CVE-2025-38349

In the Linux kernel, the following vulnerability has been resolved:

eventpoll: dont 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. Thats 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 its wrong for the much subtler next to last case when somebody else may also be dropping their reference and free the ep while were 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 its 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 werent 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 doesnt need mutex protection (thats the whole point of refcounts: unlike mutexes, they are inherently about object lifetimes).

Affected Software

References

• https://git.kernel.org/stable/c/521e9ff0b67c66a17d6f9593dfccafaa984aae4c
• https://git.kernel.org/stable/c/605c18698ecfa99165f36b7f59d3ed503e169814
• https://git.kernel.org/stable/c/6dee745bd0aec9d399df674256e7b1ecdb615444
• https://git.kernel.org/stable/c/8c2e52ebbe885c7eeaabd3b7ddcdc1246fc400d2
• https://project-zero.issues.chromium.org/issues/430541637