CVE-2026-46242

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

eventpoll: fix ep_remove struct eventpoll / struct file UAF

ep_remove() (via ep_remove_file()) cleared file->f_ep under

file->f_lock but then kept using @file inside the critical section

(is_file_epoll(), hlist_del_rcu() through the head, spin_unlock).

A concurrent __fput() taking the eventpoll_release() fastpath in

that window observed the transient NULL, skipped

eventpoll_release_file() and ran to f_op->release / file_free().

For the epoll-watches-epoll case, f_op->release is

ep_eventpoll_release() -> ep_clear_and_put() -> ep_free(), which

kfree()s the watched struct eventpoll. Its embedded ->refs

hlist_head is exactly where epi->fllink.pprev points, so the

subsequent hlist_del_rcu()'s "*pprev = next" scribbles into freed

kmalloc-192 memory.

In addition, struct file is SLAB_TYPESAFE_BY_RCU, so the slot

backing @file could be recycled by alloc_empty_file() --

reinitializing f_lock and f_ep -- while ep_remove() is still

nominally inside that lock. The upshot is an attacker-controllable

kmem_cache_free() against the wrong slab cache.

Pin @file via epi_fget() at the top of ep_remove() and gate the

critical section on the pin succeeding. With the pin held @file

cannot reach refcount zero, which holds __fput() off and

transitively keeps the watched struct eventpoll alive across the

hlist_del_rcu() and the f_lock use, closing both UAFs.

If the pin fails @file has already reached refcount zero and its

__fput() is in flight. Because we bailed before clearing f_ep,

that path takes the eventpoll_release() slow path into

eventpoll_release_file() and blocks on ep->mtx until the waiter

side's ep_clear_and_put() drops it. The bailed epi's share of

ep->refcount stays intact, so the trailing ep_refcount_dec_and_test()

in ep_clear_and_put() cannot free the eventpoll out from under

eventpoll_release_file(); the orphaned epi is then cleaned up

there.

A successful pin also proves we are not racing

eventpoll_release_file() on this epi, so drop the now-redundant

re-check of epi->dying under f_lock. The cheap lockless

READ_ONCE(epi->dying) fast-path bailout stays.