CVE-2025-37967: Linux UCSI DisplayPort Deadlock Patch and Azure Linux

The Linux kernel patch tracked as CVE-2025-37967 fixes a subtle but real deadlock in the USB Type‑C UCSI DisplayPort code — and while Microsoft’s public advisory correctly identifies Azure Linux as a Microsoft product that “includes this open‑source library and is therefore potentially affected,” that attestation is not a technical guarantee that Azure Linux is the only Microsoft artifact that might carry the vulnerable code.

Background / Overview​

CVE-2025-37967 was assigned to a Linux kernel change addressing a deadlock in the UCSI (USB Type‑C Connector System Software Interface) driver’s DisplayPort alternate‑mode handling. The patch introduces two new helpers, ucsi_con_mutex_lock and ucsi_con_mutex_unlock, and changes the driver so the connector mutex is only taken when a connection is actually established and the partner pointer is valid. The original bug allowed a circular locking/wait scenario in which the function that removes a DisplayPort partner held the connector mutex while waiting for deferred work (dp_altmode_work) to finish, while dp_altmode_work itself attempted to acquire the same mutex — producing a deadlock that can hang USB‑C DisplayPort operations and, in the worst case, make the host unresponsive for that subsystem.
This is an availability/robustness class defect rather than a confidentiality or integrity compromise: it is a local, kernel‑level deadlock that can be triggered by interacting with USB‑C/DisplayPort functionality. The practical impact reported by distributors and security trackers ranges from system hangs of the affected driver path to denial‑of‑service against processes that depend on that functionality. Distributors and vulnerability databases that track the issue classify the risk as medium and recommend installing the upstream fix when available in distribution kernels.

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What Microsoft actually said — and what that wording means​

Microsoft’s Security Response Center (MSRC) entries for a number of Linux kernel CVEs (including other kernel fixes) include a short FAQ line used repeatedly across advisories: “Is Azure Linux the only Microsoft product that includes this open‑source library and is therefore potentially affected?” The answer given in those entries emphasizes that Microsoft has attested Azure Linux as including the upstream component and that Microsoft began publishing machine‑readable CSAF/VEX attestations in October 2025 to improve transparency; it also promises to update the CVE if impact to additional Microsoft products is identified. That phrasing is deliberate and operational: it confirms Azure Linux as a known carrier, and it commits Microsoft to expand attestations as inventory work completes. (msrc.microsoft.com)
It is important to read that statement narrowly: it is an inventory attestation (Microsoft has checked and found the code in Azure Linux) and a transparency commitment (Microsoft will update records if it finds additional affected products). It is not a blanket engineering assertion that no other Microsoft artifact contains the same upstream source code. In short: Microsoft has confirmed Azure Linux; it has not — by that statement alone — established exclusivity.

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Why Azure Linux being named does not prove exclusivity​

To understand why an attestation that "Azure Linux includes the library" is not equivalent to "only Azure Linux includes it," you need to appreciate how Linux kernel artifacts are produced and distributed:

• The Linux kernel is built per‑artifact with configuration options (CONFIG_* flags). Whether a specific driver or code path is compiled in depends on those build options and the kernel version/commit used.
• Microsoft distributes multiple Linux kernel artifacts and images: the linux‑azure kernels used for many Azure VM images, kernels embedded in Marketplace images, the WSL2 kernel shipped with Windows, container/AKS node images, guest images used by managed services, and custom kernel builds for partner appliances.
• Any of those artifacts could ship the same upstream file (or an affected commit range) if they were built from a kernel tree that included the relevant UCSI/DisplayPort code and did not have the vulnerable commit removed or backported. That means the presence or absence of the vulnerability is an artifact property — not a simple product family property.

WindowsForum community analysis and multiple public advisories from vendors and distribution trackers have emphasized this distinction repeatedly: a named attestation is a good starting point for remediation, but it is not a substitute for artifact‑level verification across a vendor’s entire engineering output. Microsoft’s own wording — “If impact to additional products is identified, we will update the CVE to reflect this” — implicitly acknowledges that their initial attestation is scoped and subject to expansion after inventorying other artifacts.

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Independent verification of the technical fix and affected scope​

Multiple independent vulnerability trackers and distribution security pages document CVE‑2025‑37967 and the underlying kernel fix. Key, high‑confidence references include:

• The NIST/NVD entry which summarizes the fix and the deadlock semantics.
• Distribution/security trackers (Debian Security Tracker, Tenable plugin notes), which record the CVE and list the affected kernels and distribution packages (and in some cases the exact stable commits backported or patched). These vendor trackers also tie to upstream kernel commits that implement the fix.
• CERT/third‑party trackers that reference upstream kernel commits and provide links to stable kernel patches — those commit references are the most authoritative machine‑readable proof of exactly what changed in the kernel. The upstream commit metadata shows the specific functions added (ucsi_con_mutex_lock / ucsi_con_mutex_unlock) and the code paths adjusted.

When distributing the fix in packages, maintainers may backport the change into older kernel branches or include it in the next point release; that is how downstream distribution advisories end up showing specific patched package versions. In short: the technical fix is well documented upstream and mirrored by multiple independent trackers and distributors.

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How to determine whether your systems are affected (operational checklist)​

• Identify the kernel binary and version(s) you run.
• Check uname -a and the precise package version if you run a distribution kernel. Many advisories map CVE fixes to package versions (Debian, RHEL, Ubuntu, SUSE, Azure Linux kernels).
• If you run a Microsoft‑supplied kernel (e.g., linux‑azure on an Azure VM, WSL2 kernel, or an Azure Marketplace image), treat those kernels as candidate artifacts to verify.
• Confirm whether the UCSI DisplayPort code is present in your kernel build.
• Check whether the ucsi and ucsi_displayport modules are present: lsmod | grep ucsi or find /lib/modules/$(uname -r) -name 'ucsi'.
• If your running kernel exposes /proc/config.gz, use zcat /proc/config.gz | grep -i UCSI to see if the build enabled the relevant code path. If /proc/config.gz is not present, refer to the distribution kernel config or package changelog. (Note: the presence of a module does not prove the exact commit; it only shows the code path is present.)
• Match your kernel commit/version to upstream CVE tracking.
• Use distribution advisories (Debian, Tenable, vendor security trackers) to see whether your packaged kernel version is listed as patched or vulnerable.
• If you built your own kernel or run a third‑party kernel, compare your commit ID to the upstream commits referenced in CVE trackers (the kernel stable commits that include the ucsi_con_mutex_lock fix). If your commit predates the fix, you are vulnerable until you rebuild or upgrade.
• If you identify vulnerable kernels, apply vendor updates or upstream patches.
• Follow your distribution’s recommended package updates. If you run Azure Linux, apply the Microsoft‑published kernel updates for the Azure Linux variant. Microsoft has published CSAF/VEX attestations for Azure Linux; consult those attestations to confirm the patched product artifact. (msrc.microsoft.com)
• Temporary mitigations and operational cautions.
• Some operators will consider blacklisting or unloading affected kernel modules as a stopgap. That can reduce exposure to the buggy code path, but it will likely disable USB‑C DisplayPort functionality for affected ports and may impact docks or HCI devices. Test in a lab before applying such mitigations broadly. Where possible, prefer applying a patched kernel rather than removing support for whole classes of hardware.

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Is Azure Linux the only Microsoft product that could be affected? (short technical answer)​

No — not necessarily. Azure Linux is the Microsoft product Microsoft has publicly attested to include the implicated upstream component for this CVE, but that attestation is scoped and is not equivalent to a complete, exhaustive scan of every Microsoft‑published kernel artifact. Other Microsoft kernels or images (linux‑azure builds for VM SKUs, kernels embedded in Marketplace images, WSL2 kernel binaries, AKS/VM scale set node images, vendor partner appliances supplied by Microsoft, etc.) could include the same vulnerable upstream file depending on kernel version and build configuration. Microsoft says it will update the CVE/VEX data if it identifies additional impacted products; until then, artifact‑level verification is the only reliable way to prove absence.

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Critical analysis — strengths and risks of Microsoft’s approach​

Strengths​

• Transparency by design: Microsoft naming Azure Linux and publishing CSAF/VEX attestations is a material improvement in supply‑chain hygiene; it gives customers a concrete starting point for remediation and enables automated tooling to consume Microsoft’s attestations. That is a clear, measurable win for customers who rely on Azure Linux images. (msrc.microsoft.com)
• Focused remediation: By confirming a product that is widely used (Azure Linux) and committing to patch it, Microsoft reduces risk for a large class of cloud customers quickly, which is especially valuable for managed infrastructure and VM images.

Risks and limitations​

• Partial attestation window: Publishing attestations for a single product family creates a temporal window where other Microsoft artifacts remain unverified. Customers who assume a named attestation implies exclusivity could under‑inventory their exposure and miss vulnerable images (e.g., Marketplace VMs or packaged appliances). Microsoft’s promise to update CVEs if additional products are found is necessary but reactive; it does not remove the need for customers to verify artifact inventories proactively.
• Artifact complexity: Linux kernel inclusion is conditional on both upstream commit ranges and CONFIG_* settings. Two Microsoft kernels with the same version number might differ in enabled drivers and therefore differ in exposure. This heterogeneity complicates automatic assumptions about risk across a vendor’s product lineup.
• Operational tradeoffs with mitigations: Kernel module unloading or driver blacklisting can reduce exposure but often disables hardware features, which is an unacceptable trade for many production workloads. That forces operators to choose between availability of hardware features and applying a full kernel update.

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Practical guidance for Microsoft customers and cloud operators​

• Treat the MSRC attestation for Azure Linux as a verified, high‑confidence indicator for that product family — and as a prompt to conduct a broader inventory of Microsoft‑supplied kernel artifacts in your estate. Specifically:
• Inventory all VM images, Marketplace images, WSL distributions, AKS node images, and vendor appliances you run that were produced or packaged by Microsoft.
• For each artifact, collect kernel version, kernel package metadata, and (where possible) the actual kernel config/commit ID. If you cannot obtain kernel config, consult vendor package changelogs or the artifact manifest.
• Use distribution vendor advisories and CVE trackers as the canonical sources for whether a given packaged kernel has been patched; for custom kernels, compare commit IDs to the upstream kernel patch references. Multiple trackers (NVD, Debian, Tenable, distribution security trackers) track CVE‑2025‑37967 and reference stable kernel commits — use those to map your artifact.
• Where Microsoft is the vendor of your image:
• Check the Azure Linux (or other Microsoft image) VEX/CSAF data the vendor publishes to see whether the specific artifact (for example, a particular linux‑azure kernel version) is listed as affected or patched. Microsoft has started publishing this machine‑readable inventory; use it where available. (msrc.microsoft.com)
• If you cannot find coverage for a Microsoft artifact you rely on, open a support case with Microsoft and request artifact‑level verification or the relevant kernel commit IDs. Don’t assume absence of a public attestation equals absence of vulnerable code.
• For immediate operational risk reduction:
• Prioritize patching Azure Linux instances (if you use that distro) because Microsoft has explicitly attested they include the upstream component and will publish fixes for that product first.
• Test upgrades in a staging environment to validate USB‑C docking, external display behavior, and hardware compatibility before rolling to production, because kernel updates can affect hardware subsystems.

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What vendors and platform teams should do differently going forward​

• Vendors that publish attestations should accelerate artifact‑by‑artifact coverage of their product catalog. A single product attestation is helpful but insufficient; customers need a persistent machine‑readable map of which kernel artifacts include which upstream components.
• Microsoft’s move to publish CSAF/VEX attestations is the right direction. To close the transparency gap, vendors should:
• Publish the exact kernel commit IDs and enabled CONFIG_* options for each kernel artifact.
• Provide a cross‑reference showing which CVEs map to which artifacts and which artifacts are patched or backported — ideally in continuous, machine‑consumable form.

These measures make it possible for enterprise defenders to write deterministic rules that check artifact metadata against a vulnerability inventory, reducing the human guesswork that currently drives uncertainty.

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Conclusion​

CVE‑2025‑37967 is a well‑documented Linux kernel deadlock fix in the UCSI DisplayPort code. Microsoft’s public advisory is useful — it confirms that Azure Linux includes the affected component and that Microsoft will publish VEX/CSAF attestations and update CVE mappings as it completes additional inventory work — but that confirmation is scoped, not exhaustive. Customers should treat Azure Linux’s attestation as a high‑priority remediation target while performing artifact‑level verification across all Microsoft‑supplied kernels and images in their estate. Use kernel version/commit checks, distribution advisories, and published VEX/CSAF data as the authoritative mapping between CVE and artifact, and when in doubt, escalate to vendor support for explicit artifact verification.

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Practical checklist (recap)

• Inventory: list every Microsoft‑supplied kernel artifact in use.
• Detect: check module presence (ucsi, ucsi_displayport) and kernel config/commit.
• Map: compare kernel versions/commits to upstream CVE patch commits documented by distributors.
• Remediate: apply vendor‑provided kernel updates or backports; test hardware functions before broad rollout.
• Verify: consume vendor CSAF/VEX attestations where available and request artifact information for any images not covered.

If your environment relies on Azure Linux images, patch those images promptly and follow Microsoft’s published guidance; if you run other Microsoft kernels, do not assume they are unaffected until you have verified artifact‑level evidence.

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Source: MSRC Security Update Guide - Microsoft Security Response Center