Understanding CVE-2025-37983: Linux qibfs leak, Azure Linux Attestation & Microsoft risk

A small, specific memory-leak fix in the Linux kernel’s qibfs module has been assigned CVE‑2025‑37983, and Microsoft’s public attestation currently names the Azure Linux distribution as a confirmed carrier of the affected upstream code — but that attestation does not mean Azure Linux is the only Microsoft product that could possibly include the vulnerable component. Read on for a detailed, practical take: what this bug actually is, why Microsoft’s wording matters (and what it doesn’t mean), which Microsoft artifacts could contain the same code, how to check your environment, and step‑by‑step mitigation and operational guidance for defenders and administrators.

Background / Overview​

qibfs is a small filesystem helper in the Linux kernel that accompanies the QLogic InfiniBand driver (the qib family). The bug fixed and tracked as CVE‑2025‑37983 is not a remote code‑execution flaw or privilege escalation; it is a resource management defect: under a particular failure path (failure to allocate a new inode during a create operation), a directory entry reference — a dentry — is not released, producing a leak. The Linux maintainers described it tersely: when an inode allocation fails, a dentry was left behind instead of being dopped, which can accumulate and, under extreme memory pressure, contribute to exhaustion and potential instability or denial‑of‑service.
This is an important but narrowly scoped class of bug: it requires local interaction and extremely low free‑memory conditions to trigger the problematic allocation path in many deployments. Still, because it exists inside the kernel and affects dentry/inode accounting, it is worthy of attention in environments where InfiniBand drivers or qib hardware support are compiled into the kernel (for example, HPC and enterprise clusters).
Microsoft’s Security Response Center (MSRC) has explicitly attested that Azure Linux (Microsoft’s distribution intended for Azure images and cloud customers) includes the implicated upstream component and is therefore potentially affected. Microsoft also stated that it began publishing machine‑readable CSAF/VEX attestations for third‑party components in October 2025 and that it will update CVE records if additional Microsoft products are found to include the same code.
That phrasing is precise and product‑scoped: it confirms Microsoft inspected Azure Linux artifacts and found the upstream qibfs code, and it promises to expand attestations if additional Microsoft products are identified. It does not—contrary to some casual readings—prove that only Azure Linux might ever include the vulnerable code. Inventory and build choices across Microsoft’s Linux artifacts vary; some other Microsoft‑maintained kernel images could include upstream drivers depending on version and configuration.

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What the bug is (technical summary)​

• The defect sits in drivers/infiniband/hw/qib/qib_fs.c: the qibfs helper that manages small pseudo‑filesystem entries backed by the qib InfiniBand driver.
• The concrete problem: on one error path, allocation of a new inode can fail (for example under OOM); the code failed to call dput(dentry) before returning an error, leaving the dentry reference stuck.
• Practical effect: a leak of kernel references leading to increased kernel memory consumption, and under repeated/long‑running triggers, potential kernel resource exhaustion and service disruption (DoS).
• Exploitation profile: local user or process triggering the qibfs code path repeatedly; no remote exploitation path is inherent to the bug. Privileges required are those that allow interacting with the qibfs pseudo‑filesystem and making the operations that exercise that inode allocation.
• Fix: a one‑line defensive change was applied that calls dput(dentry) before returning on the failed inode allocation; the patch was merged into the stable kernel trees and upstream mainline.

This is a housekeeping bug in kernel resource accounting. It matters in production systems that use InfiniBand, that include the qib driver in the kernel, or that build an expansive kernel config which pulls in the qibfs component.

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Microsoft’s statement: read closely​

Microsoft’s public CVE note and their broader transparency blog say two things that are both true yet easily misunderstood if read too quickly:

• Microsoft has attested that Azure Linux includes the implicated upstream component and is therefore potentially affected. That is an authoritative, product‑level statement that Azure Linux customers should treat as “in scope” for remediation and patching.
• Microsoft also said they will update the CVE/VEX mapping if additional Microsoft products are discovered to include the same upstream component.

Key takeaway: the attestation is a positive confirmation for Azure Linux; the absence of a similar attestation for other Microsoft products is simply an absence of attestation, not proof that those products are immune.
Why that matters operationally: vendor attestations (CSAF/VEX) are machine‑readable signals that reduce noise for security teams. They enable automated decisions: if an organization ingests a vendor’s VEX file and the vendor attests “Not Affected” for Product X and “Known Affected” for Product Y, teams can selectively remediate. Microsoft’s approach (starting with Azure Linux) delivers high‑quality signals for that one product; however, customers who use other Microsoft Linux artifacts should not assume immunity until Microsoft confirms it through inventory and VEX publishing.

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Is Azure Linux the only Microsoft product that could be affected?​

Short answer: No — but it is the only Microsoft product that Microsoft has publicly attested so far.
Longer, technically precise answer and reasoning:

• Attestation vs. inclusion: Microsoft’s VEX/CSAF mapping for a CVE is a product‑scoped statement of inventory. Because Microsoft started its VEX rollout with Azure Linux, that product was inventoried and attested first. That is why the MSRC page names Azure Linux explicitly.
• Absence of attestation is not evidence of absence. Microsoft explicitly committed to updating CVE entries should other Microsoft products be identified as carriers. The practical consequence is that until Microsoft finishes mapping other product families, you cannot rely on the CVE page alone to conclude other Microsoft artifacts are clean.
• Microsoft builds and publishes several Linux artifacts that could, depending on kernel versions and configuration, include the same qibfs source files:
• The publicly‑hosted WSL2 kernel source and builds (the WSL2‑Linux‑Kernel GitHub repository) are built from upstream snapshots with Microsoft’s configuration and patches. Whether any given WSL2 kernel build includes the qibfs component depends on the kernel config and whether InfiniBand drivers were enabled in that build.
• Azure VM images and specialized Azure HPC images (Azure Linux HPC variants) frequently ship kernels and modules that include InfiniBand support for high‑performance networking; those images are explicitly intended to support IPoIB, RDMA, and Mellanox/OFED stacks, and therefore are more likely to include InfiniBand drivers.
• Microsoft’s internal or service‑facing Linux kernel artifacts (for example kernels used in some Azure service backends, kernel modules included in certain managed offerings) could include or exclude qibfs depending on build choices for that image.
• Therefore, technically other Microsoft artifacts could include the vulnerable qibfs code, but only product‑level inventory (VEX/CSAF or direct binary inspection) can confirm presence.

Operationally this means: treat the MSRC attestation as authoritative for Azure Linux, and treat other Microsoft Linux artifacts as unverified until Microsoft either attests or you verify locally.

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Which Microsoft artifacts are most likely to include qibfs?​

From a practical, security‑operations perspective, the highest‑probability candidates are:

• Azure Linux images that are labeled or configured for HPC / InfiniBand/RDMA use. Those images ship kernels and modules intended for RDMA stacks. If your workload uses Azure’s InfiniBand‑enabled VM shapes (HPC instances), those kernels often include the relevant drivers.
• Any Azure Marketplace image or Azure maintained distro that advertises RDMA or InfiniBand support, or that bundles Mellanox OFED, libibverbs, or related userland packages.
• Microsoft’s published WSL2 kernels could include upstream drivers, but the default WSL2 kernel config commonly excludes many hardware drivers that are irrelevant inside a VM without direct hardware passthrough. In short: WSL2 kernels are less likely to include qibfs unless a customer or Microsoft explicitly enabled InfiniBand support in that build.
• Other Microsoft artifacts that intentionally ship general‑purpose or “near upstream” kernels (for example, certain kernel images used for Azure appliance or container host scenarios) could include the qibfs code depending on the chosen kernel version and config.

Remember: the only authoritative public Microsoft attestation for CVE‑2025‑37983 at the time of publication named Azure Linux. If you maintain or depend on other Microsoft Linux artifacts, verify locally.

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How to check if your environment includes the vulnerable component (practical steps)​

If you run any Microsoft‑supplied Linux image or Microsoft‑manufactured Linux kernel (WSL2, Azure images, or others), use the following checklist to determine whether the qibfs code is present and whether the kernel version is in an affected commit range:

• Identify the kernel image and version:
• On a Linux VM or WSL2 shell, run: uname -r
• Capture the package kernel version your distro uses (for example, apt/dpkg or rpm queries).
• Inspect modules and drivers for qib/qibfs:
• Check whether the qib module or qibfs pseudo‑filesystem appears in modules: lsmod | grep qib or find /lib/modules/$(uname -r) -type f -name 'qib'
• Search for either the userland entries or the qib_fs.c path in installed kernel source or build trees if available.
• Search kernel config for InfiniBand/qib options:
• If you have the kernel config, look for CONFIG_INFINIBAND and CONFIG_IB_QIB or similar entries.
• Example: zcat /proc/config.gz | grep -i 'INFINIBAND|QIB'
• If you control the kernel build:
• Inspect the source tree for drivers/infiniband/hw/qib/qib_fs.c or grep the tree for qibfs/dentry/dput occurrences.
• For WSL2:
• WSL2 users can check the WSL kernel release and the published WSL2‑Linux‑Kernel tag used by Microsoft. Use wsl --status and the kernel release string; you can also update the WSL kernel via wsl --update to pick up Microsoft’s published kernel builds.
• If you use Azure images:
• Check the image release notes and the kernel version shipped with your image. HPC‑targeted images that list RDMA, Mellanox OFED, or similar in their release notes are the most likely to include qibfs.

If your inventory reveals presence of the qib driver or qibfs helper in the kernel, treat the node as in‑scope for remediation.

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Mitigation and remediation guidance​

Short version: patch the kernel or upgrade the distro’s kernel packages; where immediate patching is infeasible, reduce exposure and monitor for DoS signs.

• Patch or upgrade:
• Apply the vendor kernel update that contains the qibfs fix. The fix is small and has been merged into upstream stable trees and distributed via vendor kernel packages in mainstream distros. Kernel updates are the correct fix.
• For Azure Linux customers, apply Microsoft’s Azure Linux updates as they are published; MSRC’s attestation signals that Azure Linux builds will be handled by Microsoft’s patch pipeline.
• For WSL2 users, update WSL via the standard mechanisms (wsl --update) or switch to a patched/custom kernel image that includes the upstream fix.
• For Azure VM images derived from vendor distributions (Ubuntu, Red Hat, SUSE), follow the distro’s kernel security advisories and apply the kernel package updates provided by that vendor.
• Temporary mitigations when patching is not immediately possible:
• Limit local user access to nodes that expose infrequent kernel interfaces for testing; the exploit path is local and relatively low reward, so reducing the number of low‑privileged local users is meaningful.
• Use cgroups, resource limits, and kernel memory cgroup configurations to reduce the risk that a single process can repeatedly exercise allocation failure conditions across the entire kernel. These are partial mitigations and not a substitute for kernel updates.
• Monitor for symptoms: rising dentry counts, kernel memory pressure, and OOM events traceable to the qibfs path. Frequent inode/dentry growth tied to user actions on InfiniBand namespaces is a red flag.
• Detection and monitoring:
• Monitor kernel logs for error messages near qibfs or qib driver stack frames.
• Instrument your nodes to raise alerts when dentry counts grow abnormally or when the slab caches related to dentry/inode allocations increase persistently.
• For organizations with large fleets:
• Prioritize Azure Linux images first (MSRC attested).
• Inventory other Microsoft Linux kernels and Azure marketplace images for InfiniBand/kernel versions.
• Use configuration management to schedule kernel updates and reboots in a controlled maintenance window.
• If you rely on vendor kernels (e.g., RHEL, Ubuntu), coordinate with those vendors’ security advisories and apply their supplied fixes.

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Operational risk assessment — how worried should you be?​

• Attack surface: low. The vulnerability is local and requires exercising a specific qibfs code path under memory allocation failure conditions. It is not a remote, client‑facing memory corruption vulnerability that allows arbitrary code execution.
• Impact category: denial of service / resource exhaustion. In high‑availability HPC environments or multi‑tenant hosts with InfiniBand, repeated leaks could degrade service quality or eventually cause node instability.
• Likelihood of exploitation: low in general enterprise systems (most servers will not see qibfs exercised), but higher in HPC or InfiniBand‑enabled systems where qib drivers and qibfs pseudo‑filesystem are present and accessible to local users or workloads.
• Urgency ranking: medium for affected InfiniBand/HPC hosts; lower for general servers where InfiniBand is not included in the kernel configuration.
• Practical exposure across Microsoft artifacts:
• Azure Linux: confirmed in‑scope — act promptly and follow Microsoft’s updates.
• WSL2: likely low exposure unless InfiniBand drivers were intentionally included — check kernel config and update WSL.
• Azure HPC images: high priority to check and patch because they frequently include RDMA stacks.

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Why vendor VEX/CSAF matters — and its current limits​

Microsoft’s move to publish machine‑readable CSAF and VEX attestations is a critical improvement for enterprise vulnerability management: it allows teams to automate the “is this vendor product affected?” question with high fidelity. However, early rollouts tend to be incremental; Microsoft chose to start with Azure Linux, which is sensible operationally but means other product families will appear later in the mapping.
Two practical implications:

• Security teams should ingest VEX/CSAF files where available and treat affirmative “Known Affected” attestations as authoritative for those products.
• Teams must not treat absence of a VEX attestation as definitive proof of non‑exposure. Until a vendor attests or you confirm locally, assume the need to inventory and/or verify.

In short: VEX/CSAF reduces uncertainty when present, but absence of data is just that—absence—until replaced by explicit attestations.

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Recommended action checklist for admins and security teams​

• Immediately:
• Verify whether Azure Linux images are in your environment. If yes, apply Microsoft’s Azure Linux kernel updates as they become available.
• If you run Azure HPC images or any images with InfiniBand/RDMA features, prioritize kernel verification and updates.
• Within 24–72 hours:
• Inventory all Microsoft‑provided Linux kernel artifacts (WSL2 kernels, Azure published kernels, marketplace images).
• Run the local checks listed above to confirm whether qibfs or qib modules are present.
• Within 7 days:
• Patch kernels where the qibfs vulnerability is present. Schedule maintenance and reboots for production nodes if required.
• Implement monitoring for kernel slabs and dentry growth where immediate patching is delayed.
• Ongoing:
• Integrate vendor VEX/CSAF feeds into your vulnerability management system so future attestations are auto‑ingested.
• Maintain a policy to treat vendor attestations as authoritative, but require local verification for any un‑attested critical images.

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Final assessment and guidance​

Microsoft’s statement that “Azure Linux includes this open‑source component and is therefore potentially affected” is the right and responsible first step: it gives Azure customers a clear signal to remediate. That statement should not be read as “Azure Linux is the only Microsoft product that could possibly include the qibfs code.” Other Microsoft artifacts — notably some Azure images, specialized HPC kernels, and potentially WSL2 builds depending on their configuration — could include the same upstream driver files depending on which kernel commits and configs were used.
Practical advice for teams:

• Treat Azure Linux as confirmed in‑scope and patch accordingly.
• Inventory all other Microsoft Linux artifacts in your estate and verify locally: check kernel configs, module lists, and image release notes, especially for products designed to support InfiniBand or RDMA.
• Use Microsoft’s CSAF/VEX feeds when available to automate decisioning, but always complement vendor attestations with local verification for critical systems.
• Given the bug’s exploitation profile (local, OOM path, DoS), prioritize systems where qib drivers are present or where InfiniBand is actively used.

This CVE is an example of why supply‑chain clarity and machine‑readable attestations matter: Microsoft publishing a product‑level VEX for Azure Linux gives customers a reliable onramp to remediation. But until VEX/CSAF coverage is broadened across Microsoft’s product portfolio, defenders must continue to combine vendor attestations with practical, in‑place inventory and verification to ensure they are not surprised by embedded upstream components in unexpected places.

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Appendix — Quick reference commands (copy/paste)​

• Identify kernel version:
• uname -r
• Check for qib modules or files:
• lsmod | grep -i qib
• find /lib/modules/$(uname -r) -type f -name 'qib'
• Search kernel config for InfiniBand:
• zcat /proc/config.gz | grep -Ei 'INFINIBAND|IB_QIB|QIB'
• For WSL2:
• wsl --status
• wsl --update
• If you maintain kernel sources locally:
• grep -R "qib_fs.c" /usr/src/linux || grep -R "drivers/infiniband/hw/qib" /usr/src/linux

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When vendors publish targeted attestations, use them — but avoid complacency. Attestations are a trustable signal, not a universal proof of absence. For CVE‑2025‑37983, Azure Linux customers should act now; everyone else running Microsoft‑supplied or Microsoft‑maintained Linux kernels should inventory, verify, and patch where needed.

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Source: MSRC Security Update Guide - Microsoft Security Response Center