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.

Fixes: 58c9b01 ("epoll: use refcount to reduce ep_mutex contention")
Reported-by: Jaeyoung Chung <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Christian Brauner (Amutable) <[email protected]>

Author: brauner

fs/eventpoll.c

@@ -912,22 +912,26 @@ static bool ep_remove_epi(struct eventpoll *ep, struct epitem *epi)
  */
 static void ep_remove(struct eventpoll *ep, struct epitem *epi)
 {
-	struct file *file = epi->ffd.file;
+	struct file *file __free(fput) = NULL;
 
 	lockdep_assert_irqs_enabled();
 	lockdep_assert_held(&ep->mtx);
 
 	ep_unregister_pollwait(ep, epi);
 
-	/* sync with eventpoll_release_file() */
+	/* cheap sync with eventpoll_release_file() */
 	if (unlikely(READ_ONCE(epi->dying)))
 		return;
 
-	spin_lock(&file->f_lock);
-	if (epi->dying) {
-		spin_unlock(&file->f_lock);
+	/*
+	 * If we manage to grab a reference it means we're not in
+	 * eventpoll_release_file() and aren't going to be.
+	 */
+	file = epi_fget(epi);
+	if (!file)
 		return;
-	}
+
+	spin_lock(&file->f_lock);
 	ep_remove_file(ep, epi, file);
 
 	if (ep_remove_epi(ep, epi))