netfilter: ctnetlink: must check mark attributes vs NULL

else we will oops (null deref) when the attributes aren't present.

Also add back the EOPNOTSUPP in case MARK filtering is requested but
kernel doesn't support it.

Fixes: 59c08c6 ("netfilter: ctnetlink: Support L3 protocol-filter on flush")
Reported-by: syzbot+e45eda8eda6e93a03959@syzkaller.appspotmail.com
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_netlink.c

@@ -832,15 +832,22 @@ ctnetlink_alloc_filter(const struct nlattr * const cda[], u8 family)
 {
 	struct ctnetlink_filter *filter;
 
+#ifndef CONFIG_NF_CONNTRACK_MARK
+	if (cda[CTA_MARK] && cda[CTA_MARK_MASK])
+		return ERR_PTR(-EOPNOTSUPP);
+#endif
+
 	filter = kzalloc(sizeof(*filter), GFP_KERNEL);
 	if (filter == NULL)
 		return ERR_PTR(-ENOMEM);
 
 	filter->family = family;
 
 #ifdef CONFIG_NF_CONNTRACK_MARK
-	filter->mark.val = ntohl(nla_get_be32(cda[CTA_MARK]));
-	filter->mark.mask = ntohl(nla_get_be32(cda[CTA_MARK_MASK]));
+	if (cda[CTA_MARK] && cda[CTA_MARK_MASK]) {
+		filter->mark.val = ntohl(nla_get_be32(cda[CTA_MARK]));
+		filter->mark.mask = ntohl(nla_get_be32(cda[CTA_MARK_MASK]));
+	}
 #endif
 	return filter;
 }