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

ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock

list_for_each_entry() iterates ci->i_cap_flush_list but drops
i_ceph_lock to send cap messages.  During the unlock window,
handle_cap_flush_ack() can acquire i_ceph_lock, detach cf entries
with tid <= flush_tid from the list, release i_ceph_lock, and free
them via ceph_free_cap_flush() outside any lock.  When the original
thread reacquires i_ceph_lock and the for-loop macro advances via
cf = list_next_entry(cf, i_list), it dereferences cf->i_list.next
on freed memory.

The race timeline:

  __kick_flushing_caps()              handle_cap_flush_ack()
  -----------------------             -----------------------
  holds i_ceph_lock        <---
  iterates to cf (tid=10)
  prepares FLUSH message
  drops i_ceph_lock        <---
  __send_cap() ── FLUSH(tid=10)
	                              MDS sends FLUSH_ACK(tid=10)
                           --->       acquires i_ceph_lock
                                      cf->tid(10) <= flush_tid(10),
                                      detaches cf from i_cap_flush_list
                                      drops i_ceph_lock
                                      ceph_free_cap_flush(cf) <- frees it!
  acquires i_ceph_lock     <---
  for-loop advances:
    cf = list_next_entry(cf, i_list)
      -- UAF on freed cf->i_list.next

The cf was just sent by __kick_flushing_caps itself via __send_cap().
The MDS may respond with FLUSH_ACK quickly enough that
handle_cap_flush_ack() frees cf before __kick_flushing_caps can
finish the iteration.

Fix by converting to a manual while loop: save the next pointer
under i_ceph_lock before dropping it, then use the saved pointer
after reacquiring, so the potentially-freed cf is never accessed again.