netfilter: nf_ct_ftp: add sequence tracking pickup facility for injected entries

This patch allows the FTP helper to pickup the sequence tracking from
the first packet seen. This is useful to fix the breakage of the first
FTP command after the failover while using conntrackd to synchronize
states.

The seq_aft_nl_num field in struct nf_ct_ftp_info has been shrinked to
16-bits (enough for what it does), so we can use the remaining 16-bits
to store the flags while using the same size for the private FTP helper
data.

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

include/linux/netfilter/nf_conntrack_ftp.h

···
 
 #define FTP_PORT	21
 
+#define NF_CT_FTP_SEQ_PICKUP	(1 << 0)
+
 #define NUM_SEQ_TO_REMEMBER 2
 /* This structure exists only once per master */
 struct nf_ct_ftp_master {
 	/* Valid seq positions for cmd matching after newline */
 	u_int32_t seq_aft_nl[IP_CT_DIR_MAX][NUM_SEQ_TO_REMEMBER];
 	/* 0 means seq_match_aft_nl not set */
-	int seq_aft_nl_num[IP_CT_DIR_MAX];
+	u_int16_t seq_aft_nl_num[IP_CT_DIR_MAX];
+	/* pickup sequence tracking, useful for conntrackd */
+	u_int16_t flags[IP_CT_DIR_MAX];
 };
 
 struct nf_conntrack_expect;

net/netfilter/nf_conntrack_ftp.c

···
 
 	/* Look up to see if we're just after a \n. */
 	if (!find_nl_seq(ntohl(th->seq), ct_ftp_info, dir)) {
+		/* We're picking up this, clear flags and let it continue */
+		if (unlikely(ct_ftp_info->flags[dir] & NF_CT_FTP_SEQ_PICKUP)) {
+			ct_ftp_info->flags[dir] ^= NF_CT_FTP_SEQ_PICKUP;
+			goto skip_nl_seq;
+		}
+
 		/* Now if this ends in \n, update ftp info. */
 		pr_debug("nf_conntrack_ftp: wrong seq pos %s(%u) or %s(%u)\n",
 			 ct_ftp_info->seq_aft_nl_num[dir] > 0 ? "" : "(UNSET)",
···
 		goto out_update_nl;
 	}
 
+skip_nl_seq:
 	/* Initialize IP/IPv6 addr to expected address (it's not mentioned
 	   in EPSV responses) */
 	cmd.l3num = nf_ct_l3num(ct);
···
 	return ret;
 }
 
+static int nf_ct_ftp_from_nlattr(struct nlattr *attr, struct nf_conn *ct)
+{
+	struct nf_ct_ftp_master *ftp = nfct_help_data(ct);
+
+	/* This conntrack has been injected from user-space, always pick up
+	 * sequence tracking. Otherwise, the first FTP command after the
+	 * failover breaks.
+	 */
+	ftp->flags[IP_CT_DIR_ORIGINAL] |= NF_CT_FTP_SEQ_PICKUP;
+	ftp->flags[IP_CT_DIR_REPLY] |= NF_CT_FTP_SEQ_PICKUP;
+	return 0;
+}
+
 static struct nf_conntrack_helper ftp[MAX_PORTS][2] __read_mostly;
 
 static const struct nf_conntrack_expect_policy ftp_exp_policy = {
···
 			ftp[i][j].expect_policy = &ftp_exp_policy;
 			ftp[i][j].me = THIS_MODULE;
 			ftp[i][j].help = help;
+			ftp[i][j].from_nlattr = nf_ct_ftp_from_nlattr;
 			if (ports[i] == FTP_PORT)
 				sprintf(ftp[i][j].name, "ftp");
 			else

net/netfilter/nf_conntrack_netlink.c

···
 	if (help) {
 		if (help->helper == helper) {
 			/* update private helper data if allowed. */
-			if (helper->from_nlattr && helpinfo)
+			if (helper->from_nlattr)
 				helper->from_nlattr(helpinfo, ct);
 			return 0;
 		} else
···
 				goto err2;
 			}
 			/* set private helper data if allowed. */
-			if (helper->from_nlattr && helpinfo)
+			if (helper->from_nlattr)
 				helper->from_nlattr(helpinfo, ct);
 
 			/* not in hash table yet so not strictly necessary */

net/netfilter/nfnetlink_cthelper.c

···
 {
 	const struct nf_conn_help *help = nfct_help(ct);
 
+	if (attr == NULL)
+		return -EINVAL;
+
 	if (help->helper->data_len == 0)
 		return -EINVAL;