cls_u32: fix use after free in u32_destroy_key()

Li Shuang reported an Oops with cls_u32 due to an use-after-free
in u32_destroy_key(). The use-after-free can be triggered with:

dev=lo
tc qdisc add dev $dev root handle 1: htb default 10
tc filter add dev $dev parent 1: prio 5 handle 1: protocol ip u32 divisor 256
tc filter add dev $dev protocol ip parent 1: prio 5 u32 ht 800:: match ip dst\
 10.0.0.0/8 hashkey mask 0x0000ff00 at 16 link 1:
tc qdisc del dev $dev root

Which causes the following kasan splat:

 ==================================================================
 BUG: KASAN: use-after-free in u32_destroy_key.constprop.21+0x117/0x140 [cls_u32]
 Read of size 4 at addr ffff881b83dae618 by task kworker/u48:5/571

 CPU: 17 PID: 571 Comm: kworker/u48:5 Not tainted 4.15.0+ #87
 Hardware name: Dell Inc. PowerEdge R730/072T6D, BIOS 2.1.7 06/16/2016
 Workqueue: tc_filter_workqueue u32_delete_key_freepf_work [cls_u32]
 Call Trace:
  dump_stack+0xd6/0x182
  ? dma_virt_map_sg+0x22e/0x22e
  print_address_description+0x73/0x290
  kasan_report+0x277/0x360
  ? u32_destroy_key.constprop.21+0x117/0x140 [cls_u32]
  u32_destroy_key.constprop.21+0x117/0x140 [cls_u32]
  u32_delete_key_freepf_work+0x1c/0x30 [cls_u32]
  process_one_work+0xae0/0x1c80
  ? sched_clock+0x5/0x10
  ? pwq_dec_nr_in_flight+0x3c0/0x3c0
  ? _raw_spin_unlock_irq+0x29/0x40
  ? trace_hardirqs_on_caller+0x381/0x570
  ? _raw_spin_unlock_irq+0x29/0x40
  ? finish_task_switch+0x1e5/0x760
  ? finish_task_switch+0x208/0x760
  ? preempt_notifier_dec+0x20/0x20
  ? __schedule+0x839/0x1ee0
  ? check_noncircular+0x20/0x20
  ? firmware_map_remove+0x73/0x73
  ? find_held_lock+0x39/0x1c0
  ? worker_thread+0x434/0x1820
  ? lock_contended+0xee0/0xee0
  ? lock_release+0x1100/0x1100
  ? init_rescuer.part.16+0x150/0x150
  ? retint_kernel+0x10/0x10
  worker_thread+0x216/0x1820
  ? process_one_work+0x1c80/0x1c80
  ? lock_acquire+0x1a5/0x540
  ? lock_downgrade+0x6b0/0x6b0
  ? sched_clock+0x5/0x10
  ? lock_release+0x1100/0x1100
  ? compat_start_thread+0x80/0x80
  ? do_raw_spin_trylock+0x190/0x190
  ? _raw_spin_unlock_irq+0x29/0x40
  ? trace_hardirqs_on_caller+0x381/0x570
  ? _raw_spin_unlock_irq+0x29/0x40
  ? finish_task_switch+0x1e5/0x760
  ? finish_task_switch+0x208/0x760
  ? preempt_notifier_dec+0x20/0x20
  ? __schedule+0x839/0x1ee0
  ? kmem_cache_alloc_trace+0x143/0x320
  ? firmware_map_remove+0x73/0x73
  ? sched_clock+0x5/0x10
  ? sched_clock_cpu+0x18/0x170
  ? find_held_lock+0x39/0x1c0
  ? schedule+0xf3/0x3b0
  ? lock_downgrade+0x6b0/0x6b0
  ? __schedule+0x1ee0/0x1ee0
  ? do_wait_intr_irq+0x340/0x340
  ? do_raw_spin_trylock+0x190/0x190
  ? _raw_spin_unlock_irqrestore+0x32/0x60
  ? process_one_work+0x1c80/0x1c80
  ? process_one_work+0x1c80/0x1c80
  kthread+0x312/0x3d0
  ? kthread_create_worker_on_cpu+0xc0/0xc0
  ret_from_fork+0x3a/0x50

 Allocated by task 1688:
  kasan_kmalloc+0xa0/0xd0
  __kmalloc+0x162/0x380
  u32_change+0x1220/0x3c9e [cls_u32]
  tc_ctl_tfilter+0x1ba6/0x2f80
  rtnetlink_rcv_msg+0x4f0/0x9d0
  netlink_rcv_skb+0x124/0x320
  netlink_unicast+0x430/0x600
  netlink_sendmsg+0x8fa/0xd60
  sock_sendmsg+0xb1/0xe0
  ___sys_sendmsg+0x678/0x980
  __sys_sendmsg+0xc4/0x210
  do_syscall_64+0x232/0x7f0
  return_from_SYSCALL_64+0x0/0x75

 Freed by task 112:
  kasan_slab_free+0x71/0xc0
  kfree+0x114/0x320
  rcu_process_callbacks+0xc3f/0x1600
  __do_softirq+0x2bf/0xc06

 The buggy address belongs to the object at ffff881b83dae600
  which belongs to the cache kmalloc-4096 of size 4096
 The buggy address is located 24 bytes inside of
  4096-byte region [ffff881b83dae600, ffff881b83daf600)
 The buggy address belongs to the page:
 page:ffffea006e0f6a00 count:1 mapcount:0 mapping:          (null) index:0x0 compound_mapcount: 0
 flags: 0x17ffffc0008100(slab|head)
 raw: 0017ffffc0008100 0000000000000000 0000000000000000 0000000100070007
 raw: dead000000000100 dead000000000200 ffff880187c0e600 0000000000000000
 page dumped because: kasan: bad access detected

 Memory state around the buggy address:
  ffff881b83dae500: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ffff881b83dae580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 >ffff881b83dae600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                             ^
  ffff881b83dae680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  ffff881b83dae700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ==================================================================

The problem is that the htnode is freed before the linked knodes and the
latter will try to access the first at u32_destroy_key() time.
This change addresses the issue using the htnode refcnt to guarantee
the correct free order. While at it also add a RCU annotation,
to keep sparse happy.

v1 -> v2: use rtnl_derefence() instead of RCU read locks
v2 -> v3:
  - don't check refcnt in u32_destroy_hnode()
  - cleaned-up u32_destroy() implementation
  - cleaned-up code comment
v3 -> v4:
  - dropped unneeded comment

Reported-by: Li Shuang <shuali@redhat.com>
Fixes: c0d378ef1266 ("net_sched: use tcf_queue_work() in u32 filter")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_u32.c

@@ -398,10 +398,12 @@ static int u32_init(struct tcf_proto *tp)
 static int u32_destroy_key(struct tcf_proto *tp, struct tc_u_knode *n,
 			   bool free_pf)
 {
+	struct tc_u_hnode *ht = rtnl_dereference(n->ht_down);
+
 	tcf_exts_destroy(&n->exts);
 	tcf_exts_put_net(&n->exts);
-	if (n->ht_down)
-		n->ht_down->refcnt--;
+	if (ht && --ht->refcnt == 0)
+		kfree(ht);
 #ifdef CONFIG_CLS_U32_PERF
 	if (free_pf)
 		free_percpu(n->pf);
@@ -659,16 +661,15 @@ static void u32_destroy(struct tcf_proto *tp, struct netlink_ext_ack *extack)
 
 		hlist_del(&tp_c->hnode);
 
-		for (ht = rtnl_dereference(tp_c->hlist);
-		     ht;
-		     ht = rtnl_dereference(ht->next)) {
-			ht->refcnt--;
-			u32_clear_hnode(tp, ht, extack);
-		}
-
 		while ((ht = rtnl_dereference(tp_c->hlist)) != NULL) {
+			u32_clear_hnode(tp, ht, extack);
 			RCU_INIT_POINTER(tp_c->hlist, ht->next);
-			kfree_rcu(ht, rcu);
+
+			/* u32_destroy_key() will later free ht for us, if it's
+			 * still referenced by some knode
+			 */
+			if (--ht->refcnt == 0)
+				kfree_rcu(ht, rcu);
 		}
 
 		idr_destroy(&tp_c->handle_idr);