[PATCH v5 12/15] tcp, udp: Bind outbound listening sockets by interface instead of address

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

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 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 exists 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 this a different way, however.  Don't bind to a specific
address, but _do_ use SO_BINDTODEVICE to restrict the sockets to the "lo"
interface.  We fall back to the old behaviour for older kernels where
SO_BINDTODEVICE is not available unprivileged.

Note that although traffic to a local but non-loopback address is passed
over the 'lo' interface (as seen by netfilter and dumpcap), it doesn't
count as attached to that interface for the purposes of SO_BINDTODEVICE
(information from the routing table overrides the "physical" interface).
So, this change doesn't help for bug 100.

It's also not a complete fix for bug 113, it does however:
 * Get us a step closer to fixing bug 113
 * Slightly simplify the code
 * Make things a bit easier to allow more flexible binding on the guest in
   in future

Link: https://bugs.passt.top/show_bug.cgi?id=113

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 conf.c |  6 ++++++
 pif.c  |  6 ------
 tcp.c  |  5 +++++
 udp.c  | 30 +++++++++++++++++++++++-------
 4 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/conf.c b/conf.c
index 6bd9717b..02a4b65a 100644
--- a/conf.c
+++ b/conf.c
@@ -235,6 +235,12 @@ static void conf_ports(const struct ctx *c, char optname, const char *optarg,
 		if (c->mode != MODE_PASTA)
 			die("'auto' port forwarding is only allowed for pasta");
 
+		if ((optname == 'T' || optname == 'U') && c->no_bindtodevice) {
+			warn(
+"'-%c auto' enabled without unprivileged SO_BINDTODEVICE", optname);
+			warn(
+"Forwarding from addresses other than 127.0.0.1 will not work");
+		}
 		fwd->mode = FWD_AUTO;
 		return;
 	}
diff --git a/pif.c b/pif.c
index 85904f35..db447b4f 100644
--- a/pif.c
+++ b/pif.c
@@ -81,12 +81,6 @@ 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) {
 		ref.fd = sock_l4_dualstack(c, type, port, ifname);
 	} else {
diff --git a/tcp.c b/tcp.c
index 2abb8be4..aacc5b20 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2627,6 +2627,11 @@ static void tcp_ns_sock_init(const struct ctx *c, in_port_t port)
 {
 	ASSERT(!c->no_tcp);
 
+	if (!c->no_bindtodevice) {
+		tcp_sock_init(c, PIF_SPLICE, NULL, "lo", port);
+		return;
+	}
+
 	if (c->ifi4)
 		tcp_sock_init_one(c, PIF_SPLICE, &inany_loopback4, NULL, port);
 	if (c->ifi6)
diff --git a/udp.c b/udp.c
index 3d097fbb..4b625b78 100644
--- a/udp.c
+++ b/udp.c
@@ -1182,6 +1182,26 @@ static void udp_splice_iov_init(void)
 	}
 }
 
+/**
+ * udp_ns_sock_init() - Init socket to listen for spliced outbound connections
+ * @c:		Execution context
+ * @port:	Port, host order
+ */
+static void udp_ns_sock_init(const struct ctx *c, in_port_t port)
+{
+	ASSERT(!c->no_udp);
+
+	if (!c->no_bindtodevice) {
+		udp_sock_init(c, PIF_SPLICE, NULL, "lo", port);
+		return;
+	}
+
+	if (c->ifi4)
+		udp_sock_init(c, PIF_SPLICE, &inany_loopback4, NULL, port);
+	if (c->ifi6)
+		udp_sock_init(c, PIF_SPLICE, &inany_loopback6, NULL, port);
+}
+
 /**
  * udp_port_rebind() - Rebind ports to match forward maps
  * @c:		Execution context
@@ -1213,14 +1233,10 @@ 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_ns_sock_init(c, port);
+			else
 				udp_sock_init(c, PIF_HOST, NULL, NULL, port);
-			}
 		}
 	}
 }
-- 
2.52.0