CVE-2025-39742: Linux RDMA hfi1 Divide-by-Zero Fix and Azure Linux Attestation

The Linux kernel received a targeted fix for a divide‑by‑zero condition in the RDMA hfi1 driver — tracked as CVE‑2025‑39742 — and Microsoft’s public advisory has confirmed that Azure Linux is a product the company has inventoried and attested as “potentially affected,” while promising to expand its machine‑readable CSAF / VEX mappings if further Microsoft products are found to include the same upstream component. (msrc.microsoft.com)

Background / Overview​

CVE‑2025‑39742 is a correctness bug in the Linux kernel’s RDMA hfi1 driver: the function find_hw_thread_mask() performed a division using a value that could be zero, and the original code checked the divisor after performing the division. That ordering creates a classic divide‑by‑zero runtime error, which in kernel context can lead to a panic or other severe stability failure. The upstream fix simply moves the zero check before the division and reorganizes the logic to avoid the runtime error.
HFI1 is a hardware‑specific RDMA driver (found under drivers/infiniband/hw/hfi1 in the kernel source) used for certain host fabric interfaces, historically associated with Intel Omni‑Path (OPA) and related HPC fabric devices. It implements hardware‑dependent RDMA and OPA VNIC features and is compiled only when the kernel’s configuration exposes that hardware support. That makes the driver present on a narrower set of systems (HPC and specialized RDMA hosts) rather than on general‑purpose desktop or cloud images by default.
Put simply: the bug is real, the upstream fix is small and low‑risk, and the practical impact for most servers depends on whether the hfi1 driver is present or can be loaded on your hosts.

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The technical defect, explained​

What the code did wrong​

At the heart of CVE‑2025‑39742 is a simple ordering error. The function in question computed a per‑socket core count by dividing the number of online CPUs by the number of core siblings and the number of online nodes. The code performed that division, and only afterwards checked whether the divisor (num_core_siblings) was zero. If num_core_siblings can be zero in any valid runtime topology seen by the driver, the division is undefined and the kernel can trigger a fatal error. The upstream fix moves the check for affinity->num_core_siblings == 0 ahead of the division and returns early when that value is zero, preventing the divide operation entirely.

Why this matters in kernel context​

A divide‑by‑zero inside a kernel driver is not a benign bug. Unlike application processes, a kernel panic or oops can affect system availability, crash services, and in extreme cases leave systems unusable until rebooted. For RDMA and low‑latency fabrics used in HPC and specialized clusters, such stability failures are particularly disruptive to long‑running jobs or multi‑tenant services. Public advisories consequently treat the issue as an availability and integrity risk rather than a remote code execution vector. Several distributors assign a medium‑to‑high severity to the fix because of the potential for kernel crashes.

Exploitability: local vs remote​

Available advisories and CVE records characterize the flaw as a local weakness: it requires local code execution or module load to trigger the path that performs the problematic calculation. There is no public evidence of a remote exploit chain that leverages this divide‑by‑zero for remote code execution as of this writing; rather, documented impact is instability and kernel crash. That said, the kernel can be a powerful target — local bugs can sometimes be combined with other flaws or configuration features to escalate privileges — so vendors and operators treat such fixes as important to apply where hfi1 is present.

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Where the fix has landed and who has published advisories​

Multiple Linux distributors and vendors have cataloged CVE‑2025‑39742 and published patches or backports:

• SUSE listed the fix in its SLES/LTSS advisories and backport/livepatch announcements, marking the issue as resolved in recent kernel updates.
• Ubuntu published a security notice mapping the CVE to its kernel packages and stable trees, and included the upstream description and tracking details.
• Oracle Linux, Amazon Linux and other vendors have integrated the upstream change into their kernel advisories or tracking pages; Amazon Linux’s security tracker lists affected kernels and pending fixes.
• The upstream patch and a set of stable backports are visible in Linux kernel commit lists and CVE aggregators; change diffs show the simple safety check insertion and rearrangement in drivers/infiniband/hw/hfi1/affinity.c.

Each distribution’s precise set of affected package names aries; operators should consult their vendor’s security feed and apply the appropriate kernel update or livepatch for their kernel line.

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Microsoft’s advisory, CSAF/VEX attestation, and the key question​

The user’s central question asks whether Azure Linux is the only Microsoft product that includes the vulnerable open‑source component and therefore might be affected. Microsoft’s public wording in its Security Update Guide for this and many other kernel CVEs is consistent and deliberate: Microsoft states that “Azure Linux includes this open‑source library and is therefore potentially affected”, and also notes the company began publishing machine‑readable CSAF / VEX attestations in October 2025 and will update the CVE mapping if additional Microsoft products are identified as carriers. That is the text Microsoft publishes to tell customers which of Microsoft’s managed Linux artifacts the company has completed inventorying and attesting. (msrc.microsoft.comnpack what that wording actually means, operationally:

• Affirmative attestation for Azure Linux: Microsoft is stating that its Azure Linux product family has been inventoried and that the hfi1 component (or the upstream kernel code that contains it) is present in those images. Customers running Azure Linux images therefore have a confirmed scope: they should apply the update or follow Microsoft’s remediation guidance. (msrc.microsoft.com)
• Not a global negative for other Microsoft products: The advisory does not claim Microsoft examined every Microsoft kernel, image, or binary and proven the component absent everywhere else. Microsoft explicitly commits to adding additional CSAF/VEX attestations when inventory work identifies other products as carriers. In practical terms, a lack of attestation for a product is “not yet mapped,” not “guaranteed safe.” Multiple independent analyses of Microsoft’s attestation wording reach the same conclusion: attestation is product‑scoped, not exclusive.

So: Microsoft’s published statement is accurate, but deliberately scoped.

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Direct answer: Is Azure Linux the only Microsoft product that includes the library?​

Short answer: No — but practicallsso far.

• Microsoft has confirmed (and published a machine‑readable attestation for) Azure Linux as a product that includes the implicated kernel code and is therefore potentially affected. Treat that as the authoritative “yes” for Azure Linux images. (msrc.microsoft.com)
• Microsoft has not asserted that no other Microsoft product contains the code. Because Microsoft maintains multiple Linux artifacts (Azure Linux images, linux‑azure kernel builds, WSL2 kernel binaries shipped with Windows, curated Marketplace images, AKS node images, container base images, and internal appliance kernels), any of those artifacts could — depending on kernel version, build configuration (CONFIG_INFINIBAND_HFI1), and presence of backports — carry the same upstream driver. Until Microsoft adds additional CSAF/VEX attestations or an independent inspection of a particular artifact confirms absence, it should be treated as “unknown.” This interpretation is consistent with expert commentary and forum analysis of Microsoft’s attestation approach.
• Practically, many Microsoft customers operating standard Azure marketplace images will find their Azure Linux images covered and patched through Microsoft’s channels; other Microsoft‑distributed images (for example, the WSL2 kernel that ships as part of Windows or specialized marketplace kernels) require a per‑artifact check to determine inclusion of hfi1. If Microsoft identifies additional products that ship the same upstream component, the company has committed to update the CVE record and the VEX/CSAF files to reflect that wider mapping. (msrc.microsoft.com)

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How you can verify whether a given Microsoft artifact (or any kernel image) includes hfi1​

To move from uncertainty to certainty for your environment, perform a short set of checks on the artifact(s) you care about:

• Inventory the kernel binary and build metadata for the artifact or image you run (for example, the Azure Linux image, a Marketplace VM image, or a WSL2 kernel). Look for the kernel version and vendor patch level. Vendor advisories map CVEs to kernel versions.
• Check kernel configuration for CONFIG_INFINIBAND_HFI1:
• If you have the vmlinuz (or packaged kernel) and a corresponding /proc/config.gz or the kernel config file in /boot, search for CONFIG_INFINIBAND_HFI1=y or =m. If it is not present, the driver is not compiled in. If it is =m (module), the module could still be loaded at runtime.
• On a running system, check for module presence and loadability:
• lsmod | grep hfi1
• modinfo hfi1
• If the module exists, note its version and build provenance (modinfo can help). If the module is absent but CONFIG_INFINIBAND_HFI1 exists in the kernel config as a module, the module can be present in the distribution packages and loaded by the system when hardware is detected.
• Inspect installed kernel package changelogs and vendor CVE mappings:
• Use your distribution’s security feed (apt‑listchanges, zypper lp, yum changelog) to find whether the kernel build includes the hfi1 fix for CVE‑2025‑39742. This is the definitive vendor mapping for that distro/kernel line.
• If you need to block the risk quickly and hfi1 is not used in your environment:
• Blacklist the hfi1 module in /etc/modprobe.d/ (for example, add “blacklist hfi1”) and rebuild the initramfs so the module is not auto‑loaded; then schedule a maintenance reboot to ensure the module never loads. This is a practical mitigation when you do not rely on that RDMA hardware. (Be cautious: blacklisting on systems that require hfi1 will break hardware functionality.)

Follow these steps on every image/artifact you obtain from Microsoft (Azure Linux, Marketplace, WSL2 kernel image, and so on) if you need to move from “unknown” to “verified safe or patched.”

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Practical remediation recommendations (operational checklist)​

• For Azure Linux customers:
• Confirm the distribution update that Microsoft or the Azure Linux team published for CVE‑2025‑39742, then apply the kernel package update or livepatch as recommended. Azure Linux is the product Microsoft has attested to include the vulnerable code; treat that as priority. (msrc.microsoft.com)
• For on‑prem or self‑managed distributions:
• Apply your vendor’s kernel security update (Ubuntu, SUSE, Oracle, Amazon Linux) that contains the upstream hfi1 fix. Each vendor’s advisory shows the affected kernel versions and the package names to update.
• For mixed environments where hfi1 is not used:
• Blacklist the hfi1 module and rebuild initramfs; schedule reboots to remove the attack surface immediately while awaiting vendor patches.
• For Windows environments that run Linux artifacts (e.g., WSL2, Azure‑hosted Linux containers):
• Verify the specific kernel image shipped in the artifact. WSL2 uses a Microsoft‑built kernel binary — check the WSL kernel version and consult Microsoft’s VEX/CSAF mapping or the kernel config for CONFIG_INFINIBAND_HFI1 if available. If WSL or container base images are internally curated, treat them as separate artifacts that need per‑artifact verification.
• Continuous supply‑chain step:
• Automate scanning of SBOMs, kernel configs, and module lists for 1 and CVE‑2025‑39742 signatures so new images are flagged proactively.

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Risk analysis and context​

• Strengths of the fix and vendor response:
• The upstream fix is minimal and narrowly scoped; it’s low‑risk for regression and straightforward to backport to stable kernel branches. That makes vendor rollouts relatively simple and fast. The change reduces code complexity and eliminates the runtime fatal path.
• Multiple distributors have already mapped or backported the fix into their kernel streams (SUSE, Ubuntu, Oracle, Amazon Linux, and others), which provides immediate remediation paths for most environments.
• Limitations and operational risks:
• The bug is hardware/driver specific: only hosts with the hfi1 driver compiled or installed are realistically at risk. Many general‑purpose cloud images do not include this driver, but specialized HPC or OPA hosts do. Mis‑inventory — assuming a driver is absent when it is present — is the real operational hazard.
• Microsoft’s attestation covers Az family; other Microsoft artifacts remain an inventory gap until Microsoft publishes additional CSAF/VEX entries. Relying solely on Microsoft’s published product list without per‑artifact verification exposes customers to surprises where Microsoft‑distributed images happen to include the driver.
• Exploit likelihood:
• Public trackers show evidence of exploitation, consistent with a kernel stability fault that is local in nature. Nevertheless, local faults that crash kernels are serious in multi‑tenant and high‑availability contexts and therefore warrant prompt patching where the driver exists.

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What Microsoft’s CSAF/VEX rollout means for customers​

Microsoft’s move to publish machine‑readable CSAF (Common Security Advisory Framework) / VEX (Vulnerability Exploitability eXchange) attestations — begun in October 2025 for Azure Linux — is an operational improvement: it provides automation‑friendly metadata to help customers map CVEs to Microsoft product artifacts. However, the rollout is phased. The published attestations are authoritative only for the product families they name; they do not imply universal coverage of all Microsoft artifacts today. If Microsoft identifies additional Microsoft products that ship the implicated component, the company will update CVE records and publish new attestations to reflect that mapping. That is the policy Microsoft repeats across multiple kernel CVE entries and it should be read as atement rather than a global all‑clear. (msrc.microsoft.com)
Operationally, that means:

• Prioritize Azure Linux images for patching when Microsoft attests the product as affected.
• Treat other Microsoft artifacts (WSL2 kernel, marketplace images, container base images, AKS node images) as requiring per‑artifact verification until Microsoft attests them or you confirm via kernel config/module checks.

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Final guidance for administrators and security teams​

• Immediate triage (first 24–72 hours)
• Confirm which hosts in your estate have hfi1 present (lsmod/modinfo and kernel config checks). Prioritize systems where hfi1 is present and in use.
• For Azure Linux systems, apply Microsoft’s recommended kernel update or livepatch without delay: Microsoft has attested Azure Linux as a carrier for the upstream component. (msrc.microsoft.com)
• For other distributions, deploy vendor kernel updates (SUSE, Ubuntu, Oracle, Amazon Linux) that include the hfi1 fix.
• Short‑term mitigations (if a quick patch is not immediately available)
• Blacklist the hfi1 module if you do not require the hardware. Rebuild initramfs and schedule a maintenance reboot to ensure the module cannot be loaded. This is a service‑impacting mitigation if the hardware is required, so use with care.
• Ongoing activities
• Automate artifact inspection (SBOM, kernel config scanning) for CONFIG_INFINIBAND_HFI1 and CVE mappings.
• Track Microsoft’s CSAF/VEX feed for new attestations beyond Azure Linux; update operations playbooks accordingly.

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Conclusion​

CVE‑2025‑39742 is a straightforward but important kernel fix: a divide‑by‑zero safety check moved to the correct place in the hfi1 RDMA driver. The fix is small and backportable, and multiple distributors have published advisories and patches. Microsoft has confirmed that Azure Linux includes the implicated component and has committed to publish CSAF/VEX attestations — a pragmatic, phased approach that gives Azure Linux customers an immediate, authoritative signal to act on. However, Microsoft’s attestation is product‑scoped and not an exhaustive inventory of every Microsoft artifact; other Microsoft‑distributed kernels or images could include the same hfi1 driver depending on build configuration. Operators should therefore verify the kernel configuration and module presence for each artifact they run and apply vendor patches or mitigations as appropriate.
If you manage Linux systems in Azure, prioritize Azure Linux images first; if you manage mixed or hybrid estates, treat each Microsoft‑distributed artifact as a separate verification item and automate checks for CONFIG_INFINIBAND_HFI1 and the CVE mapping so you know exactly where the risk resides.

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Source: MSRC Security Update Guide - Microsoft Security Response Center