CVE-2025-38349 in Linux
Summary
by MITRE • 07/18/2025
In the Linux kernel, the following vulnerability has been resolved:
eventpoll: don't decrement ep refcount while still holding the ep mutex
Jann Horn points out that epoll is decrementing the ep refcount and then doing a
mutex_unlock(&ep->mtx);
afterwards. That's very wrong, because it can lead to a use-after-free.
That pattern is actually fine for the very last reference, because the code in question will delay the actual call to "ep_free(ep)" until after it has unlocked the mutex.
But it's wrong for the much subtler "next to last" case when somebody *else* may also be dropping their reference and free the ep while we're still using the mutex.
Note that this is true even if that other user is also using the same ep mutex: mutexes, unlike spinlocks, can not be used for object ownership, even if they guarantee mutual exclusion.
A mutex "unlock" operation is not atomic, and as one user is still accessing the mutex as part of unlocking it, another user can come in and get the now released mutex and free the data structure while the first user is still cleaning up.
See our mutex documentation in Documentation/locking/mutex-design.rst, in particular the section [1] about semantics:
"mutex_unlock() may access the mutex structure even after it has internally released the lock already - so it's not safe for another context to acquire the mutex and assume that the mutex_unlock() context is not using the structure anymore"
So if we drop our ep ref before the mutex unlock, but we weren't the last one, we may then unlock the mutex, another user comes in, drops _their_ reference and releases the 'ep' as it now has no users - all while the mutex_unlock() is still accessing it.
Fix this by simply moving the ep refcount dropping to outside the mutex: the refcount itself is atomic, and doesn't need mutex protection (that's the whole _point_ of refcounts: unlike mutexes, they are inherently about object lifetimes).
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Analysis
by VulDB Data Team • 02/10/2026
The vulnerability identified as CVE-2025-38349 represents a critical race condition in the Linux kernel's eventpoll subsystem that can lead to use-after-free conditions and potential privilege escalation. This flaw exists in the epoll implementation where the kernel incorrectly manages reference counting operations in relation to mutex locking mechanisms. The issue was discovered by security researcher Jann Horn and demonstrates a fundamental misunderstanding of how mutexes and reference counting should interact in concurrent kernel code.
The technical flaw occurs when the eventpoll subsystem attempts to release an epoll file descriptor structure. The problematic code sequence first decrements the reference count for the epoll structure and then releases the mutex lock. This ordering creates a window where another thread can acquire the mutex, observe that the reference count has dropped to zero, and proceed to free the epoll structure while the original thread is still in the process of unlocking the mutex. This violates fundamental concurrency principles and creates a classic use-after-free scenario that can be exploited by malicious actors.
This vulnerability directly maps to CWE-367, which describes Time-of-Check to Time-of-Use (TOCTOU) race conditions, and more specifically relates to CWE-416, which covers use-after-free conditions. The flaw also aligns with ATT&CK technique T1068, which involves exploiting local privilege escalation opportunities through kernel vulnerabilities. The root cause stems from improper understanding of mutex semantics as outlined in the Linux kernel documentation, particularly in the mutex-design.rst file that explicitly warns against assuming mutex_unlock() operations are atomic with respect to structure access.
The operational impact of this vulnerability is severe as it can allow local users to potentially escalate privileges or cause system instability through carefully crafted concurrent access patterns. The race condition can be triggered by multiple threads simultaneously accessing epoll file descriptors, making it particularly dangerous in multi-threaded applications or systems with high concurrent I/O loads. The vulnerability affects all Linux kernel versions that implement the problematic epoll reference counting logic and can be exploited without requiring special privileges beyond normal user access.
The fix for CVE-2025-38349 involves a simple but critical change to the reference counting order. The solution moves the reference count decrement operation outside of the mutex protection, allowing the reference count to be safely decremented atomically without requiring mutex synchronization. This approach leverages the atomic nature of reference counting operations, which are specifically designed to handle concurrent access without explicit locking mechanisms. The mutex is then unlocked before any potential freeing of the structure, ensuring that no other thread can access the structure after the mutex is released. This change aligns with kernel development best practices that emphasize using atomic operations for object lifetime management rather than relying on mutexes for reference counting.
The fix demonstrates the importance of understanding the fundamental differences between mutexes and atomic operations in kernel development. Mutexes provide mutual exclusion but do not guarantee object lifetime safety, while atomic reference counting operations are specifically designed for managing object lifetimes in concurrent environments. This vulnerability highlights the need for kernel developers to carefully consider the semantics of synchronization primitives and their interaction with object lifecycle management. The solution also reinforces the principle that reference counting should be handled atomically outside of critical sections, which is a well-established pattern in kernel design for avoiding exactly this class of race conditions.