net: sched: do not use tcf_proto 'tp' argument from call_rcu

Using the tcf_proto pointer 'tp' from inside the classifiers callback
is not valid because it may have been cleaned up by another call_rcu
occuring on another CPU.

'tp' is currently being used by tcf_unbind_filter() in this patch we
move instances of tcf_unbind_filter outside of the call_rcu() context.
This is safe to do because any running schedulers will either read the
valid class field or it will be zeroed.

And all schedulers today when the class is 0 do a lookup using the
same call used by the tcf_exts_bind(). So even if we have a running
classifier hit the null class pointer it will do a lookup and get
to the same result. This is particularly fragile at the moment because
the only way to verify this is to audit the schedulers call sites.

Reported-by: Cong Wang <xiyou.wangconf@gmail.com>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Acked-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 2 deletions

diff --git a/net/sched/cls_basic.c b/net/sched/cls_basic.c
index 90647a8..cd61280 100644
--- a/net/sched/cls_basic.c
+++ b/net/sched/cls_basic.c
@@ -91,9 +91,7 @@ static int basic_init(struct tcf_proto *tp)
 static void basic_delete_filter(struct rcu_head *head)
 {
 	struct basic_filter *f = container_of(head, struct basic_filter, rcu);
-	struct tcf_proto *tp = f->tp;
 
-	tcf_unbind_filter(tp, &f->res);
 	tcf_exts_destroy(&f->exts);
 	tcf_em_tree_destroy(&f->ematches);
 	kfree(f);
@@ -106,6 +104,7 @@ static void basic_destroy(struct tcf_proto *tp)
 
 	list_for_each_entry_safe(f, n, &head->flist, link) {
 		list_del_rcu(&f->link);
+		tcf_unbind_filter(tp, &f->res);
 		call_rcu(&f->rcu, basic_delete_filter);
 	}
 	RCU_INIT_POINTER(tp->root, NULL);
@@ -120,6 +119,7 @@ static int basic_delete(struct tcf_proto *tp, unsigned long arg)
 	list_for_each_entry(t, &head->flist, link)
 		if (t == f) {
 			list_del_rcu(&t->link);
+			tcf_unbind_filter(tp, &t->res);
 			call_rcu(&t->rcu, basic_delete_filter);
 			return 0;
 		}
@@ -222,6 +222,7 @@ static int basic_change(struct net *net, struct sk_buff *in_skb,
 
 	if (fold) {
 		list_replace_rcu(&fold->link, &fnew->link);
+		tcf_unbind_filter(tp, &fold->res);
 		call_rcu(&fold->rcu, basic_delete_filter);
 	} else {
 		list_add_rcu(&fnew->link, &head->flist);