CVE-2025-21900
📋 TL;DR
This CVE describes a deadlock vulnerability in the Linux kernel's NFSv4 client when recovering state on a sillyrenamed file. The deadlock can cause system hangs or crashes, affecting systems using NFSv4 file sharing. Only Linux systems with NFSv4 client enabled are vulnerable.
💻 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 →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
Complete system deadlock requiring hard reboot, causing denial of service and potential data loss or corruption.
Likely Case
System hangs or crashes when specific NFSv4 operations occur during server reboot scenarios, leading to temporary denial of service.
If Mitigated
Minor performance impact during NFSv4 state recovery operations with no security compromise.
🎯 Exploit Status
Exploitation requires specific timing conditions during NFS server reboot and client state recovery. More of a reliability issue than security vulnerability.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Kernel versions containing commits 4fe4ae6c2e01d028856b73b6328b12b8945df871, 8f8df955f078e1a023ee55161935000a67651f38, f41a60bc43e7abbc636fee78bed0d74c31e738b0
Vendor Advisory: https://git.kernel.org/stable/c/4fe4ae6c2e01d028856b73b6328b12b8945df871
Restart Required: Yes
Instructions:
1. Update Linux kernel to patched version. 2. Check distribution-specific security advisories. 3. Reboot system after kernel update.
🔧 Temporary Workarounds
Disable NFSv4 client
linuxTemporarily disable NFSv4 client functionality if not required
echo "options nfs nfs4_disable_idmapping=1" > /etc/modprobe.d/nfs.conf
reboot
Use NFSv3 instead
linuxConfigure NFS mounts to use version 3 instead of version 4
mount -t nfs -o vers=3 server:/export /mnt
🧯 If You Can't Patch
- Avoid NFS server reboots while clients have sillyrenamed files open
- Monitor NFS client logs for deadlock indicators and restart affected services
🔍 How to Verify
Check if Vulnerable:
Check kernel version and verify if NFSv4 is in use: 'uname -r' and 'cat /proc/mounts | grep nfs4'Check Version:
uname -rVerify Fix Applied:
Verify kernel version contains the fix commits: 'uname -r' and check with distribution package manager📡 Detection & Monitoring
Log Indicators:
- Kernel panic messages
- NFS client hang/timeout messages in system logs
- Processes stuck in D state (uninterruptible sleep)
Network Indicators:
- NFSv4 protocol timeouts
- Increased NFS retransmission attempts
SIEM Query:
source="kernel" AND ("deadlock" OR "NFSv4" OR "delegreturn")