netfilter: ipt_ULOG: fix non-null terminated string in the nf_log path

If nf_log uses ipt_ULOG as logging output, we can deliver non-null
terminated strings to user-space since the maximum length of the
prefix that is passed by nf_log is NF_LOG_PREFIXLEN but pm->prefix
is 32 bytes long (ULOG_PREFIX_LEN).

This is actually happening already from nf_conntrack_tcp if ipt_ULOG
is used, since it is passing strings longer than 32 bytes.
Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/ipt_ULOG.c

@@ -231,8 +231,10 @@ static void ipt_ulog_packet(struct net *net,
 	put_unaligned(tv.tv_usec, &pm->timestamp_usec);
 	put_unaligned(skb->mark, &pm->mark);
 	pm->hook = hooknum;
-	if (prefix != NULL)
-		strncpy(pm->prefix, prefix, sizeof(pm->prefix));
+	if (prefix != NULL) {
+		strncpy(pm->prefix, prefix, sizeof(pm->prefix) - 1);
+		pm->prefix[sizeof(pm->prefix) - 1] = '\0';
+	}
 	else if (loginfo->prefix[0] != '\0')
 		strncpy(pm->prefix, loginfo->prefix, sizeof(pm->prefix));
 	else