netfilter: nfnetlink_queue: enable classid socket info retrieval

This enables associating a socket with a v1 net_cls cgroup. Useful for
applying a per-cgroup policy when processing packets in userspace.

Signed-off-by: Eric Sage <eric_sage@apple.com>

include/uapi/linux/netfilter/nfnetlink_queue.h

@@ -62,6 +62,7 @@ enum nfqnl_attr_type {
 	NFQA_VLAN,			/* nested attribute: packet vlan info */
 	NFQA_L2HDR,			/* full L2 header */
 	NFQA_PRIORITY,			/* skb->priority */
+	NFQA_CGROUP_CLASSID,		/* __u32 cgroup classid */
 
 	__NFQA_MAX
 };

net/netfilter/nfnetlink_queue.c

@@ -29,6 +29,7 @@
 #include <linux/netfilter/nfnetlink_queue.h>
 #include <linux/netfilter/nf_conntrack_common.h>
 #include <linux/list.h>
+#include <linux/cgroup-defs.h>
 #include <net/sock.h>
 #include <net/tcp_states.h>
 #include <net/netfilter/nf_queue.h>
@@ -301,6 +302,19 @@ static int nfqnl_put_sk_uidgid(struct sk_buff *skb, struct sock *sk)
 	return -1;
 }
 
+static int nfqnl_put_sk_classid(struct sk_buff *skb, struct sock *sk)
+{
+#if IS_ENABLED(CONFIG_CGROUP_NET_CLASSID)
+	if (sk && sk_fullsock(sk)) {
+		u32 classid = sock_cgroup_classid(&sk->sk_cgrp_data);
+
+		if (classid && nla_put_be32(skb, NFQA_CGROUP_CLASSID, htonl(classid)))
+			return -1;
+	}
+#endif
+	return 0;
+}
+
 static u32 nfqnl_get_sk_secctx(struct sk_buff *skb, char **secdata)
 {
 	u32 seclen = 0;
@@ -407,6 +421,9 @@ nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 		+ nla_total_size(sizeof(struct nfqnl_msg_packet_hw))
 		+ nla_total_size(sizeof(u_int32_t))	/* skbinfo */
 		+ nla_total_size(sizeof(u_int32_t));	/* cap_len */
+#if IS_ENABLED(CONFIG_CGROUP_NET_CLASSID)
+		+ nla_total_size(sizeof(u_int32_t));	/* classid */
+#endif
 
 	tstamp = skb_tstamp_cond(entskb, false);
 	if (tstamp)
@@ -599,6 +616,9 @@ nfqnl_build_packet_message(struct net *net, struct nfqnl_instance *queue,
 	    nfqnl_put_sk_uidgid(skb, entskb->sk) < 0)
 		goto nla_put_failure;
 
+	if (nfqnl_put_sk_classid(skb, entskb->sk) < 0)
+		goto nla_put_failure;
+
 	if (seclen && nla_put(skb, NFQA_SECCTX, seclen, secdata))
 		goto nla_put_failure;