[CVE-2026-74627][MODERATE 7.0] net: devmem: prevent net-iov / page mixing

[CVE-2026-74627][MODERATE 7.0] net: devmem: prevent net-iov / page mixing

[AL-KERNEL]

CVE: CVE-2026-74627
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: devmem: prevent net-iov / page mixing
Commit: e9bfe12b1d04c34c6fedcba21d9709b65c08aa33
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e9bfe12b1d04c34c6fedcba21d9709b65c08aa33
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74627
Analysis date: Sat, 22 Aug 2026 14:14:51 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local process with access to the net devmem TX zerocopy path may create an skb mixing net_iov backed and page backed fragments, violating skb fragment ownership rules and potentially causing a kernel crash, with weak but review-worthy memory corruption concerns due to downstream lifetime handling.

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

The original Linux kernel CVE announcement for CVE-2026-74627 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74627

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-2026-74627	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-843;*CWE-20;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'Mixing net_iov backed and page backed fragments in one skb violates an skb frag invariant in the net devmem TX zero copy path. A local process that can use the devmem send path may be able to build an skb with existing readable page backed frags and then append devmem backed frags, after which lower network stack code can mis-handle the fragment backing type and crash the kernel. For the CVSS the PR:L is used because reliable triggering requires local code execution with access to the devmem TX interface and related dma buf resources, but not full administrator privileges in all deployments. The issue is not directly network reachable from a remote peer because the malformed skb is produced by a local sender. Impact is at least denial of service via kernel crash. For the paranoid score, the invariant violation is treated as a memory corruption candidate because later skb fragment handling may apply page and net_iov lifetime rules to the wrong backing object.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74627 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE SIMPLEFIX SKB MEMORY  DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6 DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged or low-privileged trigger: +1
  A local process that can use the devmem TX zerocopy path and related dma-buf resources may construct the affected skb state.
* Memory corruption, weak/indirect corruption candidate: +1
  The bug violates the skb fragment backing invariant by allowing net_iov backed and page backed fragments in the same skb. This is a plausible object interpretation or lifetime handling hazard, but the patch does not show an arbitrary write, controlled overwrite, or proven UAF reclaim primitive.
* Memory layout invariant restoration: +1
  The patch restores a required consistency rule for skb fragment layout and backing type.
* Reliable kernel crash / strong DoS: +1
  The commit states that mixed backing types blow up down the stack, which supports at least a crash or strong DoS interpretation.
* Privileged kernel/device-management or DMA-adjacent memory path: +1
  The affected path involves devmem, dma-buf style backing, net_iov objects, and skb fragment ownership rules.
* Rare or non-default configuration: -1
  devmem TX zerocopy is not a normal default networking path and requires specific runtime support and resource setup.

Paranoid additional signal:

* Weak LPE concern: +1
  Downstream skb handling may apply page-backed lifetime or access rules to net_iov-backed data, or the reverse. This is not enough to claim a strong LPE primitive, but it is enough to avoid auto-closing without manual review.

Not awarded:

* Remote reachable: +0
  The malformed skb is produced by a local sender, not by remote packet input.
* Strong memory corruption primitive: +0
  No controlled overwrite, arbitrary write, double free, or demonstrated UAF reclaim is shown.
* Confidentiality or integrity impact: +0 conservative
  The patch supports a crash and invariant violation, but does not demonstrate data disclosure or unauthorized modification.

## 3. Reachability analysis

The trigger is local and depends on access to the net devmem TX zerocopy path and suitable dma-buf or devmem resources. This is not directly network-triggerable by a remote peer. A remote peer may receive traffic generated from the malformed skb, but the construction of the skb happens on the sending host.

In typical deployments this path is uncommon and likely requires explicit device, driver, and application support. Privilege requirements may vary. Full host root may be needed to configure the environment, but the actual trigger may be reachable by a less privileged local process if devmem resources or dma-buf file descriptors are delegated to it. Containers or service accounts with delegated device access could therefore matter for practical exposure.

Call-site confidence: medium. The patch shows the invariant check in zerocopy_fill_skb_from_devmem(), but does not include all downstream skb consumers. The helper name, skb fragment semantics, and commit wording justify manual review.

## 4. Severity interpretation

This is more than an ordinary Moderate correctness fix because it affects skb fragment backing invariants in a DMA-adjacent networking memory path. Realistically, the demonstrated impact is local DoS via kernel crash. The paranoid interpretation treats it as an Actionable Moderate because mixed skb fragment backing can plausibly lead to wrong lifetime handling or object interpretation downstream.

It should not be classified as a Strong Important candidate based only on the supplied patch, because there is no demonstrated reclaim primitive, arbitrary write, controlled type confusion, or clear privilege escalation path.

## 5. One-sentence report phrase

A local process with access to the net devmem TX zerocopy path may create an skb mixing net_iov backed and page backed fragments, violating skb fragment ownership rules and potentially causing a kernel crash, with weak but review-worthy memory corruption concerns due to downstream lifetime handling.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is in networking skb memory ownership logic and restores a fragment backing invariant involving devmem and dma-buf style objects. The known impact is DoS, but the downstream lifetime consequences are important enough that this should not be auto-closed.

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

Patch: net: devmem: prevent net-iov / page mixing
Commit: e9bfe12b1d04c34c6fedcba21d9709b65c08aa33
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e9bfe12b1d04c34c6fedcba21d9709b65c08aa33

Commit description:

We should either have net_iov or page backed frags in a single skb,
otherwise it blows up down the stack. Don't allow mixing in
zerocopy_fill_skb_from_devmem().

Fixes: bd61848 ("net: devmem: Implement TX path")
Cc: stable@vger.kernel.org
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/e3199788c4732545627a4721097ebb71ad737bab.1785150502.git.asml.silence@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/core/datagram.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=e9bfe12b1d04c34c6fedcba21d9709b65c08aa33

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

The original Linux kernel CVE announcement for CVE-2026-74627 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74627

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:N/I:N/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: 4
  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).