[PATCH v3 8/8] [RFC, DO NOT APPLY] tcp, udp: Bind outbound listening sockets by interface instead of address

[David Gibson <david@gibson.dropbear.id.au>]

NOTE: I've discovered this approach doesn't work to address bug 100,
although it may still be worthwhile for secondary reasons.

Currently, outbound forwards (-T, -U) are handled by sockets bound to the
loopback address.  Typically we create two sockets, one for 127.0.0.1 and
one for ::1.

This has some disadvantages:
 * The guest can't connect to these services using its global IP address,
   it must explicitly use 127.0.0.1 or ::1 (bug 100)
 * The guest can't even connect via 127.0.0.0/8 addresses other than
   127.0.0.1
 * We can't use dual-stack sockets, we have to have separate sockets for
   IPv4 and IPv6.

The restriction exist for a reason though.  If the guest has any interfaces
other than pasta (e.g. a VPN tunnel) external hosts could reach the host
via the forwards.  Especially combined with -T auto / -U auto this would
make it very easy to make a mistake with nasty security implications.

We can achieve both goals, however, if we don't bind the outbound listening
sockets to a particular address, but _do_ use SO_BINDTODEVICE to restrict
them to the "lo" interface.

Link: https://bugs.passt.top/show_bug.cgi?id=100

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 pif.c |  6 ------
 tcp.c | 19 ++-----------------
 udp.c | 10 +++-------
 3 files changed, 5 insertions(+), 30 deletions(-)

diff --git a/pif.c b/pif.c
index 5fb1f455..20b5cb99 100644
--- a/pif.c
+++ b/pif.c
@@ -79,12 +79,6 @@ int pif_sock_l4(const struct ctx *c, enum epoll_type type, uint8_t pif,
 
 	ASSERT(pif_is_socket(pif));
 
-	if (pif == PIF_SPLICE) {
-		/* Sanity checks */
-		ASSERT(!ifname);
-		ASSERT(addr && inany_is_loopback(addr));
-	}
-
 	if (!addr)
 		return sock_l4_dualstack(c, type, port, ifname, data);
 
diff --git a/tcp.c b/tcp.c
index a19f0ba9..841c60fa 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2602,21 +2602,6 @@ int tcp_sock_init(const struct ctx *c, uint8_t pif,
 	return 0;
 }
 
-/**
- * tcp_ns_sock_init() - Init socket to listen for spliced outbound connections
- * @c:		Execution context
- * @port:	Port, host order
- */
-static void tcp_ns_sock_init(const struct ctx *c, in_port_t port)
-{
-	ASSERT(!c->no_tcp);
-
-	if (c->ifi4)
-		tcp_sock_init_one(c, PIF_SPLICE, &inany_loopback4, NULL, port);
-	if (c->ifi6)
-		tcp_sock_init_one(c, PIF_SPLICE, &inany_loopback6, NULL, port);
-}
-
 /**
  * tcp_ns_socks_init() - Bind sockets in namespace for outbound connections
  * @arg:	Execution context
@@ -2635,7 +2620,7 @@ static int tcp_ns_socks_init(void *arg)
 		if (!bitmap_isset(c->tcp.fwd_out.map, port))
 			continue;
 
-		tcp_ns_sock_init(c, port);
+		tcp_sock_init(c, PIF_SPLICE, NULL, "lo", port);
 	}
 
 	return 0;
@@ -2815,7 +2800,7 @@ static void tcp_port_rebind(struct ctx *c, bool outbound)
 		if ((c->ifi4 && socks[port][V4] == -1) ||
 		    (c->ifi6 && socks[port][V6] == -1)) {
 			if (outbound)
-				tcp_ns_sock_init(c, port);
+				tcp_sock_init(c, PIF_SPLICE, NULL, "lo", port);
 			else
 				tcp_sock_init(c, PIF_HOST, NULL, NULL, port);
 		}
diff --git a/udp.c b/udp.c
index 5754c436..4f796077 100644
--- a/udp.c
+++ b/udp.c
@@ -1206,14 +1206,10 @@ static void udp_port_rebind(struct ctx *c, bool outbound)
 
 		if ((c->ifi4 && socks[V4][port] == -1) ||
 		    (c->ifi6 && socks[V6][port] == -1)) {
-			if (outbound) {
-				udp_sock_init(c, PIF_SPLICE,
-					      &inany_loopback4, NULL, port);
-				udp_sock_init(c, PIF_SPLICE,
-					      &inany_loopback6, NULL, port);
-			} else {
+			if (outbound)
+				udp_sock_init(c, PIF_SPLICE, NULL, "lo", port);
+			else
 				udp_sock_init(c, PIF_HOST, NULL, NULL, port);
-			}
 		}
 	}
 }
-- 
2.51.0