CVE-2025-21944 in Linux
Summary
by MITRE • 04/01/2025
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: fix bug on trap in smb2_lock
If lock count is greater than 1, flags could be old value. It should be checked with flags of smb_lock, not flags. It will cause bug-on trap from locks_free_lock in error handling routine.
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Analysis
by VulDB Data Team • 02/01/2026
The vulnerability CVE-2025-21944 affects the Linux kernel's ksmbd implementation, which provides SMB2/SMB3 server functionality for Linux systems. This issue specifically resides within the smb2_lock handling mechanism where a critical logic flaw exists in how lock flags are processed during multi-lock operations. The vulnerability represents a classic case of improper state management and validation that can lead to system instability and potential security implications.
The technical flaw manifests when the system processes multiple locks within a single smb2_lock operation where the lock count exceeds one. Under these conditions, the implementation fails to properly update the flags variable with the current smb_lock flags, instead retaining outdated flag values from previous operations. This occurs during the lock processing routine where the system should validate that all lock operations are properly synchronized and that flag states reflect the current lock context rather than stale values from earlier operations.
The operational impact of this vulnerability is significant as it can trigger a kernel bug-on trap condition during error handling routines, specifically within the locks_free_lock function. This occurs because the system attempts to free lock structures while maintaining inconsistent flag states that violate internal kernel assumptions. When the error handling path executes, it encounters corrupted lock state information that causes the kernel to panic and terminate the system. This vulnerability affects systems running ksmbd as an SMB server, particularly those handling concurrent lock operations or applications that rely on proper SMB locking semantics.
The vulnerability aligns with CWE-129, Improper Validation of Array Index, and CWE-128, Improper Validation of Input, as it involves incorrect handling of lock count validation and flag state management. From an ATT&CK perspective, this vulnerability could be leveraged by adversaries to achieve system compromise through denial-of-service attacks or potentially as a stepping stone for more sophisticated exploitation. The issue demonstrates poor defensive programming practices where proper state validation and flag synchronization are not enforced during multi-operation sequences.
Mitigation strategies should focus on immediate kernel updates that address the specific flag handling logic in the smb2_lock implementation. System administrators should prioritize patching affected ksmbd installations and monitoring for any lock-related system panics or unexpected terminations. Additionally, implementing proper lock validation routines and ensuring that all lock operations maintain consistent flag states throughout their lifecycle would prevent similar issues. Network segmentation and access controls should be maintained to limit exposure, while regular security auditing of kernel modules should include validation of state management and synchronization mechanisms. Organizations should also consider implementing intrusion detection systems that can monitor for kernel panic events or lock-related anomalies that may indicate exploitation attempts.