CVE-2025-39833
📋 TL;DR
This CVE describes a kernel timer initialization bug in the Linux kernel's mISDN hfcpci driver that causes a warning/panic when unloading the module with CONFIG_DEBUG_OBJECTS_TIMERS enabled. It affects systems using the hfcpci driver for ISDN hardware. The vulnerability doesn't allow remote exploitation but can cause system instability.
💻 Affected Systems
- Linux kernel
📦 What is this software?
Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →⚠️ Risk & Real-World Impact
Worst Case
Kernel panic or system crash when unloading the hfcpci module, leading to denial of service on affected systems.
Likely Case
Warning messages in kernel logs and potential module unloading failures when CONFIG_DEBUG_OBJECTS_TIMERS is enabled.
If Mitigated
Minor logging noise without functional impact if the debug configuration is disabled.
🎯 Exploit Status
Exploitation requires local privileged access to load/unload kernel modules and specific kernel configuration. Not remotely exploitable.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Kernel versions with commits 43fc5da8133badf17f5df250ba03b9d882254845 and 97766512a9951b9fd6fc97f1b93211642bb0b220 applied
Vendor Advisory: https://git.kernel.org/stable/c/43fc5da8133badf17f5df250ba03b9d882254845
Restart Required: No
Instructions:
1. Update Linux kernel to version containing the fix commits. 2. For custom kernels, apply patches from kernel git repository. 3. Rebuild and install updated kernel. 4. Reboot into new kernel.
🔧 Temporary Workarounds
Disable debug timer objects
allDisable CONFIG_DEBUG_OBJECTS_TIMERS kernel configuration option to prevent the warning/panic.
# Reconfigure kernel without CONFIG_DEBUG_OBJECTS_TIMERS
# Rebuild and install kernel
Avoid unloading hfcpci module
allPrevent unloading of the hfcpci kernel module if it's loaded.
# Prevent module unloading: echo 1 > /sys/module/hfcpci/refcnt
# Or use kernel module locking mechanisms
🧯 If You Can't Patch
- Ensure CONFIG_DEBUG_OBJECTS_TIMERS is disabled in kernel configuration
- Avoid using or loading the hfcpci driver if possible
🔍 How to Verify
Check if Vulnerable:
Check if hfcpci module is loaded and kernel has CONFIG_DEBUG_OBJECTS_TIMERS enabled: lsmod | grep hfcpci && grep CONFIG_DEBUG_OBJECTS_TIMERS /boot/config-$(uname -r)Check Version:
uname -rVerify Fix Applied:
Check kernel version includes fix commits: uname -r and verify with distribution patch notes📡 Detection & Monitoring
Log Indicators:
- Kernel logs containing 'ODEBUG: assert_init not available'
- Warnings about timer objects during module unloading
- Stack traces mentioning hfcpci or HFC_cleanup
Network Indicators:
- None - this is a local kernel issue
SIEM Query:
source="kernel" AND ("ODEBUG" OR "assert_init" OR "hfcpci" OR "HFC_cleanup")