[PATCH net v2 2/2] ipvs: reject FTP control ports as data ports

[Zihan Xi <zihanx@nebusec.ai>]

ip_vs_ftp_out() creates a wildcard data connection from the
server-advertised passive port. If that port is one of the configured FTP
control ports, ip_vs_conn_new() binds the FTP helper to the new connection
again. A subsequent wildcard lookup can then extend a controlled-connection
chain.

Reject zero and configured control ports before creating passive
connections. For active mode, reject a zero client port and a data port
derived from a configured control port.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: LLM
Co-developed-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
---
changes in v2:
  - Reject the data port derived from a configured control port in
    ip_vs_ftp_in() to cover the active-mode bypass.
  - v1 Link:
    https://lore.kernel.org/all/cover.1789110326.git.zihanx@nebusec.ai/

 net/netfilter/ipvs/ip_vs_ftp.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/net/netfilter/ipvs/ip_vs_ftp.c b/net/netfilter/ipvs/ip_vs_ftp.c
index 9e3e005a82635..4822a1a75212d 100644
--- a/net/netfilter/ipvs/ip_vs_ftp.c
+++ b/net/netfilter/ipvs/ip_vs_ftp.c
@@ -62,6 +62,17 @@ static unsigned short ports[IP_VS_APP_MAX_PORTS] = {21, 0};
 module_param_array(ports, ushort, &ports_count, 0444);
 MODULE_PARM_DESC(ports, "Ports to monitor for FTP control commands");
 
+static bool is_control_port(u16 port)
+{
+	unsigned int i;
+
+	for (i = 0; i < ports_count; i++) {
+		if (ports[i] == port)
+			return true;
+	}
+	return false;
+}
+
 
 static char *ip_vs_ftp_data_ptr(struct sk_buff *skb, struct ip_vs_iphdr *ipvsh)
 {
@@ -319,6 +330,10 @@ static int ip_vs_ftp_out(struct ip_vs_app *app, struct ip_vs_conn *cp,
 		return 1;
 	}
 
+	/* Do not redirect data to control ports */
+	if (!port || is_control_port(ntohs(port)))
+		return 0;
+
 	/* Now update or create a connection entry for it */
 	{
 		struct ip_vs_conn_param p;
@@ -529,6 +544,9 @@ static int ip_vs_ftp_in(struct ip_vs_app *app, struct ip_vs_conn *cp,
 		return 1;
 	}
 
+	if (!port || is_control_port(ntohs(cp->vport) - 1))
+		return 0;
+
 	/* Passive mode off */
 	cp->app_data = (void *) IP_VS_FTP_ACTIVE;
 
-- 
2.43.0