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February 1, 2025
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How To Fix CVE-2025-21672: Linux Kernel AFS Module Lock Handling Vulnerability?
This article provides a detailed analysis of the CVE-2025-21672 vulnerability, a critical flaw found within the Linux kernel's AFS (Andrew File System) module. This vulnerability can lead to a denial-of-service condition by causing a lock to be held indefinitely, preventing proper system function. We will explore the vulnerability's specifics, affected versions, and, most importantly, provide a clear path for security professionals to remediate this issue, ensuring the stability and security of their Linux systems. This article is tailored for security professionals including those in DevSecOps, application security, product security, vulnerability management, penetration testing, and security operations and engineering teams.
A Brief Overview of the Linux Kernel AFS Module
The Andrew File System (AFS) is a distributed network file system that allows client systems to access and share files across a network. In the Linux kernel, the AFS module provides the functionality to act as an AFS client, enabling Linux systems to mount and interact with AFS file shares. The AFS module handles complex operations involving file access, caching, and server communication, making it an essential component for organizations utilizing AFS infrastructure. Proper lock handling within this module is critical to avoid race conditions and other concurrency-related issues, ensuring stability and reliability.
Summary of CVE-2025-21672
CVE ID: CVE-2025-21672
Description: Improper lock handling in the Linux kernel AFS module can cause a lock to be held when returning to userspace.
CVSS Score: N/A (NVD assessment not yet provided)
CVSS Vector: N/A (NVD assessment not yet provided)
The vulnerability arises from a flaw in the afs_proc_addr_prefs_write function within the AFS module. Specifically, when processing user-supplied address preferences, the function attempts to split a string of addresses. If the number of arguments (argc) resulting from the split is less than zero, the function returns directly without releasing an acquired inode lock. This leaves the lock held when returning to user space, causing a lock-held-on-return-to-userspace condition. This specific type of flaw can lead to system instability and denial of service by preventing other processes that need the same lock to proceed. The fix involves storing the error in a return variable and then jumping to the cleanup section to release the lock properly, regardless of the error condition.
Impact of CVE-2025-21672
The primary impact of CVE-2025-21672 is the potential for a denial-of-service (DoS) condition. When the vulnerable code path is triggered, the inode lock remains held indefinitely, blocking other processes that require access to the same file or resource. This can manifest as system hangs, unresponsive applications, and degraded performance. Specifically, if a process attempts to access the AFS filesystem while the lock is held by a different process, it will be blocked indefinitely. This condition can be triggered by a crafted request to the /proc/fs/afs/addr_prefs virtual file that sends a specially formatted string. For instance, an attacker with local access can trigger this flaw by writing crafted content to the /proc/fs/afs/addr_prefs file. The vulnerability is not directly exploitable remotely, but local exploitation can still cause significant disruption.
Products Affected by CVE-2025-21672
The vulnerability affects the Linux kernel itself, specifically the AFS module. The following table lists the affected kernel versions and the versions where the fix is available:
|
Affected Product
|
Affected Versions
|
Fixed Versions
|
|---|---|---|
|
Linux Kernel
|
Prior to the fix commit in v6.13-rc3
|
v6.13-rc3 and later
|
It's crucial to note that this vulnerability is present in the mainline Linux kernel. Therefore, any Linux distribution that includes the vulnerable kernel version with the AFS module enabled, is affected. Users of custom kernels should carefully inspect their configuration to determine whether they include the affected code.
How to Check Your Product is Vulnerable?
Determining if your Linux system is vulnerable to CVE-2025-21672 involves checking your kernel version and, more importantly, observing the system for specific symptoms:
1. Kernel Version Check:
Use the command
uname -rto determine your kernel version.If your kernel version is prior to
6.13-rc3, your system is potentially vulnerable.Be aware that distributions may backport fixes, so even if the kernel version seems older, it might contain the fix. Always check the specific patch or commit logs within your distribution's kernel.
2. Check the AFS Module:
Verify if the
afsmodule is loaded by runninglsmod | grep afs. If the module is present, your system is utilizing AFS functionality.
3. Look for kernel warnings and errors:
Use
dmesgor check your system logs for messages indicating lock-held-on-return-to-userspace involving the afs module. This output may look likeWARNING: lock held when returning to user space!and show a backtrace involvingafs_proc_addr_prefs_write.
4. Testing using echo command:
Try to send the crafted input to
/proc/fs/afs/addr_prefsand look for the warnings. Use the commandecho "invalid-string" > /proc/fs/afs/addr_prefsand see if you get any warnings as mentioned indmesg.
5. Check for symptoms:
Observe whether AFS file operations are causing hangs or unresponsiveness. If you are using AFS and experiencing such issues, this may be an indicator of the vulnerability, especially combined with a vulnerable kernel version.
6. Use audit logging:
Enable audit logging for the
/proc/fs/afs/addr_prefsand check the logs.Use
auditctl -w /proc/fs/afs/addr_prefs -p wato enable auditing. Then, check logs withausearch -f /proc/fs/afs/addr_prefs.
How to Fix CVE-2025-21672?
The primary method to fix CVE-2025-21672 is to apply the necessary kernel patch or upgrade to a kernel version that includes the fix. Here's a comprehensive guide to remediating the vulnerability:
1. Upgrade Your Kernel:
The most effective solution is to upgrade to a Linux kernel version that includes the fix for this vulnerability (v6.13-rc3 or later).
Use your distribution's package management tools (e.g.,
apt,yum,dnf) to install the latest kernel updates.Always back up your system before performing kernel upgrades.
Reboot the system after the upgrade for the new kernel to take effect.
Verify the new kernel using
uname -r.
2. Apply Patch if Upgrade not Possible:
If you cannot immediately upgrade your kernel, locate the commit that fixes the vulnerability (typically available on the kernel's git repository or distribution's patch list).
Apply the patch to your currently installed kernel.
This approach should only be taken by experienced system administrators since errors can cause system instability and is highly dependent on the system and configurations.
3. Mitigation (if patching is not an immediate option):
If neither upgrading nor patching is immediately feasible, consider mitigating the risk by disabling the AFS module if your system does not depend on it.
Use the command
modprobe -r afsto unload the module.Blacklist the module to prevent it from loading at boot time. Create a file in
/etc/modprobe.d/(e.g.,blacklist-afs.conf) containingblacklist afsand regenerate your initramfs, for example, withupdate-initramfs -u.Note that if your system depends on AFS, disabling the module will disrupt AFS-related functionality. This mitigation is only suitable if AFS is not a critical service.
4. Monitoring:
Continue to monitor systems for unusual behavior, especially after applying the patch or mitigation steps.
Set up alerts for any lock-held conditions related to AFS, as this is a direct symptom of the vulnerability and will serve as an indicator that the fix is ineffective or if there is an attempt to exploit the vulnerability in unpatched systems.
5. Access control:
Ensure appropriate access control of
/proc/fs/afs/addr_prefs. This file can be written by the root user and the user who mounted the AFS. Limit the access to this file by the minimum number of required users.
6. Regularly update kernels:
Establish a schedule for regular kernel updates to stay ahead of security vulnerabilities.
By following these steps, security professionals can effectively address CVE-2025-21672, mitigating the risks posed by this vulnerability and ensuring a more stable and secure environment. Understanding the CVSS base metrics of vulnerabilities is crucial for effective remediation. Also, having a strong patch management strategy in place is essential for addressing such issues promptly. Furthermore, it's important to perform regular vulnerability assessments to identify and fix security flaws.
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Arun KL
Arun KL is a cybersecurity professional with 15+ years of experience in IT infrastructure, cloud security, vulnerability management, Penetration Testing, security operations, and incident response. He is adept at designing and implementing robust security solutions to safeguard systems and data. Arun holds multiple industry certifications including CCNA, CCNA Security, RHCE, CEH, and AWS Security.
