i915 hwmon devm fix: patch fixes CVE-2024-39479 UAF risk

A small change in the Intel i915 graphics stack — a decision to “get rid of devm” in the hwmon path — produced a classic kernel lifecycle bug with outsized operational impact: tracked as CVE‑2024‑39479, the defect creates a use‑after‑free (UAF) and local denial‑of‑service vector by letting hwmon sysfs attributes be touched after their driver backing data has already been released.

Background / Overview​

The affected code lives in the Linux kernel’s Intel DRM driver, commonly referenced as drm/i915, and touches the hwmon (hardware monitoring) glue that exposes device sensors through sysfs. In modern kernels, device resources are often managed with the device-managed resource helpers (the devm_ family), which simplify cleanup by deferring frees to the device’s teardown path. In this case the interaction between two device‑managed objects — hwmon and the hwmon’s drvdata (the driver data hwmon depends on) — created a race where the release order could vary between code paths. When drvdata* is freed before the hwmon sysfs representation is torn down, later sysfs accesses can follow a dangling pointer and trigger a kernel oops or crash.
This is a classic availability‑first vulnerability: an attacker with the ability to interact with hwmon sysfs can provoke kernel instability or host unavailability. Upstream maintainers chose a conservative, surgical fix: remove device‑managed cleanup for the affected objects and perform explicit, ordered release during device unbind so that the hwmon side is always cleaned up before its backing drvdata. The change was accepted into the stable trees and landed as fixes for multiple kernel branches.

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Why this matters: hwmon, devm, and lifecycle ordering​

What hwmon does and why it’s sensitive​

The hwmon subsystem exposes sensor readings (temperatures, voltages, fan speeds) through sysfs. Those attributes are widely readable on desktop, server, and embedded platforms; many monitoring tools, containerized workloads, and userland utilities interact with hwmon to gather telemetry. Because hwmon lives in the kernel and touches shared driver state, mistakes in cleanup order can surface as UAFs or races that crash the whole system.

Device-managed resources (devm_) are convenient — until they’re not​

The devm_* helpers are used throughout the kernel to reduce the boilerplate of freeing resources during teardown. When used correctly, they guarantee that resources allocated on behalf of a device are released when the device is removed. But devm can hide subtle ordering constraints: if two devm objects have an implicit dependency (A depends on B), devm doesn’t enforce the dependency ordering across separate code paths. When A must be released before B, and the codebase has multiple unbind/teardown paths that free A and B independently, devm’s convenience becomes a hazard. The i915 hwmon case is a textbook example.

The precise failure mode​

Upstream diagnostics and the NVD summary outline two code paths invoked during device unbind that can free either hwmon or drvdata first. If drvdata is freed while the hwmon sysfs interface still exists, a subsequent sysfs read or write that accesses the already‑freed drvdata pointer produces a use‑after‑free. The kernel trace shown in public advisories demonstrates the relevant call frames: devres_release_group, component_unbind_all, i915_driver_remove, pci_device_remove, device_release_driver_internal, and the sysfs unbind_store path that triggers the sequence. This combination allows a local unprivileged actor (or an untrusted containerized workload that can reach the device sysfs) to induce a kernel oops.

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Technical analysis of the fix​

What “Get rid of devm” actually means​

The upstream patch series removes the devm_* allocations for the hwmon device and its associated drvdata, replacing them with explicit allocations and, crucially, explicit release steps in the driver’s remove/unbind path. The intent is simple and deliberate:

• Stop relying on device-managed auto‑cleanup where the system cannot guarantee release order across multiple code paths.
• Explicitly free hwmon sysfs entries and unregister hwmon devices in the correct sequence during i915 device removal.
• Ensure that two separate teardown flows cannot release backing data before sysfs nodes are removed.

This change reduces subtle inter‑path ordering dependencies and turns a brittle, implicit contract into a straightforward, auditable sequence of operations. Mailing‑list discussion and the patchwork CI results show multiple revisions, code cleanup, and reviewer feedback before the final revision was accepted.

Why this approach is safer than alternatives​

There are a few ways the kernel community could have handled the problem:

• Add explicit locking or reference counting to cover every ordering — expensive and error prone.
• Keep devm but restructure all callers so the devres group is guaranteed to release in the right order — invasive and fragile.
• Remove devm for the objects in question and implement explicit teardown ordering — surgical, minimal scope, and localizes the lifecycle logic where it belongs.

The third option is the one chosen. It is conservative: fewer surprises, easier to reason about, and simpler to audit. The upstream maintainers also iterated on the patch to clean up static‑analyzer warnings and edge‑case handling during probe error paths.

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Affected versions, severity, and real‑world impact​

What kernels are fixed and where to look​

Vendor advisories and the stable trees mark this flaw as fixed in the following points (representative list compiled from vendor trackers and upstream announcements):

• Fixed in stable maintenance candidates: kernel 6.6.34, kernel 6.9.5, and included as a fix in the 6.10 development stream. Distribution backports vary; some vendors have included the patch in point releases and security errata. Operators should consult their distribution security advisories and kernel changelogs for exact package releases.

Multiple vulnerability databases classify the issue as a use‑after‑free that can lead to kernel crashes and sustained availability loss during exploitation; some scoring engines described the defect as high severity driven by the potential for local kernel instability. The practical exploitability remains constrained by the attack surface: an attacker needs local access or the ability to execute code in a namespace that can interact with the i915 device nodes or hwmon sysfs. That said, in multi‑tenant environments and containerized hosts where devices are exposed to untrusted workloads, the operational impact is material.

Real‑world scenarios where this matters​

• Desktop or laptop systems with local unprivileged users who can interact with hwmon via sysfs or userland utilities.
• Multi‑tenant servers or hosting nodes that expose GPU or DRM device nodes to guest containers.
• Developer machines, CI runners, or cloud images that mount host /sys or /dev interfaces into untrusted workloads.
• Embedded appliances that permit userland access to hwmon for telemetry but do not isolate kernel devices.

In each of these scenarios, an unprivileged actor could craft a sequence of operations that triggers the release inversion and cause host instability or repeated reboots. The problem is not a silent data exfiltration bug — it’s an availability/denial‑of‑service issue with a deterministic crash primitive in the wrong hands.

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Recommended remediation and mitigations​

Primary remediation: patch the kernel​

• Install vendor kernel updates that include the i915 hwmon fix. Look for kernel packages that reference the i915 hwmon change or the CVE identifier CVE‑2024‑39479 in their errata. Many major distributions included the patch in their stable kernels following the upstream merge.
• If you maintain your own kernel builds, pick a stable tree that includes the fix (for example, the 6.6 or 6.9 stable branches after the respective point releases noted above) and rebuild/test. Verify the patch is present by inspecting the i915 hwmon code path for explicit unbind/free calls rather than devm-managed calls.

Mitigations where patching is slow or impossible​

• Limit access to hwmon and DRM device nodes. Restrict read/write permission on /sys/class/hwmon and /dev/dri to trusted users only. Remove world‑read or container‑mount exposure where possible.
• Do not expose host device nodes to untrusted containers. Avoid bind‑mounting /dev/dri or /sys/class/hwmon into workloads that run untrusted code.
• Unload or blacklist the i915 module if the host does not require the integrated Intel graphics (note: this is disruptive — devices will lose graphics functionality and might require a reboot).
• Compartmentalize telemetry: use userland agents that poll hwmon on behalf of monitoring systems and export sanitized metrics, rather than letting multiple processes access the kernel sensors directly.
• Monitor kernel logs for repeated i915 oopses or WARNs indicating an exploitation attempt or repeated instability; preemptively isolate or reboot hosts in controlled ways rather than allowing cascading failures.

Those mitigations are stopgaps; the only robust remedy is to apply the upstream fix through kernel updates.

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Operational guidance for sysadmins and cloud operators​

A practical checklist​

• Inventory exposed kernel device interfaces across hosts and containers. Identify machines that expose /dev/dri or /sys/class/hwmon to untrusted workloads.
• Prioritize patching of machines that are multi‑tenant, run CI/CD workloads, or host third‑party containers.
• Apply vendor kernel patches as soon as they are available and validated in your environment. Test on a staging cluster before widespread rollout.
• Where immediate patching is infeasible, apply mitigations (restrict permissions, remove device bindings, unload i915).
• Add log detection rules to flag i915‑related kernel oopses and repeated sysfs errors; integrate alerts into your incident handling playbook.

What to expect when you patch​

Patching kernel code in the DRM stack requires reboots or live‑patching infrastructure that supports out‑of‑tree live kernel patching (and note that many live‑patch systems do not cover deep driver logic). Expect at least one controlled reboot per host when you upgrade the kernel package unless your environment supports and trusts kernel live patches.

Verifying the fix​

After updating, verify:

• The kernel version matches the patched point release from your vendor.
• No i915 hwmon devm allocations remain in the driver (if you inspect the source): the hwmon allocation and the drvdata release should be explicitly ordered in the remove/unbind path.
• No i915 tracebacks or WARN messages appear in the kernel log under typical workloads.

If you cannot verify source changes directly, rely on your vendor’s security advisory text that references CVE‑2024‑39479 or the descriptive commit messages included in kernel changelogs.

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Security and risk assessment​

Exploitability​

The vulnerability is exploitable with local access to the i915 device or hwmon sysfs — it is not a remote network service bug. That lowers the bar compared with remote RCE defects, but the threat is meaningful in environments where untrusted code runs locally or where containers share host devices.

Impact profile​

• Availability: High. The primary impact is host instability or sustained denial of service via kernel oopses and crashes.
• Confidentiality / Integrity: The public advisories focus on availability/UAF; while UAF can sometimes be escalated into code‑execution on highly targeted architectures, the documented outcome and vendor guidance have been availability‑centred. Treat the immediate risk as DoS and plan accordingly.

Broader implications for device-managed resources​

This bug is a reminder that convenience APIs in kernel subsystems (devm_* helpers) are not a panacea. They reduce code size and make resource leaks less likely, but they can mask implicit object dependencies and create fragility when multiple teardown flows exist. Driver authors and reviewers should scrutinize devm usage whenever an object has a strict release ordering requirement.

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Developer‑facing notes: review and hardening tips​

Coding practices to prevent similar bugs​

• Avoid devm_* for objects with strict cross‑object dependencies that must be torn down in a defined order.
• When devm is used, ensure a single owner/teardown path — do not scatter devm allocations across code paths that may be released in different sequences.
• Use explicit unregistration/unlink calls for sysfs and device interfaces before releasing backing data.
• Add assertions and kernel debug checks that verify object lifetimes during driver remove paths; these will catch ordering regressions early in CI.

Static analysis and CI​

Static analysis tools and careful checkpatch reviews were involved in upstream revisions to this patch series. Running sparse, KUnit tests, and targeted CI checks on the i915 tree can catch lifecycle and reference counting surprises before they reach stable trees. The patch evolved over several revisions to eliminate analyzer warnings and to reconcile error paths around probe failures.

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What we learned — and what to watch for next​

This CVE is instructive for two related reasons. First, it demonstrates a common class of kernel bugs where resource management convenience intersects with complex lifecycle semantics and multiple teardown code paths. Second, it exposes an operational blind spot: device telemetry (hwmon) is often treated as a safe, read‑only channel, but exposing that channel directly to untrusted code can create deterministic crash primitives.
Operators should treat hwmon as a privileged kernel interface rather than a benign monitoring path: restrict access, patch promptly, and review all device management policies in container orchestration platforms and CI environments.

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Conclusion​

CVE‑2024‑39479 is not a flashy remote exploit, but it is a practical, high‑impact availability bug embedded in one of the most widely deployed kernel drivers. The upstream response — removing devm usage for the affected hwmon objects and enforcing explicit ordered teardown — is a surgical and correct fix. Administrators should prioritize patching affected kernels or applying the mitigations listed above, and developers must treat device‑managed lifecycles with extra care when objects have implicit dependencies.
Keeping a close eye on kernel changelogs, vendor errata, and short‑lived CI traces that mention i915 or hwmon will surface any follow‑on regressions or additional hardening patches; in the meantime, the best remedy is the one already delivered upstream: update your kernels, close the local access pathways for untrusted workloads, and stop treating hwmon as purely innocuous telemetry.

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Source: MSRC Security Update Guide - Microsoft Security Response Center