cls_cgroup: read classid atomically in classifier

Avoid reading the unsynchronized value cs->classid multiple times,
since it could change concurrently from non-zero to zero; this would
result in the classifier returning a positive result with a bogus
(zero) classid.

Signed-off-by: Paul Menage <menage@google.com>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 11 insertions, 11 deletions

diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
index 91a3db4a76f8..cc29b44b1500 100644
--- a/net/sched/cls_cgroup.c
+++ b/net/sched/cls_cgroup.c
@@ -104,8 +104,7 @@ static int cls_cgroup_classify(struct sk_buff *skb, struct tcf_proto *tp,
                                struct tcf_result *res)
 {
         struct cls_cgroup_head *head = tp->root;
-        struct cgroup_cls_state *cs;
-        int ret = 0;
+        u32 classid;
 
         /*
          * Due to the nature of the classifier it is required to ignore all
@@ -121,17 +120,18 @@ static int cls_cgroup_classify(struct sk_buff *skb, struct tcf_proto *tp,
                 return -1;
 
         rcu_read_lock();
-        cs = task_cls_state(current);
-        if (cs->classid && tcf_em_tree_match(skb, &head->ematches, NULL)) {
-                res->classid = cs->classid;
-                res->class = 0;
-                ret = tcf_exts_exec(skb, &head->exts, res);
-        } else
-                ret = -1;
-
+        classid = task_cls_state(current)->classid;
         rcu_read_unlock();
 
-        return ret;
+        if (!classid)
+                return -1;
+
+        if (!tcf_em_tree_match(skb, &head->ematches, NULL))
+                return -1;
+
+        res->classid = classid;
+        res->class = 0;
+        return tcf_exts_exec(skb, &head->exts, res);
 }
 
 static unsigned long cls_cgroup_get(struct tcf_proto *tp, u32 handle)