CVE-2024-42232 libceph: fix race between delayed_work() and ceph_monc_stop()

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

libceph: fix race between delayed_work() and ceph_monc_stop()

The way the delayed work is handled in ceph_monc_stop() is prone to
races with mon_fault() and possibly also finish_hunting(). Both of
these can requeue the delayed work which wouldn’t be canceled by any of
the following code in case that happens after cancel_delayed_work_sync()
runs – __close_session() doesn’t mess with the delayed work in order
to avoid interfering with the hunting interval logic. This part was
missed in commit b5d91704f53e (“libceph: behave in mon_fault() if
cur_mon < 0”) and use-after-free can still ensue on monc and objects
that hang off of it, with monc->auth and monc->monmap being
particularly susceptible to quickly being reused.

To fix this:

• clear monc->cur_mon and monc->hunting as part of closing the session
  in ceph_monc_stop()
• bail from delayed_work() if monc->cur_mon is cleared, similar to how
  it’s done in mon_fault() and finish_hunting() (based on monc->hunting)
• call cancel_delayed_work_sync() after the session is closed