A subtle concurrency bug in the Linux kernel’s I²C LPI2C driver — tracked as CVE‑2024‑40965 — can produce a system‑level deadlock when clock rate queries are made while a device probe or transfer is in progress, and upstream maintainers fixed it by locking and caching the clock rate instead of calling clk_get_rate repeatedly during transfers.
Background
The Linux kernel’s device driver stack coordinates dozens of subsystems when a board or peripheral is initialized: clocks, regulator frameworks, I²C buses, and the driver core itself. Those interactions are normally safe, but when two subsystems lock mutually dependent resources the result can be a deadlock that halts progress — and this is exactly the class of problem CVE‑2024‑40965 addresses.
Observers reported the deadlock in practice when adding the tlv320aic32x4 audio codec: the codec’s clock provider tried to register or modify a clock while holding the kernel’s clock mutex, and that code path needed I²C access; the LPI2C driver, in turn, was calling clk_get_rate during its transfer path and therefore attempted to take the same clk mutex again, creating a circular wait. The practical consequence is a kernel hang or severe availability impact when the affected hardware or initialization path runs.
Why this bug mattered operationally: the defect is not a remote RCE or data leak — it’s an
availability issue. When the deadlock triggers, the affected kernel threads stop making forward progress and the system may become unresponsive with respect to the I²C subsystem or device initialization that depends on it. That makes the risk immediate for embedded platforms, multimedia stacks, or any environment that probes the same clocks and I²C peripherals during boot or runtime attach operations.
What exactly was wrong: a technical overview
The code path and the mutex semantics
At the heart of the problem is an interaction between:
- the kernel’s common clock framework (CLK), which serializes some operations behind a mutex to protect clock data structures and registration, and
- the LPI2C driver — a low‑power I²C host controller driver — which, during the transfer path, queried the current parent clock rate by calling clk_get_rate.
Calling clk_get_rate is usually safe, but not when the caller already holds the clock framework’s mutex — or when a clock provider holds that mutex and, while holding it, needs to perform an I²C operation that invokes the LPI2C transfer code. Because clk_get_rate internally takes the clk mutex in some code paths, repeated calls to it from transfer contexts create the possibility of a circular lock dependency: the clock provider waits on I²C, and the I²C transfer code waits on the clock mutex. That circularity is the textbook deadlock.
The minimal fix
Rather than querying the clock rate repeatedly during every transfer — and therefore re-entering the clock framework lock — maintainers modified the LPI2C driver to:
- read and cache the clock’s rate under an appropriate lock, and
- use that cached rate for the duration of the transfer instead oe each time.
This is a conservative, robust change: it reduces the number of interactions with the clock framework from per‑transfer to per‑configuration, eliminating the circular dependency that produced the deadlock. The patch was accepted upstream and backported into stable kernel trees and distribution packages.
Impact and affected environments
Who is exposed?
- Embedded devices and SoC platforms that rely on the LPI2C host controller driver and that have clock providers which themselves perform I²C operations (for example, audio codecs like the tlv320aic32x4).
- Systems where kernel builds include the LPI2C driver (either built‑in or as a module) and that run kernel versions prior to the upstream fixes being backported by vendors/distributions.
- Custom or vendor kernels that have not been updated or that have vendor‑specific clock/I²C integration code which could reintroduce the same pattern.
Multiple distribution security advisories (Ubuntu, SUSE, Red Hat family trackers) listed the vulnerability and shipped fixes in their kernel packages, showing it had practical reach across mainstream Linux distributions.
Severity and attacker model
CVE‑2024‑40965 carries a CVSS v3.1 base score in the mid range (published scoring places it at
5.5 — Medium) because the exploit path is
local and requires the attacker to cause the kernel to follow the probe/transfer path that triggers the deadlock. The real operational severity is availability‑focused: successful triggering results in a system hang or kernel liveness failure that affects the I²C device and potentially the wider system. This maps to a high impact for availability even though confidentiality and integrity are not directly affected.
In practical terms the attacker model looks like:
- Local unprivileged user (or a local process) performs actions or loads hardware that cause the kernel to probe devices that exercise the LPI2C transfer path; or
- Malformed firmware/device interactions during hot‑plug or runtime device registration cause the clock provider and I²C paths to intermix in the problematic way.
Because it’s not a remotely triggered bug (no network vector), widespread remote exploitation is unlikely — but the bug can be triggered inadvertently during normal hardware enumeration on affected platforms, which is why operators should treat availability fixes as priority patches for affected systems.
Patches, timeline, and vendor responses
What maintainers changed
The upstream kernel commit set replaces repeated calls to clk_get_rate during transfers with a cached value guarded by a brief lock, ensuring the clock framework isn’t reentered from the transfer path. The change was merged into the stable trees and referenced in distribution advisories. Multiple kernel stable commits are associated with the fix and were linked from vulnerability aggregators and vendors’ advisories.
Distribution and vendor advisories
Major distributions issued kernel updates that include the fix or backports:
- Ubuntu published security notices and included the kernel fixes in their LTS kernels. The Ubuntu security page documents the issue and the introduced/fixed commits.
- SUSE listed the vulnerability in its update announcements and shipped patched kernel packages.
- Red Hat and other downstream vendors referenced the upstream change in their advisories; aggregator pages such as cvefeed and OSV reflect the vendor references and commit IDs used for tracking.
These vendor advisories typically describe the fix and list the kernel package versions you should install to remediate the condition on your platform. Administrators should treat kernel updates that include this fix as
availability‑critical patches for affected systems.
Backports and stable branches
The kernel change was small and surgical, which made it straightforward to backport to stable kernel branches. That means even long‑lived distribution kernels (for example, 5.15 and similar long‑term series) received vendor patches. Operators running vendor kernels should prefer vendor‑provided packages over home‑built cherry‑picks to ensure the full context of distribution backports and test coverage is included.
Practical mitigation and remediation guidance
If you manage Linux systems — from cloud VM images to embedded devices — follow this prioritized checklist.
1. Inventory: confirm whether you are affected
- Identify hosts that load the LPI2C driver or that use SoCs/drivers known to include lpi2c. Check
lsmod, dmesg logs, or your kernel configuration (CONFIG_I2C_IMX_LPI2C, or related symbols).
- Review whether your platform uses clock providers that could interact with I²C (audio codecs and certain SoC clock drivers are common culprits).
- Consult your distribution vendor’s advisory list for kernel versions that include the fix. Distribution advisories from Ubuntu and SUSE list the affected packages and fixed versions.
2. Apply patches: preferred remediation
- Upgrade the kernel to the vendor package that contains the backport. Use your distribution’s package manager to apply kernel updates and reboot according to normal maintenance windows.
- For embedded devices where vendor updates are slow or unavailable, contact the OEM for a tested update. If you build kernels yourself, merge the upstream patch (or the vendor backport) into your maintenance branch and rebuild, but prefer vendor packages when they exist.
3. Short‑term mitigations (if immediate kernel updates are infeasible)
- Avoid loading the specific hardware profile or device tree nodes that trigger the deadlock until you can patch. For example, if the tlv320aic32x4 codec is only used in an optional audio path, temporarily disable that DT node or kernel driver binding.
- If the device is only probed at runtime, consider preventing automatic probe via driver bind/unbind controls or disabling device tree binding lines. These workarounds are operationally risky and should be used only as temporary measures.
- Isolate affected hardware (physically remove or disable) on critical systems where uptime and responsiveness are vital and a reboot is planned to apply the patch at the earliest maintenance window.
4. Test and monitor after patching
- After applying the kernel update, reboot and validate that I²C devices initialize cleanly and that the previously observed deadlock scenario no longer reproduces.
- Monitor dmesg, kernel logs, and system responsiveness during device enumeration and hot‑plug events to ensure there are no regressions.
Why this fix is the right balance
The remedy — locking and caching the clock rate once instead of calling into the clock framework repeatedly — is a conservative design fix that:
- Removes the circular lock dependency by minimizing cross‑subsystem reentry, and
- Preserves runtime performance (reading the clock rate per transfer is unnecessary overhead and not materially different from using a cached, validated rate for a transfer window).
This approach is low‑risk because it does not change the semantics of clock handling for other callers; it simply reduces the number of times the LPI2C transfer path crosses the clock framework boundary. Upstream maintainers accepted that reasoning and merged the change into stable trees, making it an appropriate and tested correction.
Operational considerations and risks
Hidden dependencies and the complexity of hardware initialization
Hardware bring‑up on modern SoCs is often a choreography of many subsystems executing callbacks and probing paths in a shared, global context. That complexity can hide fragile ordering and locking assumptions. CVE‑2024‑40965 is a reminder that a small, innocuous helper call (clk_get_rate) can create a systemic failure when the interplay of driver probe sequences and framework locks is not fully anticipated.
Patch availability vs. environment constraints
While most mainstream distributions published fixes, some embedded vendors have long maintenance cycles or tightly controlled kernel trees. Operators of appliances, IoT devices, and specialized hardware must factor the patch backlog into their risk management and may need to pursue OEM updates or deploy mitigations until a vendor‑certified kernel is available. Distribution packages and upstream patches are available and were backported; act on vendor guidance and test updates on representative hardware.
Attack surface: low for remote actors but significant for device stability
Because the vulnerability requires local trigger conditions or hardware probe events, it is unlikely to be an open remote exploitation pathway. However, that does not reduce the operational urgency: inadvertent triggering during normal use — such as module insertion, late device enumeration, or certain runtime firmware updates — can still cause severe availability loss.
Recommended actions for administrators and device maintainers
- Immediately plan to install vendor kernel updates that include the backport for CVE‑2024‑40965. Prioritize devices that run audio or multimedia stacks that probe clock providers during runtime.
- For systems where immediate updates are impossible, identify and disable the minimal set of drivers or device‑tree nodes that can reproduce the condition, and schedule a maintenance window to apply vendor patches.
- For OEMs and integrators: ensure your kernel configuration and platform bring‑up do not rely on probing sequences that can produce cross‑subsystem lock inversions; run stress and dual‑path boot tests that exercise clock/I²C interactions to detect similar issues early.
- Include the CVE and its mitigation status in your SOC/incident triage runbooks for availability incidents tied to hardware enumeration or I²C device initialization.
Final analysis: strengths of the fix and remaining caveats
Strengths
- The upstream fix is small, well‑scoped, and avoids invasive rearchitecting while removing the critical circular lock dependency.
- It was backported promptly to stable kernels and integrated into vendor packages, so operators who apply vendor updates get a tested remedy.
- The fix preserves transfer‑path performance and minimizes behavioral change.
Caveats and residual risks
- Embedded and OEM kernels that lag upstream/stable trees may still carry the vulnerable code; vendors must push validated updates. Distributors’ backports exist, but patch rollout may be slow for bespoke hardware.
- The fix addresses the specific clk_get_rate reentry pattern, but similar lock inversions can exist elsewhere; platform integrators should audit other cross‑subsystem call paths (e.g., regulators, pinctrl, and firmware loaders) for comparable risk.
- Administrators must still test kernel updates on representative hardware: changes to clock handling or caching can reveal subtle platform‑specific behaviors that only show up during full device bring‑up.
Conclusion
CVE‑2024‑40965 is an instructive example of how concurrency assumptions and cross‑framework interactions in kernel code can produce an availability hazard that’s small to patch but large in operational effect. The remedy — cache the clock rate and avoid repeated clk_get_rate calls during transfers — is effective and already distributed via upstream and vendor backports. Administrators and integrators should treat this as an availability priority: inventory affected systems, apply vendor kernel updates, and validate on representative hardware. For embedded and OEM platforms that cannot update immediately, temporary mitigations (device tree masking or driver disablement) can reduce exposure until a tested kernel patch is available.
Source: MSRC
Security Update Guide - Microsoft Security Response Center