net/sched: cls_fw: fix NULL pointer dereference on shared blocks

The old-method path in fw_classify() calls tcf_block_q() and
dereferences q->handle.  Shared blocks leave block->q NULL, causing a
NULL deref when an empty cls_fw filter is attached to a shared block
and a packet with a nonzero major skb mark is classified.

Reject the configuration in fw_change() when the old method (no
TCA_OPTIONS) is used on a shared block, since fw_classify()'s
old-method path needs block->q which is NULL for shared blocks.

The fixed null-ptr-deref calling stack:
 KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f]
 RIP: 0010:fw_classify (net/sched/cls_fw.c:81)
 Call Trace:
  tcf_classify (./include/net/tc_wrapper.h:197 net/sched/cls_api.c:1764 net/sched/cls_api.c:1860)
  tc_run (net/core/dev.c:4401)
  __dev_queue_xmit (net/core/dev.c:4535 net/core/dev.c:4790)

Fixes: 1abf272022 ("net: sched: tcindex, fw, flow: use tcf_block_q helper to get struct Qdisc")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260331050217.504278-1-xmei5@asu.edu
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/sched/cls_fw.c

@@ -247,8 +247,18 @@ static int fw_change(struct net *net, struct sk_buff *in_skb,
 	struct nlattr *tb[TCA_FW_MAX + 1];
 	int err;
 
-	if (!opt)
+	if (!opt) {
-		return handle ? -EINVAL : 0; /* Succeed if it is old method. */
+		if (handle)
+			return -EINVAL;
+
+		if (tcf_block_shared(tp->chain->block)) {
+			NL_SET_ERR_MSG(extack,
+				       "Must specify mark when attaching fw filter to block");
+			return -EINVAL;
+		}
+
+		return 0; /* Succeed if it is old method. */
+	}
 
 	err = nla_parse_nested_deprecated(tb, TCA_FW_MAX, opt, fw_policy,
 					  NULL);