CVE-2025-21985: AMD Display Kernel Bug Narrowed to Azure Linux, Could Hit WSL

Microsoft's public attestation narrows the immediate scope to the Azure Linux distribution, but the underlying vulnerability—an out-of-bounds access in the Linux kernel's AMD display stack—exists in upstream kernel code and therefore can appear in any product that ships a Linux kernel build with the AMD DRM/Display (drm/amd/display) components enabled; Azure Linux is not categorically the only Microsoft product that could be affected, though it is the only Microsoft product for which Microsoft has published a formal VEX/CSAF attestation at this time.

Background / Overview​

In early April 2025 the Linux kernel tree received a patch to fix out-of-bounds array access issues in the AMD display driver (the drm/amd/display code path used by the amdgpu driver). The bug centers on array indexing and bounds checks in DC/Display Core code paths—most prominently in routines where encoder/stream mapping and stream-location indices are used. Out-of-bounds reads or writes in GPU driver code are serious because they can produce memory disclosure, system crashes (denial-of-service), or in certain cases help pave the way to local privilege escalation or arbitrary code execution when combined with other conditions.
Microsoft's security guidance for CVE-2025-21985 specifically references the Azure Linux Distribution (formerly CBL-Mariner) and emphasizes their commitment to publishing machine-readable attestations (CSAF/VEX) for Azure Linux. Microsoft notes that if impact to additional products is identified, we will update the CVE to reflect this. That wording means Microsoft has completed product-impact attestation for Azure Linux but has not yet declared other Microsoft products as affected or not affected.
The technical root of the issue is in the Linux kernel's AMD display core: functions use indices whose valid ranges differ from the array sizes they index into, and the patch adds explicit checks or adjusts assertions so indexes cannot be used out-of-bounds. Kernel distributions that include the affected upstream code and compile it into their kernel (either built-in or as modules) are the ones at risk.

---

What the CVE actually says — concise technical summary​

• The vulnerability is an out-of-bounds array access in the AMD DRM display code. The implicated code paths include mapping arrays such as hpo_stream_to_link_encoder_mapping and indices like disp_cfg_stream_location.
• The fix enforces explicit bounds checks against constants such as MAX_HPO_DP2_ENCODERS and adjusts assert conditions so the valid index ranges are respected.
• The vulnerability is present in upstream kernel trees that include the AMD display core (drm/amd/display). Affected kernel trees and specific version windows vary by how distributions rebased or backported the code; vendors and security databases list the affected kernel ranges and patch versions.
• Exploitation requires local code execution capability (local attacker or process on the machine) and depends on the ability to interact with the affected display driver path (for example, by opening device nodes or causing driver paths to execute). Impact can include sensitive-memory disclosure and denial-of-service; further exploitation complexity depends on the local environment.

---

Why Microsoft’s Azure Linux attestation matters — and what it does not mean​

Microsoft's VEX/CSAF attestation for Azure Linux is important because it provides machine-readable, product-level declarations of vulnerability status—a practical way for enterprises and security tooling to reduce false positives when scanning supply chains or telemetry.

• What it confirms: Microsoft has examined the Azure Linux distribution and published an attestation that clarifies whether the distribution is affected or has been patched. For customers using Azure Linux, this gives a high-confidence, actionable statement.
• What it does not confirm: The attestation does not state that other Microsoft products cannot contain the same vulnerable code. It is an attestation of current knowledge and scope—Microsoft has explicitly said they will update the CVE if impact to additional products is identified.
• Operational implication: Treat the Azure Linux attestation as authoritative for Azure Linux. Treat other Microsoft products as potentially affected only if they include a Linux kernel build with the AMD display components enabled or if they embed the vulnerable code via other means.

---

Microsoft products to consider (beyond Azure Linux)​

Microsoft's ecosystem is larger than a single Linux distribution. Several Microsoft products and services either ship their own Linux kernels or host Linux workloads, and any of those could, in principle, include the same drm/amd/display code path if their kernels were configured to include it.

1. Windows Subsystem for Linux (WSL)​

• Microsoft builds and distributes a Linux kernel for WSL2. That kernel is based on upstream branches but tuned for WSL. The WSL repository and release notes show a public Microsoft kernel tree and configuration files that users can inspect or build against.
• Whether a particular WSL kernel includes the AMD DRM/display code depends on the kernel configuration used by that WSL release. WSL historically shipped kernels with many drivers compiled in or available as modules; module support has evolved over recent updates.
• Important nuance: GPU functionality in WSL often relies on host-side Windows drivers and a virtualization layer. Some GPU features are provided by the Windows GPU stack and do not rely on guest amdgpu DRM in the WSL kernel. However, AMD has published instructions and tooling to install Linux-side amdgpu/ROCm stacks in WSL for compute and graphics use; that indicates the WSL environment can, in some deployments, end up running components that interact with the drm/amd/display code paths.
• Conclusion: WSL is a realistic candidate to check. Whether an individual WSL installation is affected requires checking the WSL kernel config and the presence of amdgpu/drm components.

2. Azure Virtual Machines and Marketplace images​

• Azure hosts many Linux distributions as guest VMs. Those guest kernels are supplied by independent Linux vendors (Ubuntu, Red Hat, SUSE, etc.. If those kernels include the vulnerable drm/amd/display code and are running on a VM that provides or exposes AMD GPU functionality (e.g., GPU-accelerated VMs), the guest is subject to the upstream kernel vulnerability.
• Microsoft’s attestation focuses on Azure Linux (the distro Microsoft publishes) and does not automatically cover third-party images running on Azure. Customers using vendor-supplied images should consult their distribution’s security advisories and kernel patch timelines.

3. Azure Kubernetes Service (AKS) node images and connected services​

• AKS node pools use underlying VM images and can run Linux kernels from multiple vendors. If node images (or custom node images) include the vulnerable driver code and are used with GPU-enabled nodes, the nodes could be affected.
• Container images generally do not include kernel code; vulnerability exposure is driven by the host kernel that runs containers.

4. Other Microsoft products shipping Linux components​

• Microsoft ships specialized Linux kernel builds for specific features (for example, kernel builds integrated into cloud appliances, developer toolchains, or kernel-enabled features). Each of those builds must be inspected for the DRM/AMDGPU pieces to determine impact.
• Microsoft has said it will update the CVE if additional internal products are found to ship the relevant code.

---

Technical verification checklist — how to confirm whether your Microsoft product instance is affected​

The vulnerability is present only if the system is running a kernel build that includes the AMD display code and the affected code paths are enabled. Use these checks to verify:

• Identify the running kernel:
• uname -r
• Check distribution/kernel package version to match affected ranges reported by security advisories.
• Check whether the AMD DRM or amdgpu driver is present:
• lsmod | grep amdgpu
• modinfo amdgpu (if modules are available)
• dmesg | grep -i amdgpu
• Look in /sys/class/drm or /dev/dri for exposed DRM devices.
• Inspect kernel configuration for DRM/AMD drivers:
• zless /proc/config.gz | grep DRM_AMDGPU
• zless /proc/config.gz | grep DRM_AMD_DC
• If you control a WSL instance, check the WSL2 kernel configuration file in the public Microsoft repo or the locally running kernel config.
• Cross-reference kernel version vs. vendor advisory:
• Compare the running kernel (and any backports) with distribution or vendor security advisories about CVE-2025-21985 or corresponding kernel patch commit IDs.
• For WSL users:
• Confirm the WSL kernel version (uname -r inside WSL).
• Confirm whether the WSL kernel is the Microsoft-supplied kernel or a custom kernel (wsl --status and .wslconfig kernel path).
• Ensure Windows Update has applied the latest WSL kernel refreshes that Microsoft releases (WSL kernel updates are sometimes delivered via Windows Update).

If any check indicates the presence of upstream DRM/AMD display code in an affected kernel version, treat the instance as potentially vulnerable until patched.

---

Practical mitigations and recommended actions​

• For Azure Linux (customers using Microsoft’s Azure Linux distribution)
• Apply the vendor-provided package updates that Microsoft has published for Azure Linux. The CSAF/VEX attestation will indicate status; follow the distribution’s patch instructions.
• Use Microsoft’s automated patch management or Azure Update Management to deploy available kernel updates.
• For WSL users
• If you use the Microsoft-provided WSL kernel: ensure Windows Update has installed the latest WSL kernel package; check the WSL release notes and apply any kernel updates Microsoft publishes.
• If you are using a custom WSL kernel: rebuild or obtain an updated kernel that includes the AMD DRM fixes and replace the kernel binary via .wslconfig as documented by Microsoft.
• If you rely on AMD GPU functionality inside WSL (e.g., ROCm), coordinate with AMD’s published instructions and the distribution’s kernel module availability; update the amdgpu stack as instructed by the vendor.
• For Azure VMs and AKS
• Update guest OS kernels using the distribution vendor’s advisories and your organization’s patching processes.
• For GPU-enabled VMs, validate the interaction between guest and host GPU stacks; consult cloud vendor guidance regarding passthrough or virtual GPU exposure.
• For on-prem or other Microsoft-supplied products that bundle Linux kernels
• Monitor Microsoft bulletins and VEX/CSAF feeds; Microsoft has pledged to update the CVE if further product impact is identified.
• Implement host-level mitigations such as restricting local unprivileged access to device nodes, applying container isolation, and limiting use of GPU features to trusted workloads.
• General hardening
• Restrict local access: remove or tightly control membership in groups that grant access to /dev/dri or device nodes.
• Use kernel exploit mitigation features where available (e.g., KASLR, SMEP/SMAP support as provided by the platform).
• Employ layered defenses: endpoint EDR, integrity monitoring, and strict patch discipline.

---

Risk analysis — how serious is CVE-2025-21985 in practice?​

• Attack surface: The vulnerability requires local code or process execution to interact with the driver path; it is not remotely exploitable over the network by itself. That reduces blast radius for exposed internet-facing services that do not provide local code execution.
• High-value environments: Systems that permit multiple tenants or untrusted code to access GPU devices—shared GPU hosts, developer machines, or multi-user servers—are higher risk. GPU-enabled cloud VMs or hosts with device passthrough require careful vendor-specific analysis.
• Potential impact: Kernel driver out-of-bounds bugs can produce information leakage or system crashes. In favorable conditions (for an attacker), they can contribute to privilege escalation chains; however, exploitation often depends on surrounding environment and additional conditions.
• Ease of exploitation: Public proof-of-concepts may not be available immediately; exploitation requires a local exploit path into the driver execution context. Attack complexity is often non-trivial but not necessarily prohibitive for a determined attacker with local access.

---

Why the question “Is Azure Linux the only Microsoft product that includes this open-source library?” is nuanced​

There are three distinct lenses to apply to that question:

• Product attestation lens
• Microsoft has published a product-level attestation (CSAF/VEX) specifically for Azure Linux. In the narrowest reading of the user's question—“Has Microsoft attested that Azure Linux is the only Microsoft product that includes this component?”—the answer is no: Microsoft attested for Azure Linux only and has left the door open to update the CVE if other internal products are found to be affected.
• Code-distribution lens
• The DRM/AMD display code is upstream Linux kernel code. Any Microsoft product that ships a Linux kernel build that included those drm/amd/display components could contain the vulnerable code. That includes the Microsoft-built WSL kernel and any bespoke or redistributed Linux kernel images Microsoft ships or maintains—conditional on kernel configuration.
• Operational exposure lens
• Whether a product is practically affected (i.e., vulnerable in a way that can be exploited in a deployment) depends on runtime configuration: whether the amdgpu DRM code is enabled, whether device nodes are present/exposed, whether GPU devices are present and accessible, and whether the kernel version includes the vulnerable commit or an upstream patch.

Because of these overlapping considerations, the safe, security-first answer is: Azure Linux is the only Microsoft product with a formal, published attestation about CVE-2025-21985 so far; however it is not the only Microsoft product that could theoretically include the vulnerable drm/amd/display code—WSL and other Microsoft-distributed Linux kernel builds should be inspected and patched where necessary.

---

How enterprises should treat Microsoft’s statement and what to do next​

• Treat Microsoft’s VEX/CSAF attestation for Azure Linux as authoritative for Azure Linux instances. Apply the patches Microsoft indicates for that distro as a priority.
• For other Microsoft products in your environment (WSL on developer laptops, Azure VM images, AKS nodes):
• Run the technical verification checklist above to determine presence of affected code.
• If a kernel includes the AMD DRM code and the running version falls within a known affected range, prioritize kernel updates or mitigations.
• Ingest Microsoft’s CSAF/VEX feeds into your vulnerability management pipeline. Microsoft’s machine-readable attestations are designed to let enterprises automate scope determination and reduce false positives.
• Maintain a cadence of verifying vendor advisories (Linux distro vendors, AMD, and cloud providers) for fixes and configuration guidance. Where Microsoft or other vendors provide patches, apply and verify in staging before production rollout.

---

Caveats, uncertainties, and things to watch​

• Kernel configuration variability: Whether a Microsoft-supplied kernel build includes the drm/amd/display code is a function of the kernel config Microsoft used when building that kernel. That configuration can and does change between releases. Therefore, it is not possible to say “all WSL kernels are/are not affected” without inspecting the specific kernel build or Microsoft’s attestation for that product.
• Distribution backports: Some vendors backport security fixes into older kernel branches rather than upgrading kernel version numbers. When comparing running kernels against vulnerability ranges, take vendor advisories and backports into account.
• Cloud GPU virtualization diversity: The way GPUs are presented to guests varies between clouds and VM SKUs. Some GPU features are virtualized on the host and do not run guest-side amdgpu DRM code; others provide passthrough and rely on guest drivers. Determine the specific GPU virtualization model for your Azure VM types.
• Rapid developments: Microsoft has committed to updating CVE entries and VEX attestations when additional impact is identified. Security teams should monitor those feeds closely; the scope could expand or be further clarified after additional analysis.

---

Conclusion​

Microsoft’s public VEX/CSAF work gives Azure Linux customers clear, machine-readable guidance about the status of CVE-2025-21985 for that distribution. That does not mean Azure Linux is the only Microsoft product that could include the vulnerable drm/amd/display code. The underlying vulnerable code lives in upstream Linux kernel sources; any Microsoft product that ships or uses a Linux kernel built with those AMD DRM/display components could, in principle, be affected. WSL is an especially plausible candidate for examination because Microsoft builds and ships a Linux kernel for WSL; the presence or absence of the vulnerable paths depends on the kernel configuration and the runtime environment (GPU availability and exposure). The prudent course for organizations is to inspect their Microsoft-hosted and Microsoft-supplied Linux kernels (including WSL instances and Azure guest images), apply vendor patches, and automate scope decisions using machine-readable CSAF/VEX feeds that Microsoft and other vendors now publish. Where certainty is lacking, enforce conservative mitigations (restrict device access, apply kernel updates promptly) and continue monitoring vendor advisories for updates or expanded product attestations.

---

Source: MSRC Security Update Guide - Microsoft Security Response Center