A subtle NULL-pointer bug in the Linux ACPI code — tracked as CVE-2024-56782 — has been patched upstream but remains a live operational concern for many deployments because it can trigger kernel crashes and sustained denial-of-service conditions when certain local device paths are exercised.
Background / Overview
The vulnerability lies in the x86 ACPI subsystem of the Linux kernel, in a helper that decides whether to skip serial-device (serdev) enumeration for particular controllers. The function at the heart of the issue was calling an ACPI device HID-match helper without first verifying that the ACPI device pointer (commonly named
adev) was non-NULL. That missing check can lead to a
NULL pointer dereference: if a code path calls the routine with no ACPI companion, a dereference of the NULL pointer can occur and the kernel may panic or crash, producing a hard denial of service for the host.
At a technical level, the fix is minimal — add a guard that checks whether
adev is NULL before invoking the HID match routine — but the operational consequences and the distribution-level patching picture are what make this more than a trivia fix. The issue was assigned a moderate CVSS rating based on standard scoring (reflecting its
local, low-privilege attack vector) with a high effect on availability because a kernel panic or hang is a direct way to make systems unusable.
What the bug actually does (technical summary)
- The vulnerable code runs in the kernel ACPI x86 driver stack; specifically it participates in logic that decides whether to skip creating a serdev controller device for certain ACPI-described hardware.
- Under normal circumstances the helper is called with a controller device that has an ACPI companion device structure attached. The vulnerable check assumed the ACPI companion would always be present and therefore invoked an ACPI HID match helper without a preceding NULL test.
- If that assumption does not hold — for example, because of platform differences, device enumeration ordering, or a crafted local input sequence — the code can dereference a NULL pointer in-kernel.
- A NULL pointer dereference in kernel mode commonly triggers an oops/panic or at minimum makes the kernel thread hit an unrecoverable state. The result is a crash or system lockup, i.e., a denial-of-service (availability) failure.
This is a classic example of a simple defensive-programming omission producing an outsized operational impact.
Severity, exploitability and impact
- Attack vector: Local. The vulnerability requires local access to a vulnerable machine (it is not remotely exploitable as a pure network vector).
- Privileges required: Low. The scoring and vendor analyses indicate low privileges can trigger the condition (meaning an unprivileged local user/process can cause the kernel to hit the NULL dereference under certain conditions).
- User interaction: None. An attacker or local process does not need to trick a user into doing something interactive.
- Primary impact: Availability (High). Confidentiality and integrity impacts are not indicated by the vulnerability itself.
- CVSS summary: The vulnerability received a mid-range base score under commonly used CVSS 3.x metrics because the attack is local but the availability impact is significant. Some vendors also mapped the vulnerability under CVSS v4 with a higher availability weighting.
In operational terms, successful exploitation results in either a sustained loss of availability while the attacker continues to trigger the condition, or a persistent outage if the system requires a manual reboot or repair after a kernel panic. Repeated or automated local triggers can be used to knock hosts offline repeatedly, which is a meaningful denial-of-service vector in multi-tenant or shared-hosting environments.
Affected components and who should care
- The problem exists in the Linux kernel ACPI x86 driver stack. Kernel releases prior to the upstream stable fix are the ones that carry the bug.
- Vendor remediation status varies: upstream kernel maintainers accepted a small stable-fix patch; many major distributions published advisories and have started shipping updated kernel packages or upstream backports. However, not every distributor applies the same release schedule.
- Important classes of systems to review immediately:
- Physical Linux desktops and laptops that boot Linux directly and use ACPI (particularly x86 systems).
- Linux-based servers that are accessible to untrusted local users or that host multi-tenant workloads.
- Virtual machines and container hosts where local user access is possible. (Note: the actual exploitability inside particular hypervisors or container environments depends on how and whether ACPI device enumeration is presented to guests.
- Windows hosts running the Windows Subsystem for Linux (WSL2) or other bundled/custom Linux kernels — you must verify which kernel build is used by WSL and whether that kernel includes the upstream fix; the presence or absence of ACPI handling in those Microsoft-supplied kernels affects exposure.
- Systems that do not use or expose ACPI serdev paths in practice will be less likely to be impacted in real-world operations — but the absence of exposure cannot be assumed without verification.
Vendor patching and distribution status (practical picture)
Upstream kernel maintainers accepted a minimal fix in the stable tree that adds the missing NULL check. That makes the vulnerability patchable at the kernel source level with a small change, and it has been merged into the stable releases referenced by vendor trees.
However, the real-world exposure is driven by how quickly each Linux distribution or vendor ships a kernel package that contains the fix. The patch was incorporated into the Linux stable branch and then has been pulled into vendor kernels and various backport streams at different cadences. The landscape looks like:
- Some distributions applied the stable patch and released updated kernel packages quickly for rolling and current supported releases.
- Several vendor advisories catalog the issue as moderate/medium severity but still flag the availability impact as high in the scoring rationale.
- A number of long-term support kernels or vendor-maintained kernels received backports; conversely, some vendor kernels (notably certain historically slow-maintained or custom kernels) were listed as no fix planned for some older kernel forks. That mismatch means administrators on some platforms may have to either upgrade to a newer kernel series or apply vendor-provided mitigations.
Key operational takeaways:
- If you run kernels older than the patched stable release (the upstream fix was merged into the 6.12.x stable line and backported as appropriate), you should plan to update.
- Production-critical hosts and multi-tenant systems should patch first because the primary impact is availability.
- Systems that cannot be updated quickly should apply compensating controls (see the mitigation section below).
Mitigation and incident-handling guidance
The most reliable fix is to install a kernel that includes the upstream stable patch. Beyond that, practical mitigation steps are:
- Patch and reboot on the maintenance window
- Priority: high for public-facing multi-tenant and production hosts.
- Install vendor-supplied kernel packages that include the stable fix; if a vendor has not published a packaged kernel update, upgrade to a kernel series that has the fix backported.
- Reboot to activate the newly installed kernel.
- If patching is temporarily impossible:
- Restrict local access. Because the attack is local, limiting the set of accounts that can access a machine reduces attack surface.
- Harden user separation and session limits: enforce stricter controls on who can log in locally, disable unneeded accounts, and use PAM or other local access restrictions.
- Deploy monitoring for unexpected kernel oops/panic messages in system logs and set alerts for repeated reboots or kernel crash patterns.
- Consider live-patching services where available (vendor-specific live patching solutions). Livepatch technology can apply trivial fixes without full reboots in many enterprise setups — evaluate whether the patch is suitable for livepatch workflows.
- For cloud or provider-managed kernels:
- Check your cloud vendor image/kernel advisory status. Some images use vendor kernels and may not be patched immediately; review image and AMI updates.
- For managed Kubernetes nodes or virtual machine fleets, plan a controlled rolling-update to replace node images with patched kernels.
- WSL2 and embedded/custom kernels:
- Verify the WSL kernel version on Windows hosts (for example, query the WSL kernel release or run uname -r inside a WSL2 instance). If WSL is using a build that predates the fix, consult the WSL release notes and update the WSL kernel package through the supported mechanism (Windows Update or the WSL update path).
- For custom-built or vendor-supplied kernels (embedded devices, appliances), contact the vendor or rebuild the kernel with the upstream fix applied.
- Detection:
- Monitor syslogs, kdump/ crash reports, and centralized logging for signs of kernel NULL dereference or panic stacks. Repeated, unexplained kernel crashes should be investigated and correlated with local user activity.
In short: patch first, reduce local attack surface second, and add monitoring to detect attempted or accidental triggers.
Why a small fix matters (analysis)
At first glance the change — add a NULL check before calling an HID-match helper — is an almost trivial defensive improvement. But a simple defensive omission in kernel code is dangerous because kernel-level faults bypass nearly every safety net; an unprivileged process that manages to make the kernel dereference NULL can force a panic affecting all processes and services on the host. This is exactly the kind of reliability failure that enterprises dread because it is:
- Easy to trigger under the right conditions (the attack complexity is low).
- Hard to recover from automatically in some deployments (requires manual reboot or operator action).
- Dangerous in multi-tenant environments where one (compromised or abusive) tenant could repeatedly take down a host used by others.
The patch itself is a textbook example of
fail-safe defensive coding: avoid dereferencing pointers before checking them. Accepting such a small change upstream was the correct engineering approach. The continuing risk stems from the heterogeneity of vendor kernels and the operational difficulty of keeping many systems on the latest patched kernel.
Practical checks for administrators (what to run, what to look for)
- Determine kernel version:
- Run: uname -r
- Check whether the running kernel series is older than the patched series in your vendor stream. If your vendor lists a fixed kernel package, compare installed package versions (distribution-specific commands).
- Check boot-time and dmesg logs for ACPI/serdev entries:
- Look for unusual ACPI device enumeration warnings around boot time. Repeated ACPI-related oopses or serdev messages may indicate the affected codepath being hit.
- Inspect vendor security advisories and kernel changelogs:
- If your distribution supplies official advisories, follow those update instructions and rely on the vendor’s recommended kernel packages.
- For WSL2 users:
- Query WSL kernel version via
uname -r inside WSL, and check the WSL documentation or the WSL kernel release listing to confirm whether the build you are using contains the upstream fix. If using the Microsoft-supplied kernel, use the supported WSL update path.
Threat models and risk context for WindowsForum readers
Although this is a Linux kernel issue, Windows admins and enthusiasts should pay attention for several reasons:
- Many Windows developers and sysadmins run WSL2 for development and testing. WSL uses a Microsoft-provided Linux kernel; whether a particular WSL build is impacted depends on which kernel version Microsoft shipped and whether that build integrates the upstream fix. WSL installs using older kernel builds may be exposed in principle.
- Dual-boot machines and multiboot setups that include Linux are directly affected if they run an unpatched kernel.
- Virtualized Linux guests running on Windows hosts (Hyper-V, VirtualBox, VMware) are affected if the guest kernel includes the vulnerable code; the host OS (Windows) itself is not affected by this particular kernel CVE.
- Organizations that manage mixed Windows/Linux environments should ensure Linux kernel patching is part of their broader operational patch cycles and that local user access policies are applied consistently.
Risk timeline and vendor follow-up (what to expect)
- Upstream: small stable-fix patch merged quickly; this is the usual path for a minor but critical defensive fix.
- Distributors: expected to backport the fix into supported kernel branches; however, time-to-publish varies by vendor and by kernel branch.
- Cloud images and appliance vendors: patch windows may be larger. Some vendors may advise upgrading to a newer kernel series rather than backporting.
- Exploits: as of the current review there are no public reports of in-the-wild exploitation or publicly available proof-of-concept exploit code that leverages this specific CVE. That status reduces immediate panic but should not be treated as assurance — local vulnerabilities frequently become part of chained exploit scenarios when combined with other issues.
Flag: the absence of public PoC or confirmed exploitation reports is time-sensitive. Administrators should re-check vendor advisories and trusted vulnerability feeds for updates because the situation can change.
Final assessment and recommendations
- This is a local NULL pointer dereference that leads to potential kernel panic and denial-of-service. While not a remote RCE or data-exfiltration vector, it is operationally relevant because availability loss at kernel level is severe.
- The upstream patch is straightforward and already merged. The real-world task is to ensure the patched kernels reach your systems.
- Priority of action:
- Inventory: identify all machines running affected kernel series (including WSL2 instances, VMs, and appliances).
- Patch: apply vendor-supplied kernel updates that include the fix, and reboot when practical.
- Mitigate: restrict local access where immediate patching is impossible; enable monitoring for kernel crashes.
- Verify: after patching, confirm that nodes boot cleanly and that ACPI/serdev related messages are not generating crashes.
- Additional caution: if you use custom kernels or vendor appliances that do not accept upstream patches easily, engage your vendor or build and deploy a patched kernel image as appropriate.
The vulnerability is a reminder that tiny defensive omissions in kernel code can have outsized consequences. The engineering fix is trivial; the operational challenge — getting the remediation safely and quickly to diverse fleet images and vendor kernels — is the harder work. Systems administrators should treat this one as a patch-and-verify priority for Linux systems subject to local user access, and Windows users running WSL2 or hosting Linux VMs should verify the kernel builds they use and update per vendor guidance.
Source: MSRC
Security Update Guide - Microsoft Security Response Center