net/sched: act_ife: avoid possible NULL deref

tcf_ife_encode() must make sure ife_encode() does not return NULL.

syzbot reported:

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
 RIP: 0010:ife_tlv_meta_encode+0x41/0xa0 net/ife/ife.c:166
CPU: 3 UID: 0 PID: 8990 Comm: syz.0.696 Not tainted syzkaller #0 PREEMPT(full)
Call Trace:
 <TASK>
  ife_encode_meta_u32+0x153/0x180 net/sched/act_ife.c:101
  tcf_ife_encode net/sched/act_ife.c:841 [inline]
  tcf_ife_act+0x1022/0x1de0 net/sched/act_ife.c:877
  tc_act include/net/tc_wrapper.h:130 [inline]
  tcf_action_exec+0x1c0/0xa20 net/sched/act_api.c:1152
  tcf_exts_exec include/net/pkt_cls.h:349 [inline]
  mall_classify+0x1a0/0x2a0 net/sched/cls_matchall.c:42
  tc_classify include/net/tc_wrapper.h:197 [inline]
  __tcf_classify net/sched/cls_api.c:1764 [inline]
  tcf_classify+0x7f2/0x1380 net/sched/cls_api.c:1860
  multiq_classify net/sched/sch_multiq.c:39 [inline]
  multiq_enqueue+0xe0/0x510 net/sched/sch_multiq.c:66
  dev_qdisc_enqueue+0x45/0x250 net/core/dev.c:4147
  __dev_xmit_skb net/core/dev.c:4262 [inline]
  __dev_queue_xmit+0x2998/0x46c0 net/core/dev.c:4798

Fixes: 295a6e0 ("net/sched: act_ife: Change to use ife module")
Reported-by: syzbot+5cf914f193dffde3bd3c@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6970d61d.050a0220.706b.0010.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yotam Gigi <yotam.gi@gmail.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260121133724.3400020-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/sched/act_ife.c

@@ -821,6 +821,7 @@ static int tcf_ife_encode(struct sk_buff *skb, const struct tc_action *a,
 	/* could be stupid policy setup or mtu config
 	 * so lets be conservative.. */
 	if ((action == TC_ACT_SHOT) || exceed_mtu) {
+drop:
 		qstats_drop_inc(this_cpu_ptr(ife->common.cpu_qstats));
 		return TC_ACT_SHOT;
 	}
@@ -829,6 +830,8 @@ static int tcf_ife_encode(struct sk_buff *skb, const struct tc_action *a,
 		skb_push(skb, skb->dev->hard_header_len);
 
 	ife_meta = ife_encode(skb, metalen);
+	if (!ife_meta)
+		goto drop;
 
 	spin_lock(&ife->tcf_lock);
 
@@ -844,8 +847,7 @@ static int tcf_ife_encode(struct sk_buff *skb, const struct tc_action *a,
 		if (err < 0) {
 			/* too corrupt to keep around if overwritten */
 			spin_unlock(&ife->tcf_lock);
-			qstats_drop_inc(this_cpu_ptr(ife->common.cpu_qstats));
-			return TC_ACT_SHOT;
+			goto drop;
 		}
 		skboff += err;
 	}