net: sched: fix potential use-after-free in __tcf_chain_put()

When used with unlocked classifier that have filters attached to actions
with goto chain, __tcf_chain_put() for last non action reference can race
with calls to same function from action cleanup code that releases last
action reference. In this case action cleanup handler could free the chain
if it executes after all references to chain were released, but before all
concurrent users finished using it. Modify __tcf_chain_put() to only access
tcf_chain fields when holding block->lock. Remove local variables that were
used to cache some tcf_chain fields and are no longer needed because their
values can now be obtained directly from chain under block->lock
protection.

Fixes: 726d0612 ("net: sched: prevent insertion of new classifiers during chain flush")
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -470,10 +470,9 @@ static void __tcf_chain_put(struct tcf_chain *chain, bool by_act,
 {
 	struct tcf_block *block = chain->block;
 	const struct tcf_proto_ops *tmplt_ops;
-	bool is_last, free_block = false;
+	bool free_block = false;
 	unsigned int refcnt;
 	void *tmplt_priv;
-	u32 chain_index;
 
 	mutex_lock(&block->lock);
 	if (explicitly_created) {
@@ -492,23 +491,21 @@ static void __tcf_chain_put(struct tcf_chain *chain, bool by_act,
 	 * save these to temporary variables.
 	 */
 	refcnt = --chain->refcnt;
-	is_last = refcnt - chain->action_refcnt == 0;
 	tmplt_ops = chain->tmplt_ops;
 	tmplt_priv = chain->tmplt_priv;
-	chain_index = chain->index;
-
-	if (refcnt == 0)
-		free_block = tcf_chain_detach(chain);
-	mutex_unlock(&block->lock);
 
 	/* The last dropped non-action reference will trigger notification. */
-	if (is_last && !by_act) {
-		tc_chain_notify_delete(tmplt_ops, tmplt_priv, chain_index,
+	if (refcnt - chain->action_refcnt == 0 && !by_act) {
+		tc_chain_notify_delete(tmplt_ops, tmplt_priv, chain->index,
 				       block, NULL, 0, 0, false);
 		/* Last reference to chain, no need to lock. */
 		chain->flushing = false;
 	}
 
+	if (refcnt == 0)
+		free_block = tcf_chain_detach(chain);
+	mutex_unlock(&block->lock);
+
 	if (refcnt == 0) {
 		tc_chain_tmplt_del(tmplt_ops, tmplt_priv);
 		tcf_chain_destroy(chain, free_block);