sched: cls_flow: fix panic on filter replace

The following test case causes a NULL pointer dereference in cls_flow:

  tc filter add dev foo parent 1: handle 0x1 flow hash keys dst action ok
  tc filter replace dev foo parent 1: pref 49152 handle 0x1 \
            flow hash keys mark action drop

To be more precise, actually two different panics are fixed, the first
occurs because tcf_exts_init() is not called on the newly allocated
filter when we do a replace. And the second panic uncovered after that
happens since the arguments of list_replace_rcu() are swapped, the old
element needs to be the first argument and the new element the second.

Fixes: 70da9f0bf999 ("net: sched: cls_flow use RCU")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 2 deletions

diff --git a/net/sched/cls_flow.c b/net/sched/cls_flow.c
index 76bc3a20ffdb..bb2a0f529c1f 100644
--- a/net/sched/cls_flow.c
+++ b/net/sched/cls_flow.c
@@ -425,6 +425,8 @@ static int flow_change(struct net *net, struct sk_buff *in_skb,
         if (!fnew)
                 goto err2;
 
+        tcf_exts_init(&fnew->exts, TCA_FLOW_ACT, TCA_FLOW_POLICE);
+
         fold = (struct flow_filter *)*arg;
         if (fold) {
                 err = -EINVAL;
@@ -486,7 +488,6 @@ static int flow_change(struct net *net, struct sk_buff *in_skb,
                 fnew->mask  = ~0U;
                 fnew->tp = tp;
                 get_random_bytes(&fnew->hashrnd, 4);
-                tcf_exts_init(&fnew->exts, TCA_FLOW_ACT, TCA_FLOW_POLICE);
         }
 
         fnew->perturb_timer.function = flow_perturbation;
@@ -526,7 +527,7 @@ static int flow_change(struct net *net, struct sk_buff *in_skb,
         if (*arg == 0)
                 list_add_tail_rcu(&fnew->list, &head->filters);
         else
-                list_replace_rcu(&fnew->list, &fold->list);
+                list_replace_rcu(&fold->list, &fnew->list);
 
         *arg = (unsigned long)fnew;