[CVE-2026-74664][LOW] net: openvswitch: reallocate update replies for mismatched IDs

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

CVE: CVE-2026-74664
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net: openvswitch: reallocate update replies for mismatched IDs
Commit: bd8ca84d48cd9a4f6fc63df26512c55e1d339927
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bd8ca84d48cd9a4f6fc63df26512c55e1d339927
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74664
Analysis date: Sat, 22 Aug 2026 12:26:56 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
Open vSwitch may preallocate an undersized update reply skb when a short UFID request resolves to a key identified flow, causing `ovs_flow_cmd_fill_info()` to fail with `EMSGSIZE` and hit `BUG_ON`, resulting in a local control-plane DoS.

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

The original Linux kernel CVE announcement for CVE-2026-74664 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74664

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: LOW
Manual review required: NO

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-2026-74664	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-131;CWE-754;CWE-617;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'Open vSwitch can allocate an update reply skb using the short request UFID before the actual existing flow is resolved. During an update, the UFID lookup may miss and the flow key lookup may match a key identified flow, so the echoed reply needs more space than the preallocated skb provides. ovs_flow_cmd_fill_info can then fail with EMSGSIZE and the update path can hit BUG_ON, causing a kernel crash or panic. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires access to the OVS control plane or delegated CAP_NET_ADMIN style privileges, which may be available to a container root or network service account in some deployments. The issue is not directly network reachable through ordinary packet forwarding and is triggered by control plane flow update operations. Impact is denial of service only, with no evidence of information disclosure, arbitrary write, UAF, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)	YES	WARNONLY SKB LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit explicitly says the undersized reply can make `ovs_flow_cmd_fill_info()` fail with `-EMSGSIZE` and then hit `BUG_ON(error < 0)`.
* Memory layout invariant restoration: +1. The patch restores consistency between the preallocated reply skb size and the actual matched flow identifier size.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Updating Open vSwitch flows through the control plane normally requires OVS management privileges or `CAP_NET_ADMIN`.

Paranoid delta:

* Paranoid score removes the full privilege penalty in delegated environments. If OVS control is exposed to container root, reduced-capability root, or a network service account, the practical privilege barrier may be closer to PR:L than full host-root.

Not awarded:

* No generic memory corruption. The oversized reply attempt fails with `-EMSGSIZE` before `BUG_ON`.
* No LPE signal. No UAF, overwrite, type confusion, stale callback, refcount abuse, or attacker-controlled reclaim is shown.
* No remote/network-triggerable signal. This is OVS control plane flow update handling, not ordinary packet forwarding.

## 3. Reachability analysis

The bug is triggered by creating or updating Open vSwitch flows with mismatched UFID and key identifiers so that the request-sized reply skb is too small for the actual matched flow identifier. In normal deployments this requires access to the OVS control plane, typically administrative OVS privileges or `CAP_NET_ADMIN`. It is not reachable by sending normal network packets through the datapath. Namespaces or containers may reduce the practical barrier if OVS or network administration is delegated, but that is deployment-specific.

Call-site confidence: high. The relevant allocation, size comparison, update path, and `BUG_ON` consequence are described in the commit and visible in the provided `ovs_flow_cmd_new()` patch context.

## 4. Severity interpretation

This behaves like an ordinary Moderate DoS issue rather than an Important-class vulnerability. The realistic impact is a kernel crash or panic from a control-plane operation. The patch does not support confidentiality impact, integrity impact, privilege escalation, or memory corruption beyond a failed skb fill followed by `BUG_ON`. The paranoid score remains below Actionable Moderate because even with delegated OVS administration, the primitive is still DoS-only.

## 5. One-sentence report phrase

Open vSwitch may preallocate an undersized update reply skb when a short UFID request resolves to a key identified flow, causing `ovs_flow_cmd_fill_info()` to fail with `EMSGSIZE` and hit `BUG_ON`, resulting in a local control-plane DoS.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed for urgent triage in typical deployments because the issue is DoS-only, requires OVS control-plane privileges, is not packet-path remote reachable, and shows no memory corruption or privilege escalation primitive.

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

Patch: net: openvswitch: reallocate update replies for mismatched IDs
Commit: bd8ca84d48cd9a4f6fc63df26512c55e1d339927
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bd8ca84d48cd9a4f6fc63df26512c55e1d339927

Commit description:

ovs_flow_cmd_new() preallocates the optional reply skb before it takes
ovs_mutex and before it knows which existing flow will be updated.

That is normally fine because the skb is sized from the request flow
identifier.  That identifier also becomes the inserted flow's identifier.
For updates, however, a request with a UFID may miss the UFID lookup and
then fall back to the flow key lookup.  That lookup can legitimately find
an existing key-identified flow.  UFIDs are optional and the flow key is
the primary identifier.

For echoed replies, ovs_flow_cmd_fill_info() writes the matched flow's
identifier, not the request identifier used for the preallocation.  A short
request UFID can therefore leave too little room for the key identifier.
The fill can then fail with -EMSGSIZE and hit the BUG_ON(error < 0) in the
update path.

Once the update target has been resolved, reallocate the reply skb if the
matched flow needs a larger reply than the request identifier allowed.  Do
this before replacing the actions so the request can still fail cleanly if
the rare extra allocation fails.

Fixes: 74ed7ab ("openvswitch: Add support for unique flow IDs.")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reviewed-by: Ilya Maximets <i.maximets@ovn.org>
Link: https://patch.msgid.link/f7bbd3c30ce81a39156e226b3872d73abed21d2f.1785644623.git.zhilinz@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/openvswitch/datapath.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=bd8ca84d48cd9a4f6fc63df26512c55e1d339927

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

The original Linux kernel CVE announcement for CVE-2026-74664 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74664

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 0
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).