CVE-2025-21893 Overview
CVE-2025-21893 is a Use-After-Free (UAF) vulnerability in the Linux kernel's key management subsystem, specifically within the key_put() function. The flaw occurs when a key's reference count is reduced to zero and the garbage collector thread may destroy the key at any time. However, code designed to speed up quota reclamation continues to access the key after this point, creating a race condition that leads to use-after-free memory corruption.
Critical Impact
Local attackers with low privileges can exploit this memory corruption vulnerability to potentially achieve code execution, privilege escalation, or cause system instability through accessing freed memory in the kernel's key management subsystem.
Affected Products
- Linux Kernel versions prior to patched releases
- Linux Kernel 6.14-rc1 through 6.14-rc4
- Systems running vulnerable kernel versions with keys subsystem enabled
Discovery Timeline
- 2025-03-31 - CVE-2025-21893 published to NVD
- 2025-04-08 - Last updated in NVD database
Technical Details for CVE-2025-21893
Vulnerability Analysis
This vulnerability is classified as CWE-416 (Use After Free). The flaw exists in the kernel's key management code where memory safety guarantees are violated during the key destruction process.
The core issue stems from a performance optimization in the key_put() function. When a key's reference count reaches zero, the key becomes eligible for garbage collection and destruction. The garbage collector thread may destroy the key at any point after the reference count hits zero. However, code was added to expedite quota reclamation that accesses the key structure after the reference count has been decremented to zero—violating the fundamental contract that the key must not be touched after this critical point.
This creates a race condition where the garbage collector may free the key's memory while quota reclamation code still holds a reference to the now-invalid memory location. Accessing freed memory can lead to arbitrary read/write primitives depending on how the freed memory is reallocated.
Root Cause
The root cause is improper memory lifecycle management in the key_put() function. The optimization to speed up quota reclamation introduced code that accesses key structure members after the reference count has been reduced to zero. At this point, the garbage collector thread (key_gc_work) is permitted to destroy the key at any time, making any subsequent access to the key structure undefined behavior and a use-after-free condition.
The fix implements a flag-based approach to indicate when a key can be garbage collected, rather than relying on checking the key's reference count directly. This eliminates the need to access the key structure after the critical point where it may be destroyed.
Attack Vector
The attack vector is local, requiring an attacker to have low-privilege access to the target system. The vulnerability can be triggered through the kernel's keyring interface by manipulating key reference counts through repeated key operations. An attacker could:
- Create and manipulate kernel keys to trigger the vulnerable code path
- Race the garbage collector to access freed memory
- Leverage the use-after-free condition for information disclosure or code execution
The vulnerability does not require user interaction and operates within unchanged scope boundaries, meaning successful exploitation affects only the vulnerable component's security context (the kernel).
Detection Methods for CVE-2025-21893
Indicators of Compromise
- Kernel panic or crash logs referencing key_put() or key management functions
- KASAN (Kernel Address Sanitizer) reports indicating use-after-free in key subsystem
- Unusual system instability when performing keyring operations
Detection Strategies
- Enable KASAN in development/testing environments to detect UAF conditions
- Monitor kernel logs for memory corruption errors related to the keys subsystem
- Implement kernel live patching detection to identify unpatched systems
- Use SentinelOne Singularity platform for runtime kernel exploit detection
Monitoring Recommendations
- Deploy endpoint detection and response (EDR) solutions capable of monitoring kernel-level activities
- Enable audit logging for keyring operations (keyctl commands)
- Monitor for unusual patterns of key creation and destruction that may indicate exploitation attempts
How to Mitigate CVE-2025-21893
Immediate Actions Required
- Update to a patched Linux kernel version immediately
- Review and apply vendor-specific kernel updates from your distribution
- Monitor systems for signs of exploitation until patches can be applied
- Consider temporarily restricting access to keyring operations for non-essential users
Patch Information
The Linux kernel maintainers have released patches to address this vulnerability. The fix modifies key_put() to use a flag-based mechanism for garbage collection signaling instead of accessing the key's reference count directly, preventing the use-after-free condition.
Patches are available through the following kernel commits:
Workarounds
- Limit keyring access to trusted users only through appropriate permission controls
- Enable KASAN for detection purposes in non-production environments
- Monitor systems closely for exploitation attempts while awaiting patch deployment
- Consider kernel hardening options such as enabling SLAB_FREELIST_HARDENED
# Check current kernel version
uname -r
# Update kernel package (Debian/Ubuntu)
sudo apt update && sudo apt upgrade linux-image-$(uname -r)
# Update kernel package (RHEL/CentOS)
sudo yum update kernel
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
