net/sched: clear actions pointer in miss cookie init fail

Palash reports a UAF when using a modified version of syzkaller[1].

When 'tcf_exts_miss_cookie_base_alloc()' fails in 'tcf_exts_init_ex()'
a call to 'tcf_exts_destroy()' is made to free up the tcf_exts
resources.
In flower, a call to '__fl_put()' when 'tcf_exts_init_ex()' fails is made;
Then calling 'tcf_exts_destroy()', which triggers an UAF since the
already freed tcf_exts action pointer is lingering in the struct.

Before the offending patch, this was not an issue since there was no
case where the tcf_exts action pointer could linger. Therefore, restore
the old semantic by clearing the action pointer in case of a failure to
initialize the miss_cookie.

[1] https://github.com/cmu-pasta/linux-kernel-enriched-corpus

v1->v2: Fix compilation on configs without tc actions (kernel test robot)

Fixes: 80cd22c35c ("net/sched: cls_api: Support hardware miss to tc action")
Reported-by: Palash Oswal <oswalpalash@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -3235,6 +3235,9 @@ int tcf_exts_init_ex(struct tcf_exts *exts, struct net *net, int action,
 
 err_miss_alloc:
 	tcf_exts_destroy(exts);
+#ifdef CONFIG_NET_CLS_ACT
+	exts->actions = NULL;
+#endif
 	return err;
 }
 EXPORT_SYMBOL(tcf_exts_init_ex);