[patch net] net: sched: fix use-after-free in tcf_action_destroy and tcf_del_walker

[Jiri Pirko]

From: Jiri Pirko <j...@mellanox.com>

Recent commit d7fb60b9cafb ("net_sched: get rid of tcfa_rcu") removed
freeing in call_rcu, which changed already existing hard-to-hit
race condition into 100% hit:

[  598.599825] BUG: unable to handle kernel NULL pointer dereference at 
0000000000000030
[  598.607782] IP: tcf_action_destroy+0xc0/0x140

Or:

[   40.858924] BUG: unable to handle kernel NULL pointer dereference at 
0000000000000030
[   40.862840] IP: tcf_generic_walker+0x534/0x820

Fix this by storing the ops and use them directly for module_put call.

Fixes: a85a970af265 ("net_sched: move tc_action into tcf_common")
Signed-off-by: Jiri Pirko <j...@mellanox.com>
---
 net/sched/act_api.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index fcd7dc7..da6fa82 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c
@@ -180,7 +180,7 @@ static int tcf_del_walker(struct tcf_idrinfo *idrinfo, 
struct sk_buff *skb,
        idr_for_each_entry_ext(idr, p, id) {
                ret = __tcf_idr_release(p, false, true);
                if (ret == ACT_P_DELETED) {
-                       module_put(p->ops->owner);
+                       module_put(ops->owner);
                        n_i++;
                } else if (ret < 0) {
                        goto nla_put_failure;
@@ -514,13 +514,15 @@ EXPORT_SYMBOL(tcf_action_exec);
 
 int tcf_action_destroy(struct list_head *actions, int bind)
 {
+       const struct tc_action_ops *ops;
        struct tc_action *a, *tmp;
        int ret = 0;
 
        list_for_each_entry_safe(a, tmp, actions, list) {
+               ops = a->ops;
                ret = __tcf_idr_release(a, bind, true);
                if (ret == ACT_P_DELETED)
-                       module_put(a->ops->owner);
+                       module_put(ops->owner);
                else if (ret < 0)
                        return ret;
        }
-- 
2.9.3