fwd: Direct inbound spliced forwards to the guest's external address

In pasta mode, where addressing permits we "splice" connections, forwarding
directly from host socket to guest/container socket without any L2 or L3
processing.  This gives us a very large performance improvement when it's
possible.

Since the traffic is from a local socket within the guest, it will go over
the guest's 'lo' interface, and accordingly we set the guest side address
to be the loopback address.  However this has a surprising side effect:
sometimes guests will run services that are only supposed to be used within
the guest and are therefore bound to only 127.0.0.1 and/or ::1.  pasta's
forwarding exposes those services to the host, which isn't generally what
we want.

Correct this by instead forwarding inbound "splice" flows to the guest's
external address.

Link: https://github.com/containers/podman/issues/24045
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -912,6 +912,9 @@ pasta_opts:
 		"  -U, --udp-ns SPEC	UDP port forwarding to init namespace\n"
 		"    SPEC is as described above\n"
 		"    default: auto\n"
+		"  --host-lo-to-ns-lo	DEPRECATED:\n"
+		"			Translate host-loopback forwards to\n"
+		"			namespace loopback\n"
 		"  --userns NSPATH 	Target user namespace to join\n"
 		"  --netns PATH|NAME	Target network namespace to join\n"
 		"  --netns-only		Don't join existing user namespace\n"
@@ -1289,6 +1292,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"netns-only",	no_argument,		NULL,		20 },
 		{"map-host-loopback", required_argument, NULL,		21 },
 		{"map-guest-addr", required_argument,	NULL,		22 },
+		{"host-lo-to-ns-lo", no_argument, 	NULL,		23 },
 		{"dns-host",	required_argument,	NULL,		24 },
 		{ 0 },
 	};
@@ -1467,6 +1471,11 @@ void conf(struct ctx *c, int argc, char **argv)
 			conf_nat(optarg, &c->ip4.map_guest_addr,
 				 &c->ip6.map_guest_addr, NULL);
 			break;
+		case 23:
+			if (c->mode != MODE_PASTA)
+				die("--host-lo-to-ns-lo is for pasta mode only");
+			c->host_lo_to_ns_lo = 1;
+			break;
 		case 24:
 			if (inet_pton(AF_INET6, optarg, &c->ip6.dns_host) &&
 			    !IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_host))

fwd.c

@@ -447,20 +447,35 @@ uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto,
 	    (proto == IPPROTO_TCP || proto == IPPROTO_UDP)) {
 		/* spliceable */
 
-		/* Preserve the specific loopback adddress used, but let the
+		/* The traffic will go over the guest's 'lo' interface, but by
-		 * kernel pick a source port on the target side
+		 * default use its external address, so we don't inadvertently
+		 * expose services that listen only on the guest's loopback
+		 * address.  That can be overridden by --host-lo-to-ns-lo which
+		 * will instead forward to the loopback address in the guest.
+		 *
+		 * In either case, let the kernel pick the source address to
+		 * match.
 		 */
-		tgt->oaddr = ini->eaddr;
+		if (inany_v4(&ini->eaddr)) {
+			if (c->host_lo_to_ns_lo)
+				tgt->eaddr = inany_loopback4;
+			else
+				tgt->eaddr = inany_from_v4(c->ip4.addr_seen);
+			tgt->oaddr = inany_any4;
+		} else {
+			if (c->host_lo_to_ns_lo)
+				tgt->eaddr = inany_loopback6;
+			else
+				tgt->eaddr.a6 = c->ip6.addr_seen;
+			tgt->oaddr = inany_any6;
+		}
+
+		/* Let the kernel pick source port */
 		tgt->oport = 0;
 		if (proto == IPPROTO_UDP)
 			/* But for UDP preserve the source port */
 			tgt->oport = ini->eport;
 
-		if (inany_v4(&ini->eaddr))
-			tgt->eaddr = inany_loopback4;
-		else
-			tgt->eaddr = inany_loopback6;
-
 		return PIF_SPLICE;
 	}
 

passt.1

@@ -605,6 +605,13 @@ Configure UDP port forwarding from target namespace to init namespace.
 
 Default is \fBauto\fR.
 
+.TP
+.BR \-\-host-lo-to-ns-lo " " (DEPRECATED)
+If specified, connections forwarded with \fB\-t\fR and \fB\-u\fR from
+the host's loopback address will appear on the loopback address in the
+guest as well.  Without this option such forwarded packets will appear
+to come from the guest's public address.
+
 .TP
 .BR \-\-userns " " \fIspec
 Target user namespace to join, as a path. If PID is given, without this option,
@@ -893,8 +900,9 @@ interfaces, and it would also be impossible for guest or target
 namespace to route answers back.
 
 For convenience, the source address on these packets is translated to
-the address specified by the \fB\-\-map-host-loopback\fR option.  If
+the address specified by the \fB\-\-map-host-loopback\fR option (with
-not specified this defaults, somewhat arbitrarily, to the address of
+some exceptions in pasta mode, see next section below).  If not
+specified this defaults, somewhat arbitrarily, to the address of
 default IPv4 or IPv6 gateway (if any) -- this is known to be an
 existing, valid address on the same subnet.  If \fB\-\-no-map-gw\fR or
 \fB\-\-map-host-loopback none\fR are specified this translation is
@@ -931,8 +939,15 @@ and the new socket using the \fBsplice\fR(2) system call, and for UDP, a pair
 of \fBrecvmmsg\fR(2) and \fBsendmmsg\fR(2) system calls deals with packet
 transfers.
 
-This bypass only applies to local connections and traffic, because it's not
+Because it's not possible to bind sockets to foreign addresses, this
-possible to bind sockets to foreign addresses.
+bypass only applies to local connections and traffic.  It also means
+that the address translation differs slightly from passt mode.
+Connections from loopback to loopback on the host will appear to come
+from the target namespace's public address within the guest, unless
+\fB\-\-host-lo-to-ns-lo\fR is specified, in which case they will
+appear to come from loopback in the namespace as well.  The latter
+behaviour used to be the default, but is usually undesirable, since it
+can unintentionally expose namespace local services to the host.
 
 .SS Binding to low numbered ports (well-known or system ports, up to 1023)
 

passt.h

@@ -225,6 +225,7 @@ struct ip6_ctx {
  * @no_dhcpv6:		Disable DHCPv6 server
  * @no_ndp:		Disable NDP handler altogether
  * @no_ra:		Disable router advertisements
+ * @host_lo_to_ns_lo:	Map host loopback addresses to ns loopback addresses
  * @freebind:		Allow binding of non-local addresses for forwarding
  * @low_wmem:		Low probed net.core.wmem_max
  * @low_rmem:		Low probed net.core.rmem_max
@@ -285,6 +286,7 @@ struct ctx {
 	int no_dhcpv6;
 	int no_ndp;
 	int no_ra;
+	int host_lo_to_ns_lo;
 	int freebind;
 
 	int low_wmem;