CVE-2025-38244: Azure Linux Attestation and SMB Deadlock Patch Reality

The Linux kernel vulnerability tracked as CVE-2025-38244 — described upstream as “smb: client: fix potential deadlock when reconnecting channels” — is a clear reminder that modern vendor transparency programs are useful but incomplete: Microsoft has attested that the Azure Linux distribution includes the implicated open‑source component and is therefore “potentially affected,” but that attestation should not be read as a universal guarantee that no other Microsoft product carries the same code. Security teams must treat Microsoft’s Azure Linux attestation as authoritative for Azure Linux, while still performing targeted inventory and patching across other Microsoft‑provided artifacts (for example, WSL kernels, Azure images, and appliance images) until attestations or internal scans prove otherwise.

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Background / Overview​

CVE-2025-38244 surfaced in mid‑2025 as a defect in the Linux kernel’s SMB client implementation (the CIFS/SMB client code). The flaw is a classic concurrency problem: an incorrect lock order inside the reconnect path could produce a circular locking dependency and a potential deadlock when channels are being re‑established. The immediate impact is availability loss for workflows that rely on the kernel SMB client — in some exploit scenarios that can cause hangs, stalled I/O, or service disruption. Upstream kernel maintainers applied a fix to correct the lock ordering and eliminate the circular dependency.
Put simply: the bug is not a remote code execution exploit by design, but it is serious in practice because blocked or deadlocked kernel paths in storage and network stacks can bring down services, containers, or even entire virtual machines that rely on kernel SMB mounts. Multiple Linux distributors tracked and shipped fixes or backports, and the incident highlighted differences in how vendors map open‑source CVEs onto their product lineups — and how they communicate those mappings to customers.

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What the bug actually is (technical summary)​

The deadlock: lock order and cifs_signal_cifsd_for_reconnect​

The vulnerability arises from an incorrect ordering of internal locks in the reconnect logic for the kernel SMB client. The function implicated is cifs_signal_cifsd_for_reconnect (the reconnect helper path) and the deadlock manifests when two locks — for example the session‑level server lock (tcp_ses->srv_lock) and a channel buffer lock (ret_buf->chan_lock) — are acquired in a conflicting order, producing a circular dependency in certain interleavings. Diagnostic kernel warnings captured during testing showed explicit circular locking dependencies and stack traces that point to the reconnect path. The fix applied upstream reorders lock acquisition and otherwise ensures the reconnect path follows a consistent locking discipline to eliminate the circular dependency.

A note on severity and CVSS​

Vendors and aggregators assigned different severity/score values reflecting slightly different risk models (local vs. network attack surface, required privileges, and the real‑world impact of a kernel deadlock). For example, some distro advisories and mirrors labeled the issue Important and published CVSSv3 scores in the mid‑range; other databases reported scores that vary depending on whether the attacker needs local access or privileged capabilities to trigger the condition. In operational terms, treat this as an availability risk with high operational impact for workloads that rely on kernel‑level SMB mounts or on systems where the module is loaded.

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Where this code lives — why Azure Linux was attested​

The Linux SMB client (commonly called the cifs module) is a standard part of the upstream Linux kernel and has been included for many years as the kernel VFS client for SMB2/SMB3 (and for legacy CIFS where required). Because it is part of the kernel tree, any vendor kernel build that includes the affected commit range can carry the vulnerability. Microsoft’s Azure Linux distribution is a published kernel/distribution family that Microsoft builds and maintains; MSRC’s product mapping found the implicated upstream component inside Azure Linux builds and published an attestation to that effect. Microsoft’s broader transparency program now uses machine‑readable CSAF/VEX attestations to make those product‑level mappings consumable by enterprise tooling.
Why did Microsoft name Azure Linux first? Azure Linux (the CBL‑Mariner lineage and successor artifacts) is the Microsoft‑published Linux distribution, and it’s the natural starting point for automated inventory and VEX attestations. Microsoft’s published wording — that “Azure Linux includes this open‑source library and is therefore potentially affected” — is literally a product‑level inventory mapping. It means Microsoft has confirmed Azure Linux artifacts contain the upstream component; it is not a comprehensive survey of every Microsoft artifact or kernel binary. Microsoft has explicitly said it will update the CVE/VEX records if additional Microsoft products are identified during inventory.

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The central question: Is Azure Linux the only Microsoft product that includes the vulnerable library?​

Short answer: No — not necessarily. Long answer: Azure Linux is the only Microsoft product Microsoft has publicly attested (so far) to include the upstream SMB client code implicated in CVE‑2025‑38244. That attestation is authoritative for Azure Linux images and kernels — you must treat those as in‑scope and apply vendor updates — but the attestation is product‑scoped and phased. The absence of a public attestation for other Microsoft products is not proof that those products are safe. It merely means Microsoft has not yet published an attestation for them. In practice, other Microsoft artifacts — most notably the WSL2 kernel builds and any Microsoft‑published kernel images used inside Azure offerings or appliance images — are plausible carriers of the same kernel subsystem unless specifically attested as Not Affected.
Why this distinction matters: several different Microsoft artifacts can include or ship Linux kernel code:

• Microsoft’s own WSL2 kernel trees (the microsoft/WSL2-Linux-Kernel repository) are published and used to build the WSL2 runtime kernel. That kernel tree contains many upstream subsystems, including network and filesystem drivers. Whether a specific WSL2 kernel build is vulnerable depends on the version and applied backports.
• Azure virtual machine images and marketplace images may include vendor kernels (for example, Azure‑supplied Linux kernels in certain image families) or distribution kernels that Microsoft builds or maintains. Those kernels may or may not match the exact Azure Linux attested build.
• Container base images, virtual appliance images, edge devices, and other Microsoft artifacts can embed or bundle open‑source code in many ways (statically linked agents, vendor kernel modules, or separate kernel builds).

Microsoft’s VEX/CSAF rollout is intentionally machine‑readable and incremental: the company maps each downstream product family as the internal inventory completes. Their public statement that they will update the CVE if other products are impacted is part of that phased approach. Until a product is attested either as Known Affected or Not Affected, customers should not assume absence of impact.

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Cross‑checking the evidence (how we verified the mapping)​

To answer whether Azure Linux is unique we cross‑checked multiple independent sources:

• Upstream advisories, vulnerability databases, and kernel security trackers show the technical fix and the affected commit range for the kernel SMB client. Those sources reproduce the exact stack traces and the identified function path where the wrong lock order occurred.
• Major distro and cloud vendor security notices show how the issue mapped across vendor kernels (for example, Amazon Linux ALAS entries, Ubuntu USN entries, and SUSE advisories). Those advisories demonstrate that multiple kernel families were checked and patched by various vendors — not exclusively Microsoft.
• Microsoft’s public transparency blog and several MSRC CVE pages (and their FAQ text) demonstrate Microsoft’s new CSAF/VEX attestation behavior and the exact phrasing used when Azure Linux is identified as a carrier. The identical FAQ wording appears on multiple MSRC CVE pages, indicating a standard template and process for product‑level attestations. That wording includes the phrase that Microsoft will update the CVE if additional products are identified.

Taken together, the independent evidence supports this reading: Microsoft has done the inventory work and attested Azure Linux; other vendors and upstream trackers show the bug and fixes; but the presence of the vulnerable code in other Microsoft artifacts remains a matter for targeted inventory or Microsoft’s future attestations.

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Practical implications for organizations and administrators​

If your environment uses Azure Linux images, the answer is clear: apply the Microsoft/maintainer kernel updates as soon as possible.
If your environment runs other Microsoft artifacts, take these practical steps:

• Treat Microsoft’s Azure Linux attestation as definitive for Azure Linux kernels — patch those immediately.
• Inventory Microsoft‑provided artifacts in your estate that can carry kernels or kernel modules:
• WSL2 instances and their kernel versions
• Azure VM images that use Microsoft‑supplied kernels
• Azure Marketplace images and appliance images
• Any Windows workflows that embed Linux kernels (containers, distros distributed by Microsoft)
• For each artifact, determine the kernel version and the presence of the CIFS/SMB client module. If the kernel falls into the affected upstream commit range or if the module is present and loaded, schedule patching or remediation.
• If you cannot patch immediately, consider temporary mitigations (unload or blacklist the cifs module where feasible; avoid mounting SMB shares from untrusted systems; restrict access to users who can trigger reconnect operations). Be aware that forcibly removing a kernel module on a live system that is using it can cause service disruption — plan carefully.

These steps are operationally simple but require accurate artifact inventory. That is precisely the gap Microsoft’s CSAF/VEX program is intended to shrink: VEX makes it easier for automation to decide whether a particular product image is in‑scope. But until every Microsoft artifact you use is attested, you must verify locally.

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Detection checklist — commands and signals to look for​

Below are actionable checks any Linux or hybrid administrator can run to determine exposure quickly.

• Check kernel version:
• uname -r
• Compare the running kernel to vendor advisories and the upstream affected commit ranges.
• Check whether the CIFS/SMB client module is present or loaded:
• lsmod | grep cifs
• modinfo cifs (shows module version and build info)
• Inspect kernel configuration for cifs support:
• grep CONFIG_CIFS /boot/config-$(uname -r) (or zgrep CONFIG_CIFS /proc/config.gz)
• Search for SMB mounts:
• mount | grep -i cifs
• cat /proc/mounts | grep -i // (mounts using SMB will show //server/share)
• Look for kernel oops or locking warnings in logs:
• journalctl -k | grep -i -E "cifs|smb|locking|deadlock|circular locking"
• dmesg | grep -i cifs

If any of the above show the module present, SMB mounts in use, or kernel warnings referencing the reconnect path and lock names, treat the host as potentially exposed and schedule an update or reboot into a patched kernel. The only reliable long‑term fix is to apply the upstream or downstream vendor patch that corrects the lock ordering.

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Mitigation and patching guidance​

• Patch first: Prioritize installing the vendor patch or upstream kernel version that contains the fix. For Azure Linux customers, apply Microsoft’s Azure Linux kernel update per normal patch management policies. Other distribution vendors (Ubuntu, SUSE, Red Hat, Amazon Linux) also released fixes and advisories; align with those advisories for non‑Microsoft kernels.
• If you cannot patch immediately:
• Unmount SMB shares gracefully and avoid re‑establishing connections while the host remains unpatched.
• Where the module is not required, consider blacklisting the cifs module (echo "blacklist cifs" > /etc/modprobe.d/blacklist-cifs.conf) and then rebooting; only do this where safe and compatible with your workload.
• For WSL2 instances, update the WSL kernel package or the WSL runtime if a patched kernel is published by Microsoft. Use the WSL update mechanism documented by Microsoft to ensure the right kernel is installed.
• Detection and logging: Increase logging and alerting for kernel locking warnings, module load/unload events, and SMB mount operations. This improves early detection of attempted exploitation or accidental triggering of the deadlock path.

Remember: kernel module removal and blacklisting are temporary mitigation tactics and can break legitimate operations. Patching remains the only durable resolution.

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Why Microsoft’s VEX/CSAF attestation matters — and where it falls short​

Microsoft’s decision to publish CSAF/VEX attestations for Azure Linux is an important step for enterprise automation. Machine‑readable attestations allow security teams and vendors to programmatically triage and de‑noise CVE impact for specific products. That reduces false positives and helps prioritize remediation where the vendor has confirmed the component exists. The October 2025 rollout of Microsoft’s VEX attestations formalized that process and established a predictable product‑by‑product mapping approach.
However, the attestation approach has operational limits:

• Phase‑by‑phase inventory: vendors typically attest product families in waves. The fact that Azure Linux is attested first reflects the practicalities of inventory — not an architectural claim that no other Microsoft product could carry the same code.
• Absence of an attestation ≠ proof of safety: until a product is actively attested as Not Affected or is demonstrated locally to be free of the component, customers should not assume immunity. This is precisely the confusion many organizations face when vendor attestations appear for only a subset of a vendor’s product families.

In short: VEX improves automation and reduces noisy alerts, but it shifts some burden back to customers to maintain internal artifact inventories and to verify vendor attestations against in‑house deployments.

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Risk analysis: strengths, weaknesses, and threat modeling​

• Strengths of Microsoft’s approach:
• Machine‑readable VEX attestations make it possible for enterprises and security vendors to automate impact decisions for Microsoft products that have been attested.
• Publishing standardized attestations reduces the time from vulnerability publication to enterprise triage for attested products.
• Microsoft’s explicit promise to update CVEs and attestations if additional products are identified creates an audit trail and process accountability.
• Weaknesses and potential risks:
• Customers can misinterpret a product‑level attestation as a global guarantee. That false sense of safety is the primary operational risk: teams may de‑prioritize forensic inventory or fail to patch non‑attested artifacts that still include the vulnerable code.
• The attestation process is only as fast as internal inventory completion; for complex enterprises with many Microsoft artifacts and custom images, gaps can persist.
• The specific vulnerability here — a kernel deadlock — is particularly dangerous in the operational sense because it can stall or hang critical I/O and services even without an exploit chain that yields remote code execution.
• Threat model considerations:
• Local adversaries with the ability to trigger reconnect operations (for example, local users or compromised services) are more relevant than remote anonymous attackers unless the environment exposes SMB services at a network boundary.
• In multi‑tenant cloud contexts, a misbehaving tenant that can cause repeated reconnects may trigger denial of service against a host if it mounts or manipulates SMB shares in certain ways; tenant isolation and mount policies matter.

Given these tradeoffs, the rational operational stance is to treat Microsoft’s Azure Linux attestation as binding for Azure Linux and to verify all other Microsoft artifacts in your inventory until you receive a specific Not Affected attestation or confirm they are patched or never shipped the component.

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A practical case study (how teams should respond)​

• Triage: Pull together a short list of hosts that run Microsoft‑provided Linux kernels or Microsoft images:
• Azure Linux VMs, WSL2 instances, Azure Marketplace VMs, and any Microsoft‑distributed container images.
• Detect: Run the detection checklist on representative hosts and sample images to determine kernel versions and module presence.
• Patch: For Azure Linux hosts apply Microsoft updates immediately. For other distributions, follow the vendor security notice (Ubuntu, SUSE, Amazon Linux, etc.) and apply their patches.
• Validate: After patching, reboot if necessary and recheck that the patched kernel is running and that no circular locking warnings appear in kernel logs.
• Automate: Integrate VEX/CSAF ingestion into your vulnerability management tooling; however, treat a lack of attestation for a product as a cue to run a local artifact check rather than as a clearance.

This sequence prioritizes speed and defensive breadth: patch known carriers quickly, then scan aggressively to remove blind spots.

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Final assessment and recommendations​

CVE‑2025‑38244 is a kernel SMB client deadlock fix that required an upstream kernel correction to lock ordering. Microsoft’s public attestation that Azure Linux includes the upstream component and is therefore potentially affected is accurate and actionable for Azure Linux customers; Microsoft’s CSAF/VEX rollout provides an important automation primitive for enterprise triage. But customers must not conflate Microsoft’s current attestation set with a comprehensive inventory of every Microsoft artifact.
Actionable summary for IT/security teams:

• Immediately patch Azure Linux hosts using Microsoft’s provided kernel updates.
• Inventory and scan all Microsoft‑supplied artifacts in your estate (WSL2 kernels, Azure VM image families, Marketplace appliances) for the presence of the cifs/SMB client and for affected kernel versions.
• Apply vendor patches for other distributions (Ubuntu, SUSE, Amazon Linux, etc.) in mixed environments.
• In the short term, if patching is impossible, reduce exposure by unmounting SMB shares, unloading or blacklisting the cifs module where safe, and tightening access controls around SMB endpoints.
• Integrate CSAF/VEX ingestion into vulnerability tooling but always verify attestations against local artifact scans — absence of attestation is not proof of absence.

Microsoft’s transparency improvements around CSAF and VEX are a meaningful step forward for large vendors. They shorten the time to actionable intelligence for attested products and reduce noisy alerts. But transparency is not the same as omniscience: the operational responsibility to inventory, detect, and patch remains with each organization until the vendor’s attestation set covers the full surface of artifacts you run.
CVE‑2025‑38244 is therefore both a timely technical fix and a pragmatic lesson about how modern vulnerability mapping works in a world of vendor‑supplied open‑source components: accept vendor attestations when published, but verify broadly and act decisively where your local systems show exposure.

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Source: MSRC Security Update Guide - Microsoft Security Response Center