CVE-2025-38331 Cortina Driver Fix and Azure Linux Attestation Risks

A kernel-level fix for the Cortina Ethernet driver — tracked as CVE-2025-38331 — patched a network driver behavior that could destabilize systems by mishandling TCP offload (TOE/TSO) paths, and while Microsoft has publicly attested that Azure Linux includes the upstream component and is therefore potentially affected, that attestation is a scoped inventory statement rather than proof that no other Microsoft product contains the same vulnerable code.

Background / Overview​

The vulnerability identified as CVE-2025-38331 was introduced in upstream Linux kernel networking code for the Cortina (gemini) Ethernet driver. The issue arises from the interaction between the TCP Offload Engine (TOE) and TCP Segmentation Offload (TSO) support: the driver historically assumed the hardware offload could be used for all TCP frames but did not consistently enable the coupled TOE and TSO features for non-segmented TCP frames, which could lead to hardware instability, lockups, and kernel crashes under load. Upstream trackers and distribution advisories describe the fix as enabling/ensuring TOE/TSO handling for all TCP frames handled by that driver in the affected kernel ranges. Multiple independent sources confirm:

• The issue is a Linux kernel driver defect (drivers/net/ethernet/cortina/gemini.c).
• The practical impact is availability (system lockup / kernel crash), observable after minutes-to-hours depending on network load in test harnesses.

Microsoft’s public advisory for the CVE (MSRC) states that Azure Linux is a Microsoft-maintained product that includes the implicated upstream library and is therefore potentially affected, and Microsoft pledged to update the CVE/VEX/CSAF mapping if additional Microsoft products are discovered to ship the same upstream component. That phrasing is important to parse: it is a product‑scoped attestation — authoritative for the named product(s) — but not an exclusionary claim about every Microsoft artifact.

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What CVE‑2025‑38331 actually is (technical summary)​

The bug, in plain terms​

• Component: Linux kernel Cortina Ethernet driver (gemini.c).
• Fault: TOE and TSO features were not consistently activated/handled for all TCP frames; the driver passed skb->len to the TOE/TSO offloader in a way that assumed the hardware would accept non-segmented frames without enabling the TOE behavior, causing unanticipated hardware states.
• Primary impact: reliability/availability — driver instability that can produce a kernel panic, oops, or prolonged lockups under realistic network load.

Upstream remediation​

Upstream Linux maintainers merged targeted fixes into the stable kernel trees to ensure the offload quirk is correct: the driver now ensures TOE and TSO semantics are consistent for all TCP frames, preventing the hardware from entering unstable states. Multiple vendor trackers (Ubuntu, Debian, SUSE) and vulnerability databases ingested the commit references and mapped them into distribution kernel packages.

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Why Microsoft’s Azure Linux attestation matters — and what it does and doesn’t mean​

Microsoft’s MSRC entry and accompanying machine-readable VEX/CSAF attestation say, in effect, “Azure Linux includes the implicated open-source library and is therefore potentially affected.” That message serves three operational purposes:

• It tells Azure Linux operators: your Microsoft‑supplied image family contains this upstream kernel component, so you should prioritize verification and patching according to Microsoft guidance.
• It provides automation-friendly metadata (VEX/CSAF) that security tooling can ingest to drive patch orchestration and compliance checks.
• It signals Microsoft’s process — MSRC began publishing CSAF/VEX artifacts in a phased rollout (starting in October 2025) and will expand product mappings over time; the attestation therefore represents the inventory Microsoft has completed to date, not an exhaustive inventory of every Microsoft-produced artifact.

What it does not mean

• The Azure Linux attestation is not a categorical statement that no other Microsoft product includes the same upstream source file or driver. Because Microsoft ships multiple Linux-derived artifacts (CBL‑Mariner lineage kernels, linux‑azure kernel builds, WSL2 kernels, Marketplace images, managed node images, etc., the presence of the upstream file in those artifacts is an artifact‑by‑artifact question — dependent on kernel version, build-time CONFIG_* settings, and packaging choices. Absence of a VEX entry for a given Microsoft product is absence of attestation, not proof of absence.

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Are other Microsoft products plausibly affected?​

Short answer: yes, other Microsoft products and artifacts are plausible carriers. You should not treat the Azure Linux attestation as meaning “only Azure Linux could be affected.”
Why the plausibility is real

• Microsoft builds and publishes multiple Linux kernel artifacts: Azure Linux (the attested family), linux‑azure kernel images used by some Azure VM SKUs, CBL‑Mariner (the common base lineage Microsoft uses for many internal images), and the WSL2 kernel distributed to Windows users. Any of these artifacts that were built from an upstream commit range prior to the fix and that were compiled with the Cortina driver included could be affected. Distribution trackers and some vulnerability databases have already mapped this CVE to linux-azure packages and to the generic Linux kernel trees.

Evidence seen in public trackers

• Distribution advisories (Ubuntu’s linux‑azure packages) and public vulnerability feeds list linux‑azure package updates that include fixed kernel builds for Azure-optimized kernels. That mapping demonstrates the practical path by which the vulnerable upstream file enters a Microsoft-supplied image family.
• Independent vulnerability databases list the vulnerable program file as drivers/net/ethernet/cortina/gemini.c and map affected kernel version ranges; that same program file can appear in any kernel binary built from the vulnerable upstream commit range.

What Microsoft’s VEX/CSAF rollout shows in practice

• Microsoft’s initial VEX/CSAF payloads concentrated on Azure Linux artifacts; subsequent attestation cycles for other CVEs have shown the company adding product entries (for example, CBL‑Mariner or other kernel artifacts) as inventory expanded. That operational pattern explains why a single product attestation exists first for Azure Linux while other Microsoft artifacts remain “to be attested.”

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Practical guidance for administrators and security teams​

Treat Microsoft’s attestation for Azure Linux as authoritative for those images — that is, patch Azure Linux images promptly — but immediately follow a verification workflow for other Microsoft-supplied artifacts in your estate.
Action checklist (prioritized)

• Apply vendor/OS kernel updates on Azure Linux hosts and images that Microsoft identifies in its advisory. This is the highest‑priority mitigation because Microsoft has explicitly attested these artifacts as including the component.
• Inventory Microsoft-supplied Linux artifacts in your environment:
• Azure VM images: list image SKUs and kernel package versions.
• AKS node pools: record underlying node image/kernel versions.
• WSL2 hosts: check the WSL kernel version (uname -r inside WSL).
• Any Microsoft-supplied appliances or container host images (CBL‑Mariner lineage).
• For each suspect artifact, verify the kernel version and configuration:
• uname -a
• If available: grep CONFIG_* in /boot/config-$(uname -r) or /proc/config.gz for the relevant driver or related CONFIG options.
• Check whether the Cortina driver is present as builtin or module (lsmod; modinfo). If the driver is not included or compiled out, the artifact is not exposed even if the kernel version is in the vulnerable range.
• Map running kernels to distribution/vendor advisories:
• Consult your distro’s security tracker (Ubuntu USNs, Debian tracker, vendor advisories) or the public CVE commit references to determine whether your kernel's package contains the upstream fix or a vendor backport.
• Automate detection:
• Ingest Microsoft’s CSAF/VEX outputs and your distro advisory feeds into your vulnerability management platform to get machine-readable signals when Microsoft updates product mappings.
• If immediate kernel updates cannot be applied:
• Isolate the host from high‑bandwidth/hostile network flows that exercise offload paths.
• Reduce MTU or disable offload features temporarily (ethtool --offload) for interfaces where the driver is present — note: disabling offload options may impact performance and must be tested. (Disabling offload is a stopgap — the correct fix is a kernel update.
• Maintain evidence and audit trail:
• Record the kernel package version, upstream commit IDs (when available), and remediation steps for compliance and incident response.

Why per-artifact verification matters

• Kernel exposure is a function of both which upstream commits were used and how the kernel was built (CONFIG flags, modules). Two different Microsoft artifacts built from the same upstream version can differ in included drivers and thus differ in exposure — which is why Azure Linux being attested says nothing definitive about CBL‑Mariner, WSL2, linux‑azure, or other images.

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Risk analysis — strengths and residual risks of Microsoft’s approach​

Strengths​

• Microsoft’s early attestation of Azure Linux provides a concrete, actionable signal for cloud customers who run Microsoft-supplied images. VEX/CSAF machine-readable output is a practical step forward for automation-driven vulnerability management.
• Patching is straightforward for Azure Linux operators: Microsoft supplies the fixed kernel packages and guidance for upgrading those images. Distribution vendors (Ubuntu, Debian) and other cloud providers simultaneously issued fixes and advisories, creating independent cross‑checks.

Residual risks and limitations​

• Inventory completeness: Microsoft’s VEX approach is iterative; there is a window during which only a subset of Microsoft products have been attested. That leaves un‑attested artifacts in a state of unknown, which is operationally risky for organizations that run heterogeneous Microsoft artifacts (WSL2, marketplace images, CBL‑Mariner hosts).
• Artifact-specific build variance: Because kernel exposures depend on build-time configuration, a single binary-level attestation is insufficient for broad conclusions. Operators must verify each unique image or appliance in their environment.
• Detection complexity: Public scanners and corporate asset inventories may flag the CVE against any Linux kernel binary in the vulnerable upstream range, but determining whether the problematic driver is present requires inspecting the specific kernel build and configuration.

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Short technical checklist to verify a Microsoft-supplied artifact​

• Identify the artifact name and kernel package:
• For Azure VM: image SKU, kernel package (dpkg -l | grep linux-image or rpm -qa | grep kernel).
• On the running system, confirm kernel version:
• uname -r and check package changelog for backports.
• Confirm driver presence:
• lsmod | grep gemini or grep -i cortina /lib/modules/$(uname -r) -R.
• Check kernel config:
• zcat /proc/config.gz | grep -i CORTINA || grep -i 'CONFIG_' /boot/config-$(uname -r).
• If present and vulnerable: apply the vendor-provided patched kernel and reboot.

These are practical, deterministic checks that remove guesswork and avoid relying solely on a product-level VEX statement.

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Conclusion​

Microsoft’s statement that “Azure Linux includes this open-source library and is therefore potentially affected” is accurate and important: it is a product‑scoped, machine‑readable attestation that Azure Linux images include the upstream Cortina ethernet driver code paths referenced by CVE‑2025‑38331, and Azure Linux operators should act accordingly.
However, the attestation should not be read as an exclusive claim that Azure Linux is the only Microsoft product that could carry the vulnerable code. Other Microsoft artifacts that ship Linux kernels — including CBL‑Mariner derivatives, linux‑azure kernel builds, WSL2 kernels, and various Marketplace or appliance images — are plausible carriers depending on build configuration and kernel version. Operators must therefore:

• Treat Microsoft’s Azure Linux attestation as an urgent call to patch for that family, and
• Perform artifact‑level verification for other Microsoft-supplied kernels in their environment using the practical checks listed above.

The combination of upstream kernel commits, distribution advisories, and Microsoft’s evolving VEX/CSAF attestations provide the necessary signals to triage and remediate. Integrating those signals into automated inventory and patching pipelines is the most reliable way to close the verification gap that a single-product attestation inherently leaves open.

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Appendix — Key public references (for verification)

• Upstream CVE/technical details: Linux kernel stable commit list and NVD entries for CVE‑2025‑38331.
• Distribution advisories mapping the fix into linux‑azure and vendor kernels: Ubuntu USNs and Debian/OSV trackers.
• Microsoft’s machine‑readable VEX/CSAF rollout and product attestation for Azure Linux: MSRC blog and VEX metadata (MSRC published CSAF/VEX starting Oct 2025) and Microsoft's statement that they will update CVE mappings if more products are identified.

(Operational note: follow the specific package versions and fixed-kernel advisories published by your distribution or by Microsoft for the exact upgrade packages and reboot procedures.

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Source: MSRC Security Update Guide - Microsoft Security Response Center