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

netfilter: cttimeout: prevent UAF during module unload

nf_ct_set_timeout() protects the timeout hook dereference and policy lookup
with rcu_read_lock(). cttimeout_exit(), however, unregisters the per-net
operations before it clears the hook.

This allows the following interleaving:

  CPU 0                              CPU 1
  cttimeout_exit()                   nf_ct_set_timeout()
    unregister_pernet_subsys()         rcu_read_lock()
      kfree(pernet)                     h = nf_ct_timeout_hook
                                        h->timeout_find_get()
                                          nfct_timeout_pernet()

The hook still points to ctnl_timeout_find_get() when CPU 1 looks up the
already freed per-net timeout list. KASAN reported:

  BUG: KASAN: slab-use-after-free in ctnl_timeout_find_get
  Read of size 8 by task poc/90
  Call Trace:
   ctnl_timeout_find_get+0x271/0x2a0 [nfnetlink_cttimeout]
   nf_ct_set_timeout+0x7b/0x3c0
   xt_ct_tg_check+0x724/0xb20
   xt_check_target+0x234/0xa90
   do_ipt_set_ctl+0x570/0x1270
  Allocated by task 89:
   __kmalloc_noprof+0x16e/0x460
   ops_init+0x6d/0x420
   register_pernet_operations+0x2f6/0x670
  Freed by task 91:
   kfree+0x131/0x390
   ops_undo_list+0x3d4/0x730
   unregister_pernet_operations+0x232/0x490
   unregister_pernet_subsys+0x1c/0x30
   cttimeout_exit+0x52/0x970 [nfnetlink_cttimeout]

Clear the hook and wait for existing readers before unregistering the
per-net operations. This blocks new policy lookups and ensures readers that
observed the hook finish before the per-net storage is freed.