A subtle but important Linux kernel bug has been cataloged as CVE-2025-39833: a latent fault in the mISDN hfcpci driver that triggers a
kernel debug warning / OOPS when deleting an uninitialized timer during module unload, and has been fixed upstream by initializing the timer properly and switching to the kernel timer API's safe update routine.
Overview
The vulnerability, tracked as
CVE-2025-39833, concerns the mISDN telephony subsystem's
hfcpci PCI driver. Under kernels built with kernel debug support for timers (CONFIG_DEBUG_OBJECTS_TIMERS), unloading the hfcpci module can generate an ODEBUG assertion and a kernel WARNING originating from lib/debugobjects.c. The reported root cause is that a timer object used by the driver (referred to in vendor and distro advisories as the
hfc_tl timer) was not properly initialized before it is deleted at cleanup, producing the debug-time assertion. The fix applied is straightforward: initialize the timer using the kernel-friendly DEFINE_TIMER macro and use mod_timer for scheduled updates rather than performing manual timeout arithmetic. This article explains what CVE-2025-39833 is, why it matters, which systems are likely affected, how distributions have responded, and practical steps WindowsForum readers and Linux sysadmins should take to mitigate risk and validate remediation.
Background: mISDN, hfcpci and Linux kernel timers
What are mISDN and hfcpci?
- mISDN is the multi-ISDN subsystem in the Linux kernel that supports ISDN hardware and drivers; it provides protocols and glue for ISDN telephony devices.
- hfcpci is a PCI driver in the mISDN stack that supports specific ISDN HFC (Host-Function Controller) cards. It is a kernel module that registers device operations and maintains internal state for the hardware it controls.
These components are niche — found primarily on systems using legacy ISDN hardware — but they are still included in many upstream kernels and binary distributions for backward compatibility.
How kernel timers and debug objects interact
The Linux kernel timer API exposes timer structures (timer_list) that must be properly initialized before use and safely deleted at cleanup. When kernel debug infrastructure is compiled in — specifically CONFIG_DEBUG_OBJECTS_TIMERS — the kernel tracks the lifecycle of timer objects and asserts on incorrect usage, such as deleting an object that was never initialized.
The reported crash trace for CVE-2025-39833 shows debug_print_object emitting an ODEBUG assertion followed by a kernel WARNING tied into timer deletion paths (try_to_del_timer_sync / __timer_delete_sync) during module unload. In short: deleting a timer that was not initialized triggers the debug assertion and an OOPS-like log line.
What the public advisories say
Multiple vulnerability and distribution trackers summarize the same defect and suggested fix:
- The NVD entry for CVE-2025-39833 records the problem as “mISDN: hfcpci: Fix warning when deleting uninitialized timer” and includes an example kernel splat showing the ODEBUG assert and trace through debug_print_object and timer-delete code paths.
- The Ubuntu security notice echoes the exact technical description and explicitly states the developer remedy: initialize the hfc_tl timer with DEFINE_TIMER and use mod_timer instead of manual timeout updates. Ubuntu assigns a Medium priority to the issue in their advisory.
- The OSV / downstream tracker entries list downstream advisories (SUSE, Debian, Ubuntu, others) and show the CVE published mid-September 2025; they demonstrate that several mainstream distributions incorporated the correction into their kernel packages.
- Commercial vulnerability scanners (for example, Tenable) have cataloged the CVE and flagged it with a higher severity classification in some contexts, underlining that scoring can differ by vendor.
These independent sources converge on the same facts: the problem is a debug-time kernel OOPS on module unload caused by deleting an uninitialized timer; the fix is to use DEFINE_TIMER + mod_timer.
Technical analysis: why the bug happens and why the fix works
Root cause, in plain terms
The driver allocates or declares a timer object intended to fire periodically or after a timeout. In some code paths — specifically around module unload or cleanup — the driver calls the kernel API to delete that timer. But the timer object was not guaranteed to be properly initialized in all execution flows. When the kernel’s debug-object subsystem (CONFIG_DEBUG_OBJECTS_TIMERS) is enabled, it tracks timers and asserts if a deletion is attempted on a timer that is reported as
uninitialized. That assertion path is what produces the ODEBUG message and the kernel warning shown in the public trace.
Why DEFINE_TIMER and mod_timer fix the problem
- DEFINE_TIMER: This macro provides compile-time initialization of a timer_list instance and hooks the function that will run when the timer expires. Using DEFINE_TIMER ensures the timer’s internal state is set to a valid initialized state before any code paths might interact with or delete it. That removes the class of race/ordering bugs in which cleanup tries to delete something not yet fully set up.
- mod_timer: Instead of manually computing expiry values and manipulating the timer structure fields directly, mod_timer is the sanctioned API to change or add a timer to the kernel’s timer list. Using mod_timer avoids subtle mistakes around relative-to-absolute time conversions and ensures the kernel’s internal debug state is updated. Replacing manual updates with mod_timer makes the driver follow kernel timing conventions and eliminates another category of errors.
The combination of these two changes addresses both initialization and update semantics, which is why distribution advisories recommend them as the correct remedy.
Impact assessment: who is affected, and how bad is it?
Affected systems
- Any Linux kernel that includes the mISDN hfcpci driver module may be affected. Because hfcpci is a driver for relatively specialized ISDN hardware, the population of affected hosts in the wild is small, but present in servers or appliances that still use ISDN cards, development systems that build many kernel modules, and build or test environments that enable debug configurations. Distro advisories (Ubuntu, Debian, SUSE) indicate the fix entered mainstream distro kernels, implying the issue was present in upstream kernel trees and downstream binary packages.
- Systems that have CONFIG_DEBUG_OBJECTS_TIMERS enabled are more likely to detect and log the issue. That kernel config is typically enabled in debugging or development kernels — it is less common in vanilla production kernels — which reduces the probability of a widespread production impact. Still, vendor-built debug kernel packages (kernel-debug / kernel-headers with debug options) in enterprise distributions are sometimes deployed for diagnostics and may be affected.
Severity and exploitability
- Direct exploitation risk is low. This is not a classic remote code execution or privilege-escalation pattern; it is a logic/initialization bug that triggers a kernel debug assertion when a module is unloaded. The observable effect is a kernel warning and potential OOPS during module unload. There is no public evidence that this leads to arbitrary code execution. Multiple vendor trackers and the NVD classify this as a bug resolved upstream rather than a high-confidence code-execution vulnerability.
- Denial-of-service and stability risk exist. On systems where the hfcpci module is present and used, an unverifiable or poorly ordered cleanup could cause kernel warnings and potentially hamstring module unload operations; in environments where debug kernels run, this could lead to reduced availability or noisy kernel logs. Enterprise scanners may mark this more severely if debug kernels are in use or if a system relies on module unloads for hot-swap maintenance.
- Operational impact is contextual. For most production Linux hosts that do not have ISDN hardware and that run standard release kernels (without debug-timer instrumentation), the practical risk is minimal. For developer/test platforms, embedded appliances, or systems running debug-mode kernels or vendor-provided kernel-debug packages, the impact is worth patching. Distribution advisories reflect this nuance by giving differing priorities (for example, Ubuntu marked it Medium).
Distribution and vendor responses
Major distribution security trackers and vulnerability aggregators have recorded the CVE, and many downstream advisories reference the same fix description:
- Ubuntu published an advisory listing the fix and gave it Medium priority. Ubuntu’s advisory explicitly describes the fix (DEFINE_TIMER + mod_timer) and shows an example kernel trace. System administrators running Ubuntu kernels should check their package manager and kernel-package changelogs for the fix.
- Debian and its security tracker record CVE-2025-39833 and list it in their vulnerability database; Ubuntu/Debian updates typically follow upstream kernel merges and are present in security updates for kernels that include the corrected driver.
- SUSE and other downstream vendors appear in OSV and advisory lists as having tracked or patched the issue; enterprise vendors may publish RPM/patch advisories by CVE reference.
- Commercial scanners (Tenable and others) list the CVE and may assign a higher severity to installations that run debug kernels or kernel-debug packages. That’s a reflection of scanner default policies, which often treat debug kernel issues as higher-risk because they indicate a kernel-level inconsistency that could destabilize a host.
Note: public advisories summarize the fix but, at the time of publication of this article, direct upstream commit references (specific kernel.org commit IDs or a straightforward across-the-tree link) were not always visible in those aggregator pages. That means admins relying on upstream commit IDs should consult distribution changelogs or the upstream Linux kernel tree directly for the authoritative patch commit. This absence of a clearly linked upstream commit in some advisories is a situation to treat cautiously and to verify in your own update channels.
Practical guidance: detection, mitigation and remediation
Quick detection checklist
- Check whether the hfcpci module is present on your system:
- Run: lsmod | grep hfcpci
- Inspect kernel logs for ODEBUG / debug_print_object warnings:
- Run: dmesg | grep -E 'ODEBUG|debug_print_object|hfcpci|mISDN' or scan /var/log/kern.log and /var/log/messages for similar traces.
- Determine whether you are running a debug kernel or kernel-debug package (CONFIG_DEBUG_OBJECTS_TIMERS enabled):
- Production kernels commonly omit this config; debug kernels include extra diagnostics. If you installed a vendor "kernel-debug" package, you may be running with debug objects enabled. Check your distro package name (kernel-debug) or check /boot/config-$(uname -r) for CONFIG_DEBUG_OBJECTS_TIMERS.
Short-term mitigations (if you cannot patch immediately)
- If you do not use ISDN hardware:
- Blacklist the hfcpci module to prevent it from loading (add hfcpci to /etc/modprobe.d/blacklist.conf and reboot), or remove the module using rmmod if safe and unused.
- If you run a debug kernel:
- Consider switching to a normal production kernel image that omits the debug-object timer checks if you require immediate stability and the hfcpci module is not needed.
- If hfcpci is required and you cannot patch immediate:
- Avoid unloading the module (rmmod) during critical operations; the bug manifests on unload/cleanup paths, so refrain from hot-unloading until you can install the fix.
- Monitor kernel logs actively for lines resembling the trace in public advisories (ODEBUG assert lines), and treat them as indicators to apply a kernel update as soon as possible.
Long-term remediation (recommended)
- Install vendor-provided kernel updates that include the hfcpci fix. Use your distro’s package manager (apt, dnf, zypper, etc. and check kernel changelogs for CVE-2025-39833 or the hfcpci/mISDN patch mention. Distribution trackers (Ubuntu, Debian, SUSE) list the CVE and track fix availability.
- Rebuild custom kernels: If you maintain custom kernels, ensure that the upstream patch is applied to your tree (look for INIT of hfc_tl via DEFINE_TIMER and use of mod_timer in the hfcpci code) and recompile. Verify the presence of the change by inspecting the driver source and checking for the DEFINE_TIMER usage. If no upstream commit ID is present in the distro advisory, reference the vendor patch or kernel changelog entries.
- Validate after update: After upgrading kernels, reproduce module load/unload sequences and watch dmesg for the absence of the ODEBUG assertion and timer-related warnings. That procedural validation is important before promoting fixes to production.
Developer and architecture lessons: why this bug matters beyond ISDN hardware
Although the hfcpci driver affects a narrow hardware class, CVE-2025-39833 is instructive because it highlights a recurring class of kernel coding mistakes:
incorrect lifecycle handling of kernel-managed objects (timers, workqueues, kobjects, etc.. The debug-object plumbing exists precisely to catch life-cycle errors early; while these checks are often disabled in production builds, they provide strong signals during development and backport cycles.
Key lessons:
- Always initialize kernel object structures using provided macros or helper functions (DEFINE_TIMER, INIT_WORK, kobject_init, etc. rather than performing ad-hoc zeroing or manual field fiddling.
- Prefer kernel helper APIs (mod_timer, del_timer_sync, cancel_work_sync) to manual state manipulation.
- Use debug kernels or build-time checks in CI to catch these problems before an upstream merge lands in distro kernels.
- When a kernel debug assertion surfaces in production, treat it as a sign that some lifecycle path is incorrect — even if the immediate runtime effect is only a logged warning.
These best practices reduce the likelihood of hard-to-diagnose timing issues, race conditions, and OOPSes in production systems.
Assessment of vendor messaging and verification notes
Most public advisories for CVE-2025-39833 are concise: they describe the crash trace, list the HFCP CI cleanup path in the trace (HFC_cleanup & try_to_del_timer_sync), and provide the recommended fix (DEFINE_TIMER + mod_timer). Multiple independent trackers (NVD, Ubuntu, Debian, OSV, Tenable) reflect the same technical narrative. That cross-corroboration makes the high-level claim (uninitialized timer deleted on module unload) reliable. However, a few operational points to emphasize:
- Several distribution advisories summarize the fix but do not always include a direct upstream commit URL or commit-id in the advisory text visible on the tracker page. For administrators who require the upstream commit hash for compliance or traceability, it is advisable to check the upstream Linux kernel git tree (git.kernel.org) or the distro kernel changelog for the specific commit reference. The absence of a single canonical commit link in every advisory is common for small driver fixes, but it is an important verification step for hardened environments.
- Severity labels vary by vendor (Ubuntu labels it Medium; some scanners flag higher) because the exploitable surface depends on debug kernel configuration and whether the hfcpci driver is actually in use in deployed systems. The operational context (whether debug kernels are present, whether hfcpci is loaded, and whether module unload paths are used) should guide urgency.
Action checklist for system administrators (concise)
- Inspect if your hosts have hfcpci present: lsmod | grep hfcpci.
- Check kernel logs for ODEBUG / debug_print_object / timer-list warnings: dmesg | grep -E 'ODEBUG|timer_list|hfcpci'.
- Identify whether you run a debug/kernel-debug build: grep CONFIG_DEBUG_OBJECTS_TIMERS /boot/config-$(uname -r).
- If hfcpci is unused, blacklist the module temporarily and schedule a kernel update.
- If hfcpci is required, plan to install vendor kernel updates or recompile your kernel with the upstream patch applied.
- Validate the fix by testing module unload/load cycles and confirming the absence of the debug warning in dmesg.
Conclusion
CVE-2025-39833 is a textbook kernel lifecycle bug: deleting an uninitialized timer in the mISDN hfcpci driver triggers debug assertions and kernel warnings under CONFIG_DEBUG_OBJECTS_TIMERS. The fix — initialize the timer with DEFINE_TIMER and move to mod_timer for updates — is correct and surgical. For most modern deployments the practical risk is limited (hfcpci targets legacy ISDN hardware and debug-timer checks are uncommon in production kernels), but for developers, debug-kernel deployments, or environments using vendor kernel-debug packages, the issue can cause instability and must be patched.
Administrators should verify whether hfcpci exists on their hosts, inspect kernel logs for related traces, and apply vendor-provided kernel updates. Where immediate patching is impossible, temporary mitigations (blacklisting the module or switching off debug kernels) reduce near-term risk until a proper kernel update is deployed. Because the public advisories sometimes omit a single canonical upstream commit link, teams that require absolute traceability should confirm the exact upstream patch in the kernel git history or rely on distribution changelogs for verification.
Source: MSRC
Security Update Guide - Microsoft Security Response Center