xfrm: Fix work re-schedule after cancel in xfrm_nat_keepalive_net_fini()

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

xfrm: Fix work re-schedule after cancel in xfrm_nat_keepalive_net_fini()

After cancel_delayed_work_sync() is called from

xfrm_nat_keepalive_net_fini(), xfrm_state_fini() flushes remaining

states via __xfrm_state_delete(), which calls

xfrm_nat_keepalive_state_updated() to re-schedule nat_keepalive_work.

The following is a simple race scenario:

cpu0 cpu1

cleanup_net() [Round 1]

ops_undo_list()

xfrm_net_exit()

xfrm_nat_keepalive_net_fini()

cancel_delayed_work_sync(nat_keepalive_work);

xfrm_state_fini()

xfrm_state_flush()

xfrm_state_delete(x)

__xfrm_state_delete(x)

xfrm_nat_keepalive_state_updated(x)

schedule_delayed_work(nat_keepalive_work);

rcu_barrier();

net_complete_free();

net_passive_dec(net);

llist_add(&net->defer_free_list, &defer_free_list);

cleanup_net() [Round 2]

rcu_barrier();

net_complete_free()

kmem_cache_free(net_cachep, net);

nat_keepalive_work()

// on freed net

To prevent this, cancel_delayed_work_sync() is replaced with

disable_delayed_work_sync().