CVE-2024-26987 Linux Kernel Deadlock with hugetlb vmemmap in Azure Linux

The Linux kernel fix tracked as CVE-2024-26987 addresses a subtle but real deadlock in the memory‑management path that only appears when the HugeTLB Vmemmap Optimization (the hugetlb_optimize_vmemmap feature) is enabled. Microsoft’s public advisory confirms that Azure Linux includes the affected component and is a confirmed hit — but the technical truth is more nuanced: any Microsoft product that ships or embeds a Linux kernel (or kernel-derived artifacts) with an affected kernel version and with the hugetlb optimization enabled could be vulnerable. In short, Azure Linux is the only Microsoft product Microsoft has publicly attested as including the vulnerable library so far — but it is not, from a systems architecture perspective, the only Microsoft product that could include the same code and therefore could be affected.

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Background / Overview​

CVE-2024-26987 is a medium‑severity local vulnerability in the Linux kernel’s memory‑failure path. The bug manifests as a circular locking dependency — a deadlock — triggered during certain hard offline tests on HugeTLB pages when the kernel is configured to optimize their vmemmap representation (the hugetlb vmemmap optimization). The deadlock shows up as two kernel locks being taken in conflicting orders (specifically, an interaction involving pcp_batch_high_lock and cpu_hotplug_lock), which can lead to a hung task or a system‑wide stall.
This problem was discovered and patched upstream in April 2024. Distribution vendors rapidly incorporated the fix into their kernel packages; distribution advisories and the kernel stable branch history show the relevant commit and the replacement logic used to avoid the unsafe lock ordering.
Key technical elements to understand:

• HugeTLB pages are special large pages used to reduce TLB pressure for workloads that benefit from large contiguous allocations (databases, high‑performance computing, certain virtualization and GPU scenarios).
• The hugetlb vmemmap optimization reduces memory overhead for the per‑hugepage struct page metadata by remapping or freeing the vmemmap pages in certain circumstances; it is controlled by a kernel config option and a runtime sysctl.
• The deadlock happens when memory‑failure handling (e.g., page poisoning / hard offline) interacts with hugetlb vmemmap optimization while another CPU path is taking CPU hotplug locks, producing a circular lock dependency.
• The upstream patch replaces a deterministic disable of PCP (per‑CPU cache) with a drain_all_pages() approach to avoid the broken lock ordering, removing the circular dependency while preserving the intended behavior.

This is not a remote code‑execution bug: the attacker must have local access and tends to require privileged operations to exercise the poisoned/hard offline page path. The practical impact is denial of service (system stall/hang) in environments that both use HugeTLB pages and have the vmemmap optimization enabled.

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Which kernel versions are in scope​

The vulnerable code is present in multiple kernel series and was fixed in stable updates. Published CVE tracking and vendor advisories identify the affected kernel ranges as (representative ranges reported by NVD and distribution trackers):

• 5.18 ≤ version < 6.1.88
• 6.2 ≤ version < 6.6.29
• 6.7 ≤ version < 6.8.8
• Some 6.9‑rc series kernels (early release candidates) were also listed in trackers

Distributions and vendors applied the patch in their respective stable branches; Debian, Red Hat, Oracle, Amazon Linux and others listed fixed package versions or indicated “not affected” for older LTS kernels that did not include the problematic commits.
Important caveat: the distribution kernel version and vendor backports matter more than upstream numbering alone. A particular vendor can backport the fix into a kernel branch and ship a patched binary that does not match upstream version numbering exactly. Conversely, a vendor may ship a kernel binary with a patched userland interface but with a kernel tree that still contains the old code path. Always check your distribution’s security advisory for whether the vendor has shipped a fixed kernel package for the image or product you run.

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The Microsoft statement — what it actually says (and what it does not)​

Microsoft’s advisory language explicitly confirms that Azure Linux includes the open‑source library in question and is therefore a confirmed inclusion point for the vulnerable kernel code. Microsoft’s FAQ text reiterates the company’s approach: Azure Linux will be kept up to date, and Microsoft pledged to publish CSAF/VEX attestations for transparency beginning in October 2025; their statement also promises to update CVE metadata if additional Microsoft products are found affected.
Two separate readings are useful:

• The literal reading: Microsoft has stated that Azure Linux includes the component; for other Microsoft products, Microsoft has not yet publicly attested inclusion at the time of that advisory, and Microsoft will update the CVE if it finds other impacts.
• The technical reading: Microsoft maintains and distributes other Linux artifacts — most notably the WSL2 kernel (the WSL2‑Linux‑Kernel repo), various Azure VM kernel images, and internal CBL‑Mariner / Azure Linux derivatives used across Azure services and container hosts. Any of these artifacts could contain the same kernel code depending on which upstream kernel baseline and stable‑branch commits were used, and whether the runtime sysctl was enabled. Microsoft’s current public attestation is conservative: only Azure Linux is explicitly called out.

This distinction matters. A confirmed statement that Azure Linux includes the vulnerable component does not exclude — technically or practically — other Microsoft kernel artifacts from also including it. The presence of kernel code within Microsoft‑maintained artifacts is a predictable reality of how Microsoft ships Linux‑based components across its services.

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Are other Microsoft products likely to include the library?​

Short answer: Yes — from a technical standpoint, several Microsoft products and components could include the same Linux kernel code; whether a particular Microsoft product is actually vulnerable depends on the kernel version and runtime configuration that product ships.
Products/components to examine closely:

• Azure Linux (confirmed). Microsoft has publicly attested presence in the Azure Linux images and container host. If you run Azure Linux VMs or AKS nodes that use Azure Linux, treat those images as a confirmed focus for remediation.
• Windows Subsystem for Linux (WSL2). Microsoft publishes and distributes a WSL2 Linux kernel binary and maintains the WSL2 kernel sources publicly. The WSL2 kernel is a real Linux kernel — if a specific WSL kernel binary shipped to devices falls within the vulnerable version range and is configured with the hugetlb optimization (or the config option was present and the sysctl enabled), then that WSL instance could be susceptible. Developers and organizations who rely on WSL in employee machines should check the WSL kernel version in use and update via Microsoft’s WSL update mechanisms or Windows Update. WSL kernels are frequently rebased and patched, so the presence or absence of the vulnerability is version‑dependent.
• Azure VM images and managed kernels. Microsoft builds and publishes kernels for some Azure services and may supply kernel images to cloud guests or appliances. Those kernels are functionally identical to upstream kernel trees in many areas; a given Azure kernel that matches an affected upstream version may be vulnerable until patched.
• CBL‑Mariner / internal kernel artifacts. Azure Linux is largely the public face for CBL‑Mariner heritage. Microsoft’s internal builds of Mariner derivatives are used across container hosts and some distributions used by Microsoft services. Those artifacts may contain the same code; Microsoft’s attestation covers the publicly branded Azure Linux distribution, but it is reasonable to assume similar internal images may need the same patching.
• Other Microsoft products that embed Linux code. Less obvious examples might include custom kernel images packaged inside specialized appliances, images shipped for specialized workloads, or developer‑oriented containers. Each must be assessed case‑by‑case.

Bottom line: the mechanics of the vulnerability — a kernel code path and a runtime sysctl — are not unique to Azure Linux. Microsoft’s public attestation is an important reassurance for Azure Linux customers, but it is not a definitive guarantee that no other Microsoft product could contain the affected code. Customers should treat Microsoft’s remark as a status update: Azure Linux confirmed; other products under review and Microsoft will update CVE metadata when additional impact is identified.

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How to determine whether a system is affected (step‑by‑step)​

These investigative steps help operators quickly determine exposure for Linux hosts (including WSL2 instances) and for Microsoft‑provided images:

• Check the running kernel version:
• Run: uname -r
• If the kernel version falls into the upstream affected ranges, proceed to step 2. If it is older than the earliest affected baseline or is a vendor kernel marked “not affected” by your distro, you are likely safe — but always verify vendor advisories.
• Check whether the hugetlb vmemmap optimization is present and enabled:
• Confirm the sysctl exists:
• cat /proc/sys/vm/hugetlb_optimize_vmemmap
• or: sysctl vm.hugetlb_optimize_vmemmap
• Value 1 means enabled; 0 means disabled. If the file is not present, the kernel configuration or architecture may not support the knob (the feature may be compiled out).
• Check kernel configuration for the compile‑time option:
• grep CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP /boot/config-$(uname -r)
• If this option exists and is enabled in the kernel config, the runtime knob can be present. Note: some distributions change the default behavior or backport fixes.
• In WSL2 instances:
• Open a WSL shell and run uname -r and the sysctl check above.
• Update WSL via: wsl --update (or allow Windows Update to deliver the kernel fix).
• If you manage developer machines, ensure corporate update policies push the WSL kernel updates.
• Check your distribution or product vendor advisory:
• Vendors (Ubuntu, Debian, Red Hat, Oracle, Microsoft/Azure Linux) publish security advisories and fixed package versions. Use your vendor’s security tracker and apply the recommended kernel package update.
• Audit fleet inventory:
• Search configuration management or inventory tools for kernel version strings and the presence of the sysctl on endpoints to prioritize patching.

Practical detection note: some workloads never enable hugepages or the hugetlb optimization; those systems are not exposed by this specific issue even if the kernel code is present. Conversely, some systems may have the kernel config option compiled in but the sysctl disabled — they are not currently vulnerable but could become so if the sysctl is toggled.

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Immediate mitigation options​

If you identify a system in scope and a rapid kernel package update is not immediately possible, consider these mitigations:

• Temporarily disable the hugetlb vmemmap optimization (runtime workaround):
• As root: echo 0 > /proc/sys/vm/hugetlb_optimize_vmemmap
• Or: sysctl -w vm.hugetlb_optimize_vmemmap=0
• Effect: prevents the code path that triggers the unsafe lock ordering. Note, however, that disabling the optimization changes memory/hugepage behavior and may increase memory usage or alter allocation performance for workloads that rely on HVO. Test for performance impact.
• Avoid workloads that allocate HugeTLB pages dynamically until patched. If your workload relies heavily on overcommit hugepages or on-demand hugepage behavior, consider modifying it or setting nr_hugepages explicitly to avoid reliance on optimized vmemmap behavior.
• Patch/update the kernel as the definitive fix. The upstream stable patch replaces the problematic call with a safer drain_all_pages() approach (and vendors have backported the change). Apply the official vendor kernel update at your earliest maintenance window.
• For WSL2 users: Update the WSL kernel via the supported update path or Windows Update. Encourage developers to run wsl --update and to reboot WSL instances.
• For Azure customers using managed services: Follow Azure advisories; for AKS nodes or VM images, patch the underlying node images or update node pools to a patched image.

Mitigation tradeoffs must be weighed: disabling the syscall or sysctl is safe from a correctness perspective (it avoids deadlock), but can degrade large‑page memory efficiency. The fix shipped upstream is superior: it removes the lock ordering issue without disabling HVO.

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Risk analysis — who should worry most?​

• High‑exposure scenarios:
• Systems that actively use HugeTLB pages (databases, in‑memory analytics, large‑page tuned virtualization) and where the kernel sysctl is enabled.
• Systems that stress page offline/hard offline pathways (some testing frameworks, memory poison/diagnostic tooling).
• Large multi‑CPU servers where CPU hotplug and heavy memory operations coincide.
• Moderate/low exposure:
• Typical general‑purpose hosts that do not allocate large numbers of hugepages and where HVO is disabled by default or not compiled in.
• Single‑user developer machines that do not use hugepages or that run with modern patched WSL kernels.
• Exploitability: the vulnerability requires local access and relatively specific runtime behavior to produce a system hang; it is not trivial for a remote unauthenticated attacker to weaponize. That said, in multitenant environments (cloud VMs, shared containers, hosted developer images) a local privileged or elevated process could induce the condition. Therefore tenancy and privilege boundaries matter when assessing risk.

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Critical assessment of Microsoft’s communication and transparency​

Strengths:

• Microsoft’s public confirmation that Azure Linux includes the library gives Azure customers a clear focal point for triage and remediation.
• Microsoft’s move to publish CSAF/VEX attestations (announced to begin in October 2025) is a positive step toward machine‑readable, structured vendor attestations that customers can consume programmatically.

Risks and gaps:

• Microsoft’s FAQ wording is conservative and limited: stating that Azure Linux is a confirmed inclusion does not answer whether similar Microsoft kernel artifacts (WSL2, other Azure images, kernel bins shipped into services) include the code. From a customer risk assessment perspective, conservative messaging without a product‑by‑product mapping forces customers to assume exposure until Microsoft confirms otherwise.
• The technical reality is that Microsoft does ship Linux kernels in multiple contexts; customers and defenders must therefore assume wider potential impact unless vendor attestations explicitly say otherwise.
• The timeline of vendor attestations and CSAF/VEX adoption matters: delays in publishing explicit product‑level mappings increase the operational burden on large customers to inventory and triage.

Recommendation to Microsoft (analyst view): publish a clear mapping of Microsoft products and the kernel versions / build tags they ship, and an explicit product‑level verdict (Affected/Not Affected/Fixed) for each CVE that touches kernel code. That mapping is exactly what CSAF/VEX is designed to automate; its adoption should be accelerated because kernel CVEs cross many product boundaries.

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Practical checklist for administrators (quick reference)​

• Inventory:
• Identify all Microsoft images in use: Azure Linux VMs, AKS node images, WSL2 host images, specialized appliance images.
• Collect kernel versions (uname -r) for each host.
• Detection:
• For each host: check /proc/sys/vm/hugetlb_optimize_vmemmap (1 = enabled, 0 = disabled).
• Check kernel config: grep CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP /boot/config-$(uname -r)
• Immediate actions:
• If an affected kernel version is present and sysctl is 1, either disable the sysctl or schedule an immediate kernel update.
• If kernel update is not possible immediately, disable hugetlb_optimize_vmemmap (sysctl) as a temporary measure and monitor workload performance.
• For WSL2 users: instruct developers to update WSL kernel via official update path and restart their WSL instances.
• Long term:
• Ensure patch management processes include kernel updates from Microsoft and distribution vendors.
• Subscribe to vendor CVE advisories and CSAF/VEX feeds (when published) to automate impact ingestion.
• Add checks in your fleet management tooling to detect hugetlb sysctl presence and kernel versions in the affected ranges.

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Unverifiable or time‑sensitive points — caution to readers​

• Vendor attestations change over time. Microsoft’s advisory snapshot stating Azure Linux as a confirmed inclusion reflects the status at the time of publication; Microsoft has stated it will update CVE metadata if additional impact is found. Readers should not assume this is a permanent or exhaustive statement — always check the vendor’s latest advisory and CSAF/VEX artifacts for updates.
• Whether a specific WSL kernel build or a particular Azure kernel image shipped to a customer is affected depends on the exact kernel commit, the vendor’s backporting decisions, and runtime configuration. These facts are resolvable but must be verified per product image and per build tag — they are not inferable from upstream CVE text alone.
• Distribution and vendor fixes may be delivered under different package names or with backported patches. The only authoritative source for whether your instance is patched is the vendor advisory and the package changelog for the kernel binary you run.

If precise, product‑level mapping is required for compliance or regulatory reasons, customers should request the vendor’s structured attestation (CSAF/VEX) or use vendor‑provided security trackers.

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Conclusion​

CVE-2024-26987 is a targeted, medium‑severity Linux kernel deadlock that only triggers under a specific configuration: the hugetlb vmemmap optimization enabled in kernels within certain upstream ranges. Microsoft’s public advisory confirms Azure Linux includes the affected component, which makes Azure Linux a confirmed focus for remediation. However, the underlying reality is broader: any Microsoft product that ships a Linux kernel or kernel‑derived image that matches the vulnerable upstream commits and has the hugetlb optimization present could, in principle, be affected.
Operators should act pragmatically:

• Treat Azure Linux images as confirmed and patch them according to Microsoft’s guidance.
• Inspect WSL2 instances, Azure VM images, AKS node images and any Microsoft‑provided kernel artifacts in your environment for kernel version and hugetlb sysctl status.
• Apply the vendor kernel updates where available; use the sysctl disable workaround if an emergency mitigation is required.
• Push vendors (including Microsoft) for structured, product‑level attestations (CSAF/VEX) to streamline future triage.

Finally, remember that the fix itself is straightforward and has been backported by major vendors — the operational challenge is inventorying where that kernel code sits inside complex supply chains. Assume broader exposure until a product‑by‑product attestation proves otherwise, and prioritize patching for hosts that both use HugeTLB pages and have the hugetlb optimization enabled.

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Source: MSRC Security Update Guide - Microsoft Security Response Center