net_sched: move the empty tp check from ->destroy() to ->delete()

We could have a race condition where in ->classify() path we
dereference tp->root and meanwhile a parallel ->destroy() makes it
a NULL. Daniel cured this bug in commit d936377414fa
("net, sched: respect rcu grace period on cls destruction").

This happens when ->destroy() is called for deleting a filter to
check if we are the last one in tp, this tp is still linked and
visible at that time. The root cause of this problem is the semantic
of ->destroy(), it does two things (for non-force case):

1) check if tp is empty
2) if tp is empty we could really destroy it

and its caller, if cares, needs to check its return value to see if it
is really destroyed. Therefore we can't unlink tp unless we know it is
empty.

As suggested by Daniel, we could actually move the test logic to ->delete()
so that we can safely unlink tp after ->delete() tells us the last one is
just deleted and before ->destroy().

Fixes: 1e052be69d04 ("net_sched: destroy proto tp when all filters are gone")
Cc: Roi Dayan <roid@mellanox.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 7 deletions

diff --git a/net/sched/cls_tcindex.c b/net/sched/cls_tcindex.c
index 2ab001361457..8a8a58357c39 100644
--- a/net/sched/cls_tcindex.c
+++ b/net/sched/cls_tcindex.c
@@ -150,7 +150,7 @@ static void tcindex_destroy_fexts(struct rcu_head *head)
 	kfree(f);
 }
 
-static int tcindex_delete(struct tcf_proto *tp, unsigned long arg)
+static int tcindex_delete(struct tcf_proto *tp, unsigned long arg, bool *last)
 {
 	struct tcindex_data *p = rtnl_dereference(tp->root);
 	struct tcindex_filter_result *r = (struct tcindex_filter_result *) arg;
@@ -186,6 +186,8 @@ found:
 		call_rcu(&f->rcu, tcindex_destroy_fexts);
 	else
 		call_rcu(&r->rcu, tcindex_destroy_rexts);
+
+	*last = false;
 	return 0;
 }
 
@@ -193,7 +195,9 @@ static int tcindex_destroy_element(struct tcf_proto *tp,
 				   unsigned long arg,
 				   struct tcf_walker *walker)
 {
-	return tcindex_delete(tp, arg);
+	bool last;
+
+	return tcindex_delete(tp, arg, &last);
 }
 
 static void __tcindex_destroy(struct rcu_head *head)
@@ -529,14 +533,11 @@ static void tcindex_walk(struct tcf_proto *tp, struct tcf_walker *walker)
 	}
 }
 
-static bool tcindex_destroy(struct tcf_proto *tp, bool force)
+static void tcindex_destroy(struct tcf_proto *tp)
 {
 	struct tcindex_data *p = rtnl_dereference(tp->root);
 	struct tcf_walker walker;
 
-	if (!force)
-		return false;
-
 	pr_debug("tcindex_destroy(tp %p),p %p\n", tp, p);
 	walker.count = 0;
 	walker.skip = 0;
@@ -544,7 +545,6 @@ static bool tcindex_destroy(struct tcf_proto *tp, bool force)
 	tcindex_walk(tp, &walker);
 
 	call_rcu(&p->rcu, __tcindex_destroy);
-	return true;
 }