port_fwd, util: Don't bind UDP ports with opposite-side bound TCP ports

When pasta periodically scans bound ports and binds them on the other
side in order to forward traffic, we bind UDP ports for corresponding
TCP port numbers, too, to support protocols and applications such as
iperf3 which use UDP port numbers matching the ones used by the TCP
data connection.

If we scan UDP ports in order to bind UDP ports, we skip detection of
the UDP ports we already bound ourselves, to avoid looping back our
own ports. Same with scanning and binding TCP ports.

But if we scan for TCP ports in order to bind UDP ports, we need to
skip bound TCP ports too, otherwise, as David pointed out:

- we find a bound TCP port on side A, and bind the corresponding TCP
  and UDP ports on side B

- at the next periodic scan, we find that UDP port bound on side B,
  and we bind the corresponding UDP port on side A

- at this point, we unbind that UDP port on side B: we would
  otherwise loop back our own port.

To fix this, we need to avoid binding UDP ports that we already
bound, on the other side, as a consequence of finding a corresponding
bound TCP port.

Reproducing this issue is straightforward:

  ./pasta -- iperf3 -s

  # Wait one second, then from another terminal:
  iperf3 -c ::1 -u

Reported-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
Analysed-by: David Gibson <david@gibson.dropbear.id.au>
Fixes: 457ff122e33c ("udp,pasta: Periodically scan for ports to automatically forward")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 19 insertions, 8 deletions

diff --git a/port_fwd.c b/port_fwd.c
index eac8d2f..7943a30 100644
--- a/port_fwd.c
+++ b/port_fwd.c
@@ -86,22 +86,33 @@ void port_fwd_scan_tcp(struct port_fwd *fwd, const struct port_fwd *rev)
  * port_fwd_scan_tcp() - Scan /proc to update TCP forwarding map
  * @fwd:	Forwarding information to update
  * @rev:	Forwarding information for the reverse direction
- * @tcp:	Corresponding TCP forwarding information
+ * @tcp_fwd:	Corresponding TCP forwarding information
+ * @tcp_rev:	TCP forwarding information for the reverse direction
  */
 void port_fwd_scan_udp(struct port_fwd *fwd, const struct port_fwd *rev,
-		       const struct port_fwd *tcp)
+		       const struct port_fwd *tcp_fwd,
+		       const struct port_fwd *tcp_rev)
 {
+	uint8_t exclude[PORT_BITMAP_SIZE];
+
+	bitmap_or(exclude, PORT_BITMAP_SIZE, rev->map, tcp_rev->map);
+
 	memset(fwd->map, 0, PORT_BITMAP_SIZE);
-	procfs_scan_listen(fwd->scan4, UDP_LISTEN, fwd->map, rev->map);
-	procfs_scan_listen(fwd->scan6, UDP_LISTEN, fwd->map, rev->map);
+	procfs_scan_listen(fwd->scan4, UDP_LISTEN, fwd->map, exclude);
+	procfs_scan_listen(fwd->scan6, UDP_LISTEN, fwd->map, exclude);
 
 	/* Also forward UDP ports with the same numbers as bound TCP ports.
 	 * This is useful for a handful of protocols (e.g. iperf3) where a TCP
 	 * control port is used to set up transfers on a corresponding UDP
 	 * port.
+	 *
+	 * This means we need to skip numbers of TCP ports bound on the other
+	 * side, too. Otherwise, we would detect corresponding UDP ports as
+	 * bound and try to forward them from the opposite side, but it's
+	 * already us handling them.
 	 */
-	procfs_scan_listen(tcp->scan4, TCP_LISTEN, fwd->map, rev->map);
-	procfs_scan_listen(tcp->scan6, TCP_LISTEN, fwd->map, rev->map);
+	procfs_scan_listen(tcp_fwd->scan4, TCP_LISTEN, fwd->map, exclude);
+	procfs_scan_listen(tcp_fwd->scan6, TCP_LISTEN, fwd->map, exclude);
 }
 
 /**
@@ -126,7 +137,7 @@ void port_fwd_init(struct ctx *c)
 		c->udp.fwd_in.f.scan4 = open_in_ns(c, "/proc/net/udp", flags);
 		c->udp.fwd_in.f.scan6 = open_in_ns(c, "/proc/net/udp6", flags);
 		port_fwd_scan_udp(&c->udp.fwd_in.f, &c->udp.fwd_out.f,
-				  &c->tcp.fwd_in);
+				  &c->tcp.fwd_in, &c->tcp.fwd_out);
 	}
 	if (c->tcp.fwd_out.mode == FWD_AUTO) {
 		c->tcp.fwd_out.scan4 = open("/proc/net/tcp", flags);
@@ -137,6 +148,6 @@ void port_fwd_init(struct ctx *c)
 		c->udp.fwd_out.f.scan4 = open("/proc/net/udp", flags);
 		c->udp.fwd_out.f.scan6 = open("/proc/net/udp6", flags);
 		port_fwd_scan_udp(&c->udp.fwd_out.f, &c->udp.fwd_in.f,
-				  &c->tcp.fwd_out);
+				  &c->tcp.fwd_out, &c->tcp.fwd_in);
 	}
 }