CVE-2025-38349 eventpoll: don't decrement ep refcount while still holding the ep mutex
A use-after-free vulnerability in the Linux kernel's eventpoll subsystem allows attackers to execute arbitrary code or escalate privileges.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-38349?
CVE-2025-38349 is a high-severity use after free vulnerability in Linux kernel, published on 18 Jul 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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).
Weakness: CWE-416
Which versions are affected by CVE-2025-38349?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-38349?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2025-38349 being exploited?
There is no known exploitation of CVE-2025-38349 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as A-432751421, ASB-A-432751421, AZL-65544, BELL-CVE-2025-38349, DEBIAN-CVE-2025-38349, ECHO-e238-0710-b4cc and 5 more.