netfilter: nft_meta: add cgroup support

This allows you to filter traffic by process control group (cgroup).

Signed-off-by: Ana Rey <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/uapi/linux/netfilter/nf_tables.h

···
  * @NFT_META_CPU: cpu id through smp_processor_id()
  * @NFT_META_IIFGROUP: packet input interface group
  * @NFT_META_OIFGROUP: packet output interface group
+ * @NFT_META_CGROUP: socket control group (skb->sk->sk_classid)
  */
 enum nft_meta_keys {
 	NFT_META_LEN,
···
 	NFT_META_CPU,
 	NFT_META_IIFGROUP,
 	NFT_META_OIFGROUP,
+	NFT_META_CGROUP,
 };
 
 /**

net/netfilter/nft_meta.c

···
 			goto err;
 		dest->data[0] = out->group;
 		break;
+	case NFT_META_CGROUP:
+		if (skb->sk == NULL)
+			break;
+
+		dest->data[0] = skb->sk->sk_classid;
+		break;
 	default:
 		WARN_ON(1);
 		goto err;
···
 	case NFT_META_CPU:
 	case NFT_META_IIFGROUP:
 	case NFT_META_OIFGROUP:
+	case NFT_META_CGROUP:
 		break;
 	default:
 		return -EOPNOTSUPP;