[CVE-2026-74476][IMPORTANT] veth: convert frag_list skbs before running XDP

[CVE-2026-74476][IMPORTANT] veth: convert frag_list skbs before running XDP

[AL-KERNEL]

CVE: CVE-2026-74476
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: veth: convert frag_list skbs before running XDP
Commit: 5c1c15c540fc45820ce3033c319151ec891bc10a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5c1c15c540fc45820ce3033c319151ec891bc10a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74476
Analysis date: Sat, 15 Aug 2026 18:30:45 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A veth XDP receive path can expose frag_list skb data as XDP frags when nr_frags is zero, allowing AF_XDP copy mode to trust invalid fragment metadata and crash the kernel from __xsk_rcv().

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

The original Linux kernel CVE announcement for CVE-2026-74476 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74476

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-2026-74476	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';*CWE-20;CWE-125;**CWE-476;Other CVSS 'AV:A/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.4';DESCR 'A veth receive path can expose a frag_list skb to XDP as if the packet data were stored in regular frags even when nr_frags is zero. AF_XDP copy mode may then trust the inconsistent XDP fragment metadata, walk an empty fragment entry, and crash in memcpy from __xsk_rcv. The issue is adjacent network reachable in typical container or namespace deployments where veth is used with XDP and AF_XDP copy mode. It is not a general public Internet remote vector because the vulnerable path depends on local virtual Ethernet topology and XDP setup. For the CVSS the PR:L is used because a low privileged local or container user that can send traffic through the veth peer may be sufficient in realistic deployments. Impact is at least denial of service via kernel crash. The paranoid score keeps limited confidentiality impact because the bug involves invalid fragment metadata used by a copy path, but there is no clear arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 7)	YES	DANGER SIMPLEFIX NETWORK SKB KERNEL_PANIC_PLUS_UAF KPANIC MEMORY  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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## 1. ActionableScore

ActionableScore=6
ActionableScoreLower=5

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

* +1 Local unprivileged trigger: in an already configured veth plus XDP plus AF_XDP copy-mode deployment, a low-privileged local or container-side sender may be able to trigger the receive path by sending traffic through the veth peer.
* +1 Memory corruption, weak/indirect corruption candidate: the bug exposes inconsistent XDP fragment metadata, with data_len set but nr_frags equal to zero, causing AF_XDP to walk a non-existent fragment entry.
* +1 Memory layout invariant restoration: the patch restores consistency between skb_is_nonlinear(), frag_list, nr_frags, xdp_frags_size, and XDP multi-buffer expectations.
* +1 Reliable kernel crash / strong DoS: the commit explicitly describes a crash in memcpy() from __xsk_rcv().
* +1 Common packet-processing path: veth is common in container and namespace networking, and the bug sits in an XDP receive path.
* Paranoid only +1 Confidentiality impact plausible: the failure is in a copy path using bogus fragment metadata, so limited unintended read exposure is technically worth manual review, although no concrete info leak primitive is demonstrated.

Not awarded:

* No +2 remote reachable: this is not a general Internet-reachable network bug.
* No +2 strong LPE plausibility: no UAF reclaim, arbitrary write, controlled overwrite, callback hijack, or refcount takeover is shown.
* No high-control BPF/XDP API bonus: attaching XDP/AF_XDP is normally privileged. The score assumes triggering through an existing configured path, not unprivileged XDP setup.

## 3. Reachability analysis

The realistic trigger is adjacent/local network style reachability through a veth peer, most likely in container or namespace setups where XDP and AF_XDP copy mode are already configured by an administrator or privileged service. A fully unprivileged user generally cannot create the whole vulnerable setup because XDP attachment is capability-gated, but a low-privileged workload may still send packets through an already configured veth path. This is not public remote reachability. The path is configuration-dependent but not rare enough to auto-close because veth and XDP are common in high-performance container networking.

Call-site confidence: high, because the patch and commit directly identify veth_convert_skb_to_xdp_buff(), XDP fragment metadata, AF_XDP copy mode, and the crash path in __xsk_rcv().

## 4. Severity interpretation

This behaves like an actionable Moderate rather than an ordinary Moderate because it is a packet-processing crash path with malformed skb/XDP fragment state and realistic container-network exposure. It is not an Important-class issue on current evidence because the patch supports invalid fragment interpretation and kernel crash, but not a strong write primitive or practical privilege escalation chain. The paranoid score is higher only to preserve manual-review sensitivity for possible limited read impact from the AF_XDP copy path.

## 5. One-sentence report phrase

A veth XDP receive path can expose frag_list skb data as XDP frags when nr_frags is zero, allowing AF_XDP copy mode to trust invalid fragment metadata and crash the kernel from __xsk_rcv().

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: adjacent/container-network reachability plus invalid fragment metadata in a kernel packet copy path makes this unsuitable for automatic closure, even though the demonstrated impact is primarily DoS.

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

Patch: veth: convert frag_list skbs before running XDP
Commit: 5c1c15c540fc45820ce3033c319151ec891bc10a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5c1c15c540fc45820ce3033c319151ec891bc10a

Commit description:

A frag_list skb can reach veth with data_len set but nr_frags zero.
veth_convert_skb_to_xdp_buff() only converts skbs that are shared,
locked, have frags[], or do not have enough headroom. It later uses
skb_is_nonlinear() to decide whether to set XDP_FLAGS_HAS_FRAGS and
xdp_frags_size.

That exposes frag_list data to XDP as if it were stored in frags[], but
frags[] is empty. AF_XDP copy mode can then trust the bogus XDP fragment
metadata, walk an empty fragment entry, and crash in memcpy() from
__xsk_rcv().

Route non-linear skbs through skb_pp_cow_data() before exposing them to
XDP, and only advertise XDP frags when the resulting skb has frags[].
skb_copy_bits() already handles frag_list input, and skb_pp_cow_data()
builds frags[] output with skb_add_rx_frag(), which is the
representation XDP multi-buffer expects.

Fixes: 718a18a ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
Cc: stable@vger.kernel.org
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@toke.dk>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260722191925.2192070-1-matt@readmodwrite.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/veth.c
  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=5c1c15c540fc45820ce3033c319151ec891bc10a

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

The original Linux kernel CVE announcement for CVE-2026-74476 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74476

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:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 5.4

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: 5
  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: 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).