmail.com/
Suggested-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Link: https://patch.msgid.link/20260804054040.613675-3-sun.jian.kdev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/veth.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=2f2a7f3f8b9f1bffc9b0488aa02b6951b2aec139

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

The original Linux kernel CVE announcement for CVE-2026-74612 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74612

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:L/A:H
  The Best / paranoid CVSS score: 6.5

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: 6

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).