CVE-2025-39689 in Linux
Summary
by MITRE • 09/05/2025
In the Linux kernel, the following vulnerability has been resolved:
ftrace: Also allocate and copy hash for reading of filter files
Currently the reader of set_ftrace_filter and set_ftrace_notrace just adds the pointer to the global tracer hash to its iterator. Unlike the writer that allocates a copy of the hash, the reader keeps the pointer to the filter hashes. This is problematic because this pointer is static across function calls that release the locks that can update the global tracer hashes. This can cause UAF and similar bugs.
Allocate and copy the hash for reading the filter files like it is done for the writers. This not only fixes UAF bugs, but also makes the code a bit simpler as it doesn't have to differentiate when to free the iterator's hash between writers and readers.
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Analysis
by VulDB Data Team • 02/09/2026
The vulnerability identified as CVE-2025-39689 resides within the Linux kernel's ftrace subsystem, specifically addressing a critical race condition and use-after-free scenario in the handling of filter files. This flaw affects the interaction between the reader and writer components of the ftrace functionality, which is essential for kernel tracing and debugging operations. The ftrace subsystem enables dynamic tracing of kernel functions and is widely utilized for performance analysis, debugging, and security monitoring purposes across Linux systems. When exploited, this vulnerability can compromise system stability and potentially enable privilege escalation or denial of service conditions.
The technical root cause of this vulnerability stems from inconsistent memory management practices between the writer and reader components of the ftrace filter mechanism. While the writer process properly allocates and copies hash structures when updating filter files, the reader process maintains direct references to global tracer hash structures through static pointers. This design discrepancy creates a scenario where the reader continues to reference hash data that may be modified or freed by concurrent writer operations, leading to use-after-free conditions and potential memory corruption. The vulnerability manifests when multiple threads or processes access the filter files simultaneously, with one thread holding a reference to a hash structure that another thread may be actively modifying or releasing. This particular flaw aligns with CWE-416, which addresses use-after-free vulnerabilities, and demonstrates a classic race condition in concurrent programming where proper synchronization mechanisms are absent or inadequate.
The operational impact of CVE-2025-39689 extends beyond simple system instability, as it represents a fundamental flaw in kernel memory management that could be exploited to compromise system integrity. Attackers could potentially leverage this vulnerability to execute arbitrary code with kernel privileges, particularly in environments where ftrace is actively used for tracing operations. The vulnerability affects systems running Linux kernels that implement the ftrace subsystem, making it relevant across a broad spectrum of devices including servers, desktop systems, and embedded platforms. The complexity of exploitation may vary depending on the specific system configuration and the presence of additional mitigations, but the underlying flaw provides a potential attack vector that could be leveraged for privilege escalation or system compromise. Organizations utilizing kernel tracing features for debugging or monitoring purposes face particular risk, as the vulnerability is directly tied to functionality that is actively used in production environments.
The fix implemented for CVE-2025-39689 addresses the core issue by ensuring that both reader and writer components of the ftrace subsystem handle hash structures consistently. The solution involves allocating and copying hash data for reading operations in the same manner as the writer components, eliminating the direct pointer references that created the race condition. This approach not only resolves the immediate use-after-free vulnerabilities but also simplifies the overall codebase by removing the need for special handling logic that differentiated between reader and writer operations. The mitigation strategy follows established secure coding practices that emphasize consistent resource management and proper synchronization mechanisms. From an ATT&CK perspective, this vulnerability could be categorized under T1059 for command and scripting interpreter usage, and T1547 for boot or logon autostart execution, as exploitation might involve manipulating kernel tracing functionality to establish persistent access. Organizations should prioritize patching this vulnerability through kernel updates, as the fix resolves both the immediate security concern and improves overall system stability. The implementation of this fix also reduces the attack surface for potential exploitation and aligns with the principle of least privilege by ensuring proper memory management in kernel subsystems. System administrators should verify that their kernel versions include this patch and monitor for any unusual behavior in systems that heavily utilize ftrace functionality, particularly in high-security environments where kernel integrity is paramount.