net_sched: fix reference counting of tc filter chain

This patch fixes the following ugliness of tc filter chain refcnt:

a) tp proto should hold a refcnt to the chain too. This significantly
   simplifies the logic.

b) Chain 0 is no longer special, it is created with refcnt=1 like any
   other chains. All the ugliness in tcf_chain_put() can be gone!

c) No need to handle the flushing oddly, because block still holds
   chain 0, it can not be released, this guarantees block is the last
   user.

d) The race condition with RCU callbacks is easier to handle with just
   a rcu_barrier(). Much easier to understand, nothing to hide. Thanks
   to the previous patch. Please see also the comments in code.

e) Make the code understandable by humans, much less error-prone.

Fixes: 744a4cf6 ("net: sched: fix use after free when tcf_chain_destroy is called multiple times")
Fixes: 5bc17018 ("net: sched: introduce multichain support for filters")
Cc: Jiri Pirko <jiri@mellanox.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -182,7 +182,7 @@ static struct tcf_chain *tcf_chain_create(struct tcf_block *block,
 	list_add_tail(&chain->list, &block->chain_list);
 	chain->block = block;
 	chain->index = chain_index;
-	chain->refcnt = 0;
+	chain->refcnt = 1;
 	return chain;
 }
 
@@ -194,47 +194,41 @@ static void tcf_chain_flush(struct tcf_chain *chain)
 		RCU_INIT_POINTER(*chain->p_filter_chain, NULL);
 	while ((tp = rtnl_dereference(chain->filter_chain)) != NULL) {
 		RCU_INIT_POINTER(chain->filter_chain, tp->next);
+		tcf_chain_put(chain);
 		tcf_proto_destroy(tp);
 	}
 }
 
 static void tcf_chain_destroy(struct tcf_chain *chain)
 {
-	/* May be already removed from the list by the previous call. */
-	if (!list_empty(&chain->list))
-		list_del_init(&chain->list);
-
-	/* There might still be a reference held when we got here from
-	 * tcf_block_put. Wait for the user to drop reference before free.
-	 */
-	if (!chain->refcnt)
+	list_del(&chain->list);
 	kfree(chain);
 }
 
+static void tcf_chain_hold(struct tcf_chain *chain)
+{
+	++chain->refcnt;
+}
+
 struct tcf_chain *tcf_chain_get(struct tcf_block *block, u32 chain_index,
 				bool create)
 {
 	struct tcf_chain *chain;
 
 	list_for_each_entry(chain, &block->chain_list, list) {
-		if (chain->index == chain_index)
-			goto incref;
+		if (chain->index == chain_index) {
+			tcf_chain_hold(chain);
+			return chain;
+		}
 	}
-	chain = create ? tcf_chain_create(block, chain_index) : NULL;
 
-incref:
-	if (chain)
-		chain->refcnt++;
-	return chain;
+	return create ? tcf_chain_create(block, chain_index) : NULL;
 }
 EXPORT_SYMBOL(tcf_chain_get);
 
 void tcf_chain_put(struct tcf_chain *chain)
 {
-	/* Destroy unused chain, with exception of chain 0, which is the
-	 * default one and has to be always present.
-	 */
-	if (--chain->refcnt == 0 && !chain->filter_chain && chain->index != 0)
+	if (--chain->refcnt == 0)
 		tcf_chain_destroy(chain);
 }
 EXPORT_SYMBOL(tcf_chain_put);
@@ -279,10 +273,19 @@ void tcf_block_put(struct tcf_block *block)
 	if (!block)
 		return;
 
+	/* XXX: Standalone actions are not allowed to jump to any chain, and
+	 * bound actions should be all removed after flushing. However,
+	 * filters are destroyed in RCU callbacks, we have to flush and wait
+	 * for them inside the loop, otherwise we race with RCU callbacks on
+	 * this list.
+	 */
 	list_for_each_entry_safe(chain, tmp, &block->chain_list, list) {
 		tcf_chain_flush(chain);
-		tcf_chain_destroy(chain);
+		rcu_barrier();
 	}
+
+	list_for_each_entry_safe(chain, tmp, &block->chain_list, list)
+		tcf_chain_put(chain);
 	kfree(block);
 }
 EXPORT_SYMBOL(tcf_block_put);
@@ -360,6 +363,7 @@ static void tcf_chain_tp_insert(struct tcf_chain *chain,
 		rcu_assign_pointer(*chain->p_filter_chain, tp);
 	RCU_INIT_POINTER(tp->next, tcf_chain_tp_prev(chain_info));
 	rcu_assign_pointer(*chain_info->pprev, tp);
+	tcf_chain_hold(chain);
 }
 
 static void tcf_chain_tp_remove(struct tcf_chain *chain,
@@ -371,6 +375,7 @@ static void tcf_chain_tp_remove(struct tcf_chain *chain,
 	if (chain->p_filter_chain && tp == chain->filter_chain)
 		RCU_INIT_POINTER(*chain->p_filter_chain, next);
 	RCU_INIT_POINTER(*chain_info->pprev, next);
+	tcf_chain_put(chain);
 }
 
 static struct tcf_proto *tcf_chain_tp_find(struct tcf_chain *chain,