A recently disclosed Linux-kernel defect tracked as
CVE‑2025‑23144 exposes a locking error in the LED backlight subsystem (led_bl) that can produce kernel lockdep warnings, race conditions and availability failures during LED/backlight removal; the upstream remedy is a small but necessary change that ensures the
led_access mutex is held when calling
led_sysfs_disable(), and administrators should prioritize kernel updates or vendor firmware patches to eliminate the risk.
Background / Overview
The flaw arises inside the kernel's LED class and a video/backlight driver frontend that registers LED-backed backlights. At a high level, the kernel exposes two complementary helpers —
led_sysfs_enable() and
led_sysfs_disable() — that flip an internal flag describing whether the sysfs LED interfaces are available. Those helpers must be called while holding the per‑LED
led_access lock to guarantee consistent access and to prevent concurrent thread interactions from leaving internal state inconsistent. The vulnerability description, as recorded in the public CVE entry, is explicit: kernel Lockdep detects calls to
led_sysfs_enable() and
led_sysfs_disable() that violate the expected locking discipline during led-backlight removal.
The practical upshot is
availability risk. The failure mode involves kernel warnings, possible deadlocks or crashes, and predictable instability while the affected code path is exercised. Vendors and distribution trackers who reviewed the upstream patch and kernel commit assigned the issue a medium severity (CVSS v3.1 base score ~5.5 in some vendor trackers) and recorded it as an availability‑oriented hazard requiring corrective action.
This CVE was published publicly on 1 May 2025, and upstream stable‑kernel trees received a targeted correction that adds the required mutex acquisition and release around the sysfs toggle during device remove paths. The fix is intentionally small — a handful of lines — but it enforces an important synchronization invariant that had been overlooked when the led-backlight driver was added.
Why this matters: the LED subsystem and real-world exposure
The Linux LED subsystem is used far beyond decorative keyboard LEDs. It is embedded in platform backlights, camera flash/torch drivers, audio mute indicators, power and status lights on embedded gear, and some small-form-factor display backlighting implementations. The LED class provides a uniform sysfs interface for brightness and trigger attributes, and that interface is what userland components, firmware helpers and other kernel subsystems interact with. Because the LED subsystem acts at the intersection of multiple kernel layers, a synchronization lapse in the LED code can ripple into higher-level device remove/reprobe sequences or workqueue processing and cause
host-level instability.
Key points on exposure and reach:
- Many distributions ship kernels that include the affected drivers; distribution trackers (Ubuntu, Amazon Linux, Debian and others) classified the defect as medium and noted fixes and backports in specific kernel versions.
- The bug is not remotely exploitable in the sense of a network attack against an unprivileged, unauthenticated attacker; it is a local kernel synchronization defect. Most vendor scoring yields the vector elements AV:L/PR:L (local, low privileges required), UI:N (no user interaction), and an impact focused on availability.
- Although the immediate attack surface is local, devices with multi‑tenant access (shared user accounts, exposed shell or service accounts) or remote command execution bugs could see this defect chained with other primitives to create a denial‑of‑service outcome.
Put simply: while this is not a remote wormhole, it is the kind of kernel‑level correctness bug that can crash or hang a host when exercised, and that makes it operationally important for servers, embedded devices and endpoints where uptime matters.
Technical anatomy: what the code and the fix do
The LED class defines a per‑LED lock named
led_access (a mutex) inside
struct led_classdev. The helper functions
led_sysfs_disable() and
led_sysfs_enable() assert at runtime that callers hold this lock — the code uses a lockdep assertion to ensure correct locking discipline. When a caller violates that invariant, the kernel's lock dependency checker (Lockdep) emits a warning, and the offending call pattern often appears during device removal or probe/unbind/rebind sequences involving the led_bl backlight driver. The source tree reveals the assertions and the fields involved, and the stable‑kernel patch makes the remove path acquire the mutex before toggling the sysfs state.
A functional summary of the fix:
- The vulnerable sequence iterated over LED classdevs during backlight removal and called led_sysfs_enable() / led_sysfs_disable() without taking led_access. Because the helper functions themselves expect the lock to be held, this generated Lockdep warnings and created a concurrency window.
- The patch wraps the per‑LED call with *mutex_lock(&priv->leds->led_access) and mutex_unlock(… )** so the lock is held around the sysfs toggle during the loop. That change enforces the locking discipline and removes the Lockdep warnings and potential races.
Why Lockdep matters: Lockdep does not itself make code crash, but it surfaces inconsistent lock ordering or missing lock assertions that typically indicate an underlying race or deadlock risk. When Lockdep warns in kernel boot or during operation, it is a signal that a concurrency invariant is broken and that the code path could lead to deadlocks or inconsistent state under real load.
Patch, backports and vendor responses
The upstream kernel received a minimal patch that adds the required mutex acquisition and release around the sysfs toggles in the led_bl removal path. The correction has been applied to stable branches and backport queues for several long‑term kernels; stable patch notices and review emails document the commit and the reasoning behind it.
Distribution tracking and remediation status varied across vendors:
- Ubuntu assigned the vulnerability a Medium priority and recorded a CVSS 3.1 score consistent with a local availability impact. Administrators using Ubuntu‑packaged kernels should consult their release advisory for the patched package versions and update accordingly.
- Amazon Linux (ALAS) listed the issue with a CVSS 3.1 base score of 5.5 and included it in its vulnerability tracker; ALAS entries indicate whether the distribution’s kernels are affected or not.
- Red Hat / RHEL‑related trackers and third‑party vulnerability aggregators observed the issue but the RHEL packaged status can differ depending on whether the vulnerable code is present in that specific RHEL kernel ABI; some packaging entries flagged “no-fix-redhat-rpm-package” where the upstream header or package was not shipped in the same form. Operators running vendor‑supplied RHEL kernels must rely on Red Hat advisories for exact exposure and backport guidance.
The important operational takeaway: the upstream patch is small and available, but whether a given machine is fixed depends on the vendor/kernel build you run. Vendors that maintain long‑term kernels will either backport the change into their stable packages or provide instructions to rebuild or update kernels.
Impact and exploitability: realistic scenarios
CVE‑2025‑23144 is primarily an
availability defect rather than a confidentiality or integrity issue. The common impact outcomes are:
- Lockdep warnings and kernel oops — a diagnostic symptom observed in logs that indicates the locking invariant was violated. Repeated invocation of the offending sequence may escalate to more severe failures.
- Deadlocks or hangs during device remove or workqueue processing if the underlying race is triggered under the right timing conditions. That can cause persistent service disruption until the host is rebooted or the kernel recovers.
Exploitability constraints:
- Local access required. The CVSS vector and vendor trackers mark this as local (AV:L) and requiring at least low privilege (PR:L). That means an attacker needs the ability to execute code locally or send local kernel operations that exercise the relevant device remove/probe flow.
- Not remotely exploitable on its own. There is no indication of network‑facing exploitability that allows remote attackers to trigger the vulnerable code without some local foothold.
- Chaining is possible. In complex environments a remote compromise that yields limited local command execution could be chained to intentionally trigger device removal events or manipulate sysfs entries to provoke the race; that’s why defenders should treat kernel correctness bugs seriously even when they appear local.
In short, this is not an immediate internet‑scale worm risk, but it is a credible denial‑of‑service hazard for hosts that might be exposed to local exploitation vectors, untrusted local users, or management tooling that manipulates device drivers at runtime.
Detection and hunting: what defenders should look for
Kernel lockdep warnings are the most direct indicator that this or a related locking issue has manifested. Look for the familiar kernel log pattern that references
led_sysfs_enable,
led_sysfs_disable,
led_bl_remove and the
drivers/leds/led-core.c stack traces. Example log fragments (summarized) that indicate the issue:
- "WARNING: ... at drivers/leds/led-core.c:XXX led_sysfs_enable+0x..."
- Call trace containing "led_sysfs_enable", "led_bl_remove" and "platform_remove" or "device_remove"
Operational detection steps (quick, actionable):
- Inspect kernel logs (journalctl or dmesg) for LED/sysfs relevant entries:
- journalctl -k | grep -i led_sysfs_enable
- dmesg | grep -i led_bl_remove
- Search for Lockdep warnings more broadly:
- journalctl -k | grep -E 'lockdep|led_sysfs|led_bl_remove|drivers/leds/led-core'
- Include these event patterns in SIEM rules or EDR kernel‑log collectors so occurrences are searchable and alertable.
If you see these warnings on multiple hosts, prioritize investigating the kernel version and distribution kernel package — a widespread pattern suggests either a widely shipped kernel tree or a recurring device driver activity that triggers the path (for example, frequent thermal, ACPI, or hotplug operations).
Caveat: Lockdep warnings themselves are not proof of a successful attack; they are evidence of an invariant violation. Treat them as high‑priority operational incidents because they point to kernel correctness issues that can destabilize systems.
Practical mitigations and remediation steps
This section provides prioritized, concrete actions for teams responsible for Linux infrastructure, embedded devices and vendor stacks.
- Patch first, mitigate second
- Apply vendor kernel updates that include the upstream fix. For mainstream distributions, monitor your vendor’s security advisory and install the updated kernel package during a maintenance window. Upstream patches appeared in stable branches and were backported to several LTS kernels; vendor packages (Ubuntu, Amazon Linux and others) recorded fixes in their trackers.
- For embedded / firmware‑constrained devices
- Contact your hardware vendor for firmware or kernel rebuilds that include the patch. If you manage bespoke embedded images, incorporate the upstream commit into your build tree and rebuild the kernel or the affected module. The upstream change is small, so a focused backport is usually practical.
- Temporary workarounds where patching is delayed
- If the backlight driver is modular and the system can tolerate its absence, consider unloading or blacklisting the led_bl module as a stopgap. Warning: many backlights and LED functions are integrated into platform device drivers and may be built into the kernel image, so blacklisting may not be possible. Test thoroughly before deploying this option.
- Reduce exposure to local threat actors: restrict untrusted local accounts, harden remote execution tooling, and limit who can load/unload kernel modules or manipulate device topology.
- Operational controls and monitoring
- Add detection rules for the Lockdep/log patterns described above in your log aggregation and alerting platform.
- Maintain kernel change windows and test the updated kernel in staging to ensure the backlight/LED behavior remains correct on target hardware.
- Incident response thought‑path
- If you detect Lockdep warnings referencing LED sysfs helpers, treat hosts as having experienced a kernel correctness incident. Collect full kernel logs, preserve /var/log/dmesg and /var/log/kern.log, and consider a controlled reboot after evidence collection to restore stable operation.
These steps aim to remove the defective code path (via patching), reduce the chance it will be triggered (hardening and operational controls), and improve your ability to detect when the issue occurs (logging and SIEM rules).
Risk tradeoffs, vendor nuance and supply‑chain considerations
A few important operational cautions:
- Patching kernels in production is non‑trivial for many organizations. Kernel updates change ABI and can affect drivers and third‑party kernel modules. Test updated kernels in representative staging environments before wide rollout.
- Vendor-built kernels may already contain a backport or a different remediation strategy; always consult the distribution or vendor advisory for your specific kernel package. For example, Amazon Linux and Ubuntu published package‑level tracking for this CVE, while Red Hat packaging status varies depending on whether the exact upstream header/package is included in the RHEL kernel build. Relying on vendor advisories avoids applying the wrong patch or rebuilding a kernel in an unsupported way.
- Embedded and IoT devices are frequently behind in kernel maintenance; vendors of those devices should be asked for firmware updates. If a device is end‑of‑life and cannot be updated, treat it as a higher‑risk node and isolate it from critical services.
Finally, while the immediate security classification is
availability‑centric, even availability bugs can be weaponized as part of larger intrusions (for example, to frustrate forensics or to maintain an attacker’s foothold). Treat kernel correctness fixes with the same operational urgency as other critical infrastructure updates.
Recommendations — prioritized checklist for operators
- Inventory
- Identify all hosts and devices running Linux kernels that include the LED backlight driver (search package lists and kernel configs). Confirm whether the driver in question is compiled in or modular.
- Patch
- For each affected kernel, apply the vendor‑supplied kernel updates that include the upstream fix. If vendor packages are unavailable, plan a safe kernel rebuild that incorporates the upstream commit.
- Monitor
- Add kernel log detection for Lockdep entries referencing led_sysfs_enable, led_sysfs_disable, and led_bl_remove. Alert on hits and treat them as high‑priority operational events.
- Isolate legacy devices
- For unpatchable embedded devices, restrict network reachability, segregate them onto management VLANs, and limit local operator access.
- Test and validate
- Once patched, validate that backlight and LED behavior is correct on representative hardware. Verify absence of the Lockdep warnings in boot and device remove sequences.
- Vendor follow‑up
- If you rely on vendor images (OEM laptops, SoC boards or appliance firmware), confirm with the vendor whether their published device images include the backported fix.
Follow this prioritized path: inventory → patch → monitor → isolate → test → vendor follow‑up.
Closing analysis — what this CVE teaches us about kernel correctness and defense
CVE‑2025‑23144 is a textbook example of a correctness bug that looks small in code but can have outsized operational impact. The fix required holding the established per‑LED lock around a simple flag toggle — a one‑line or a handful‑of‑lines change in context — yet the absence of that lock violated an invariant enforced by Lockdep and produced observable kernel warnings and potential availability failures. That pattern repeats across kernel subsystems: small synchronization mistakes cause large systemic headaches.
Defenders should take three enduring lessons from this disclosure:
- Instrumentation matters. Kernel developers added lockdep checks and explicit assertions that made the problem visible; equivalent instrumentation in other systems helps find similar mistakes early.
- Small fixes can be high value. Even tiny upstream commits must be tracked and applied because they close correctness holes that can be used in denial‑of‑service or follow‑on attack chains.
- Visibility + process beats panic. Rapidly inventorying affected hosts, applying vendor patches or planned backports, and instrumenting monitoring for the exact Lockdep messages yields the fastest and safest route to operational resilience.
In operational terms: treat the CVE as a maintenance high‑priority rather than an emergency requiring immediate system‑wide shutdowns. Plan updates through your normal patch windows, but ensure detection, vendor confirmation and backport options are clearly documented so you can act quickly if you observe lockdep warnings in the wild.
CVE‑2025‑23144 is a reminder that kernel‑level synchronization mistakes are both subtle and impactful. The technical remedy is straightforward and available upstream; the human and operational work lies in tracking affected kernels, managing timely updates across diverse hardware fleets, and ensuring detection capabilities are in place so faults are discovered before they become outages.
Source: MSRC
Security Update Guide - Microsoft Security Response Center*