net/sched: act_gact: disallow 'goto chain' on fallback control action

in the following command:

 # tc action add action <c1> random <rand_type> <c2> <rand_param>

'goto chain x' is allowed only for c1: setting it for c2 makes the kernel
crash with NULL pointer dereference, since TC core doesn't initialize the
chain handle.
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/act_gact.c

@@ -88,6 +88,11 @@ static int tcf_gact_init(struct net *net, struct nlattr *nla,
 		p_parm = nla_data(tb[TCA_GACT_PROB]);
 		if (p_parm->ptype >= MAX_RAND)
 			return -EINVAL;
+		if (TC_ACT_EXT_CMP(p_parm->paction, TC_ACT_GOTO_CHAIN)) {
+			NL_SET_ERR_MSG(extack,
+				       "goto chain not allowed on fallback");
+			return -EINVAL;
+		}
 	}
 #endif