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_fw.c b/net/sched/cls_fw.c
index da805aeeb65c..dbfdfd1f1a9f 100644
--- a/net/sched/cls_fw.c
+++ b/net/sched/cls_fw.c
@@ -123,9 +123,7 @@ static int fw_init(struct tcf_proto *tp)
 static void fw_delete_filter(struct rcu_head *head)
 {
 	struct fw_filter *f = container_of(head, struct fw_filter, rcu);
-	struct tcf_proto *tp = f->tp;
 
-	tcf_unbind_filter(tp, &f->res);
 	tcf_exts_destroy(&f->exts);
 	kfree(f);
 }
@@ -143,6 +141,7 @@ static void fw_destroy(struct tcf_proto *tp)
 		while ((f = rtnl_dereference(head->ht[h])) != NULL) {
 			RCU_INIT_POINTER(head->ht[h],
 					 rtnl_dereference(f->next));
+			tcf_unbind_filter(tp, &f->res);
 			call_rcu(&f->rcu, fw_delete_filter);
 		}
 	}
@@ -166,6 +165,7 @@ static int fw_delete(struct tcf_proto *tp, unsigned long arg)
 	     fp = &pfp->next, pfp = rtnl_dereference(*fp)) {
 		if (pfp == f) {
 			RCU_INIT_POINTER(*fp, rtnl_dereference(f->next));
+			tcf_unbind_filter(tp, &f->res);
 			call_rcu(&f->rcu, fw_delete_filter);
 			return 0;
 		}
@@ -280,6 +280,7 @@ static int fw_change(struct net *net, struct sk_buff *in_skb,
 
 		RCU_INIT_POINTER(fnew->next, rtnl_dereference(pfp->next));
 		rcu_assign_pointer(*fp, fnew);
+		tcf_unbind_filter(tp, &f->res);
 		call_rcu(&f->rcu, fw_delete_filter);
 
 		*arg = (unsigned long)fnew;