A recently disclosed Linux kernel bug, tracked as CVE-2024-24856, is a classic kernel‑space null pointer dereference rooted in the ACPI/ACPICA parsing code that can cause immediate loss of availability by crashing the kernel when ACPI memory allocation fails and the result is used without a defensive check. The defect is narrow, the fix is small (add a NULL check and return AE_NO_MEMORY), but the operational impact is significant for systems that exercise the vulnerable ACPI paths — especially embedded and vendor‑forked kernels where backports lag.
ACPI (Advanced Configuration and Power Interface) and the ACPICA userland/kernel helpers provide firmware parsing and runtime configuration services relied on by many kernel drivers. The affected function, acpi_db_convert_to_package (and adjacent ACPICA helpers), calls ACPI_ALLOCATE_ZEROED to obtain transient storage and then proceeds to dereference the returned pointer without verifying the allocation succeeded. If ACPI_ALLOCATE_ZEROED returns NULL, the kernel will dereference a zero pointer in privileged context, producing an oops or panic and a definitive denial‑of‑service for the host. This is the condition documented in the coordinated vulnerability record. The core technical observation is simple and important: kernel code must never assume memory allocation cannot fail. In this case the remedy is equally straightforward — add a NULL check and return an appropriate error code (AE_NO_MEMORY) instead of proceeding. The upstream corrective changes are intentionally small and surgical.
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
ACPI (Advanced Configuration and Power Interface) and the ACPICA userland/kernel helpers provide firmware parsing and runtime configuration services relied on by many kernel drivers. The affected function, acpi_db_convert_to_package (and adjacent ACPICA helpers), calls ACPI_ALLOCATE_ZEROED to obtain transient storage and then proceeds to dereference the returned pointer without verifying the allocation succeeded. If ACPI_ALLOCATE_ZEROED returns NULL, the kernel will dereference a zero pointer in privileged context, producing an oops or panic and a definitive denial‑of‑service for the host. This is the condition documented in the coordinated vulnerability record. The core technical observation is simple and important: kernel code must never assume memory allocation cannot fail. In this case the remedy is equally straightforward — add a NULL check and return an appropriate error code (AE_NO_MEMORY) instead of proceeding. The upstream corrective changes are intentionally small and surgical. What exactly is wrong — technical anatomy
The vulnerable pattern
- The code calls ACPI_ALLOCATE_ZEROED (or a wrapper that can return NULL on failure) to obtain temporary storage.
- The returned pointer is used immediately (dereferenced) without testing for NULL.
- A NULL return therefore leads to a kernel dereference of address zero inside ACPICA code paths that are executed from kernel context.
Where this shows up at runtime
The vulnerable code paths are exercised when the kernel or drivers ask ACPICA to parse and return ACPI objects — operations that can occur during device probe, udev events, hotplug, or any runtime interaction with firmware descriptors. A failing allocation is conceptually uncommon on a well‑resourced host, but in memory‑constrained or artificially stressed environments (embedded devices, CI runners, containers, heavy probe loops) the allocation can fail, and repeated or accidental triggering of the path will reliably produce an oops. The impact is immediate unavailability of the kernel and services.Verification: what upstream and distros show
Multiple independent advisories and trackers document the defect and the minimal upstream fix:- The NVD entry for CVE‑2024‑24856 enumerates the null‑check problem and marks the record for enrichment; it records the description and notes the exploitability/impact type as an availability issue.
- Ubuntu’s security page and distro advisories summarize the defect, list it as Medium priority (CVSS ~5.3) and point to the kernel/ACPICA commits that address the missing NULL check. That advisory explicitly recommends returning AE_NO_MEMORY on allocation failure.
- Kernel and ACPICA trees contain the small commits that implement the defensive checks — the upstream change is a straightforward guard around the ACPI_ALLOCATE_ZEROED return and an error return path. Distribution advisories and vendor security trackers referenced the same commits when preparing package updates.
Impact and exploitability — realistic threat model
Attack vector and prerequisites
- Attack vector: Local (AV:L) — the vulnerable code path is executed in kernel context and is reachable only by code that can invoke ACPI parsing routines on the host.
- Privileges: Low to moderate — in some environments, device management daemons, unprivileged processes with certain device/node access, or automated test harnesses can trigger the offending path; other setups require higher privilege.
- Complexity: Low — triggering the vulnerable sequence is straightforward if the environment already performs ACPI evaluation, and the only additional precondition is an allocation failure (or artificially inducing memory pressure).
Who should be most concerned
- Embedded devices and vendor kernels: Highest risk. OEM and embedded Linux images often run kernels with ACPICA code compiled in and receive patches slowly; constrained memory environments make allocation failures more plausible.
- Multi‑tenant hosts and CI runners: Where untrusted workloads can trigger ACPI evaluation or device probes, a local DoS primitive has disproportionate operational impact.
- Cloud or virtualized platforms using vendor images that expose ACPI paths or that pass device topology information from hypervisor/firmware to guests.
Detection, hunting and telemetry
Because this is a null pointer dereference (overt crash) rather than silent data leakage, the failure is noisy when it happens and therefore easier to detect — but not easy to correlate to the ACPI root cause without the right signal sources.- Look for kernel oops/panic traces in dmesg or journalctl that mention ACPICA, acpi_db_convert_to_package, or ACPI_ALLOCATE_ZEROED failure patterns. The NVD and distro notes provide indicative log traces.
- For the related ACPI memory‑leak and parsing issues in CoreSight, kmemleak output and memory‑trend telemetry were useful detection signals; while CVE‑2024‑24856 is an immediate oops, the surrounding operational guidance about kmemleak and ACPI evaluator frames remains relevant for hunting ACPI issues.
- On testbeds, reproduce device probe/rebind cycles and watch for reproducible oopses on allocation failure. Add kernel crash collection tooling (kdump, persistent journal) to capture traces and the full call stack for vendor triage.
Remediation: what to apply and when
Definitive remediation
- Install vendor/distribution kernel updates that include the upstream ACPICA or kernel commits adding the NULL check. Several distributions have already published advisories and packaged fixes — consult your distro’s security tracker and kernel package changelogs to confirm the commit is present. Ubuntu and other distros list the fix in their advisories.
- Reboot into the updated kernel — kernel fixes for this class of bug require a reboot to take effect.
Interim mitigations (when patching is delayed)
- Reduce the opportunity to trigger ACPI parsing loops: limit which processes can cause device rebinds, harden udev rules, and restrict untrusted users’ ability to cause frequent device probe/remove events.
- Where acceptable, blacklist or disable driver modules that exercise ACPICA parsing on hosts that do not need them; be careful — ACPI is fundamental for many platforms and disabling it may not be practical or safe.
- For embedded appliances where vendors have not backported, consider network isolation and strict process sandboxing to prevent untrusted workloads from exercising device probe flows.
Patch mechanics and why the fix is low‑risk
The upstream change is intentionally minimal: add a defensive check for the ACPI_ALLOCATE_ZEROED return and propagate AE_NO_MEMORY where appropriate instead of dereferencing a possibly NULL pointer. Because the fix does not change the success-path behavior or the semantics of the returned objects when allocation succeeds, the regression risk is low and the patch is a straightforward candidate for backporting into stable kernel trees and vendor kernels. Distribution advisories reflect exactly that posture.Operational checklist — step by step
- Inventory and exposure mapping
- Run uname -r and record kernel package versions.
- Check kernel config: grep CONFIG_ACPI /boot/config-$(uname -r) and note whether ACPICA support is compiled in or built as modules.
- Identify devices and hosts that perform frequent ACPI evaluation (embedded devices, testbeds, CI runners, devices that use vendor ACPI _DSD/_DSM entries).
- Confirm fix availability
- Consult your distribution’s security tracker and kernel package changelog for CVE‑2024‑24856 or the ACPICA patch commit.
- Where possible, verify the upstream commit/patch is present in your kernel package by searching the package changelog for the commit hash or CVE reference.
- Patch and validate
- Apply the vendor kernel update and reboot hosts in a controlled wave (pilot → staging → production).
- Reproduce device probe cycles in test to validate absence of oops traces.
- Monitor logs and kernel telemetry for new ACPICA oops messages for at least several days post‑patch.
- Compensating controls
- Harden udev/device trigger permissions on multi‑tenant hosts.
- Isolate high‑risk embedded devices on segmented networks until vendor images are updated.
Critical analysis — strengths and remaining risks
Notable strengths
- Small, surgical fix: The upstream patch is a single, minimal defensive insertion that returns AE_NO_MEMORY when allocation fails. That makes vendor backporting straightforward and reduces regression risk.
- Clear observability: A null dereference in kernel space manifests as an oops or panic; this makes incidents noisy and therefore easier to detect and triage when kernel logging and crash collection are configured.
- Low attack complexity to fix ratio: The code change needed is trivial and thus should be rapidly absorbed into stable kernels and distro packages.
Residual risks and caveats
- Vendor/OEM lag: Embedded images and vendor‑forked kernels are the most persistent exposure vector. Devices in the field that cannot be updated quickly will remain at risk.
- Local attack value in multi‑tenant settings: Even though the vector is local, attackers with any foothold (e.g., in containerized environments, CI systems, or on shared developer hosts) can leverage a reliable DoS primitive to create significant outages.
- Assumptions about in‑the‑wild exploitation: There is no public evidence of active exploitation at disclosure; however, absence of evidence is not evidence of absence — defenders should not assume safe merely because no PoC has been published. Flag any claims of in‑the‑wild exploitation as unverified until a credible technical write‑up or incident report appears.
Practical examples of detection commands
- Check kernel logs for ACPICA oops traces:
- sudo journalctl -k | grep -i acpi
- dmesg | grep -i acpi_db_convert_to_package
- Confirm ACPICA presence in kernel:
- grep CONFIG_ACPI /boot/config-$(uname -r)
- Monitor for kernel oops/panic entries in centralized logging and alert on ACPICA symbols in call stacks.
Closing assessment
CVE‑2024‑24856 is a classic kernel robustness fix: technically minor, operationally material. The defect is limited to missing defensive checks after ACPI memory allocation and produces a null pointer dereference — a direct denial‑of‑service condition when triggered. The remedy is trivial to implement and already present in upstream repositories and distribution advisories; the real operational work for administrators is inventory, patch orchestration, and vendor engagement for embedded or OEM images that may lag in backports. Until patches are deployed across your fleet, prioritize hosts that are multi‑tenant, memory‑constrained, or run vendor kernels, and raise monitoring for kernel oopses that reference ACPICA symbols. Conclusion: Add the defensive NULL check now, patch quickly, and treat unpatched, embedded, or vendor‑forked devices as high priority for remediation because the availability impact is immediate and entirely preventable with a simple kernel update.Source: MSRC Security Update Guide - Microsoft Security Response Center
