CVE-2025-21769 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
ptp: vmclock: Add .owner to vmclock_miscdev_fops
Without the .owner field, the module can be unloaded while /dev/vmclock0 is open, leading to an oops.
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability identified as CVE-2025-21769 affects the Linux kernel's precision time protocol implementation, specifically within the vmclock subsystem that facilitates virtual machine clock synchronization. This issue resides in the device file operations structure for the vmclock miscellaneous device, where a critical missing field prevents proper module reference counting during device access. The vulnerability represents a classic race condition scenario that can lead to system instability and potential denial of service conditions.
The technical flaw manifests through the absence of the .owner field in the vmclock_miscdev_fops structure, which is a fundamental component of the Linux kernel's device driver framework. When this field is omitted, the kernel cannot properly track module references associated with the /dev/vmclock0 device file. As a result, if a process maintains an open file descriptor to /dev/vmclock0 while the vmclock kernel module is being unloaded, the kernel's reference counting mechanism fails to prevent the module from being removed from memory. This creates a dangling pointer scenario that typically results in a kernel oops or panic when the system attempts to access freed memory locations.
The operational impact of this vulnerability extends beyond simple denial of service conditions to potentially compromise system stability and integrity. Attackers could exploit this weakness by creating a process that continuously opens and maintains the /dev/vmclock0 device file while simultaneously triggering module unloading operations. The resulting kernel oops could cause system crashes, data corruption, or provide a vector for privilege escalation depending on the system configuration and access controls. This vulnerability particularly affects virtualized environments where vmclock functionality is actively utilized for time synchronization between guest and host systems, making it a significant concern for cloud computing and containerized deployments.
Mitigation strategies should focus on implementing proper module reference management through the addition of the missing .owner field to the vmclock_miscdev_fops structure. This fix aligns with established kernel development practices and security guidelines, ensuring that device file operations properly maintain module references until all file descriptors are closed. The solution directly addresses the underlying CWE-415 double free error pattern and prevents the improper module unloading scenario. System administrators should prioritize applying the patched kernel version that includes this fix, particularly in production environments where virtualization is utilized. Additionally, monitoring for suspicious module loading/unloading patterns and implementing proper access controls for virtual machine clock devices can serve as supplementary defensive measures against exploitation attempts.