userns: Convert xt_LOG to print socket kuids and kgids as uids and gids

xt_LOG always writes messages via sb_add via printk.  Therefore when
xt_LOG logs the uid and gid of a socket a packet came from the
values should be converted to be in the initial user namespace.

Thus making xt_LOG as user namespace safe as possible.

Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Patrick McHardy <kaber@trash.net>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

init/Kconfig

@@ -945,7 +945,6 @@ config UIDGID_CONVERTED
 	depends on NET_9P = n
 	depends on NETFILTER_XT_MATCH_OWNER = n
 	depends on NETFILTER_XT_MATCH_RECENT = n
-	depends on NETFILTER_XT_TARGET_LOG = n
 	depends on AF_RXRPC = n
 	depends on NET_KEY = n
 	depends on DNS_RESOLVER = n

net/netfilter/xt_LOG.c

@@ -363,10 +363,12 @@ static void dump_ipv4_packet(struct sbuff *m,
 	/* Max length: 15 "UID=4294967295 " */
 	if ((logflags & XT_LOG_UID) && !iphoff && skb->sk) {
 		read_lock_bh(&skb->sk->sk_callback_lock);
-		if (skb->sk->sk_socket && skb->sk->sk_socket->file)
+		if (skb->sk->sk_socket && skb->sk->sk_socket->file) {
+			const struct cred *cred = skb->sk->sk_socket->file->f_cred;
 			sb_add(m, "UID=%u GID=%u ",
-				skb->sk->sk_socket->file->f_cred->fsuid,
-				skb->sk->sk_socket->file->f_cred->fsgid);
+				from_kuid_munged(&init_user_ns, cred->fsuid),
+				from_kgid_munged(&init_user_ns, cred->fsgid));
+		}
 		read_unlock_bh(&skb->sk->sk_callback_lock);
 	}
 
@@ -719,10 +721,12 @@ static void dump_ipv6_packet(struct sbuff *m,
 	/* Max length: 15 "UID=4294967295 " */
 	if ((logflags & XT_LOG_UID) && recurse && skb->sk) {
 		read_lock_bh(&skb->sk->sk_callback_lock);
-		if (skb->sk->sk_socket && skb->sk->sk_socket->file)
+		if (skb->sk->sk_socket && skb->sk->sk_socket->file) {
+			const struct cred *cred = skb->sk->sk_socket->file->f_cred;
 			sb_add(m, "UID=%u GID=%u ",
-				skb->sk->sk_socket->file->f_cred->fsuid,
-				skb->sk->sk_socket->file->f_cred->fsgid);
+				from_kuid_munged(&init_user_ns, cred->fsuid),
+				from_kgid_munged(&init_user_ns, cred->fsgid));
+		}
 		read_unlock_bh(&skb->sk->sk_callback_lock);
 	}