udp: Improve detail of UDP endpoint sanity checking

In udp_flow_new() we reject a flow if the endpoint isn't unicast, or it has
a zero endpoint port.  Those conditions aren't strictly illegal, but we
can't safely handle them at present:
 * Multicast UDP endpoints are certainly possible, but our current flow
   tracking only makes sense for simple unicast flows - we'll need
   different handling if we want to handle multicast flows in future
 * It's not entirely clear if port 0 is RFC-ishly correct, but for socket
   interfaces port 0 sometimes has a special meaning such as "pick the port
   for me, kernel".  That makes flows on port 0 unsafe to forward in the
   usual way.

For the same reason we also can't safely handle port 0 as our port.  In
principle that's also true for our address, however in the case of flows
initiated from a socket, we may not know our address since the socket
could be bound to 0.0.0.0 or ::, so we can only verify that our address
is unicast for flows initiated from the tap side.

Refine the current check in udp_flow_new() to slightly more detailed checks
in udp_flow_from_sock() and udp_flow_from_tap() to make what is and isn't
handled clearer.  This makes this checking more similar to what we do for
TCP connections.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

udp_flow.c

@@ -75,16 +75,10 @@ void udp_flow_close(const struct ctx *c, struct udp_flow *uflow)
 static flow_sidx_t udp_flow_new(const struct ctx *c, union flow *flow,
 				int s_ini, const struct timespec *now)
 {
-	const struct flowside *ini = &flow->f.side[INISIDE];
 	struct udp_flow *uflow = NULL;
 	const struct flowside *tgt;
 	uint8_t tgtpif;
 
-	if (!inany_is_unicast(&ini->eaddr) || ini->eport == 0) {
-		flow_trace(flow, "Invalid endpoint to initiate UDP flow");
-		goto cancel;
-	}
-
 	if (!(tgt = flow_target(c, flow, IPPROTO_UDP)))
 		goto cancel;
 	tgtpif = flow->f.pif[TGTSIDE];
@@ -189,6 +183,7 @@ flow_sidx_t udp_flow_from_sock(const struct ctx *c, union epoll_ref ref,
 			       const union sockaddr_inany *s_in,
 			       const struct timespec *now)
 {
+	const struct flowside *ini;
 	struct udp_flow *uflow;
 	union flow *flow;
 	flow_sidx_t sidx;
@@ -210,7 +205,19 @@ flow_sidx_t udp_flow_from_sock(const struct ctx *c, union epoll_ref ref,
 		return FLOW_SIDX_NONE;
 	}
 
-	flow_initiate_sa(flow, ref.udp.pif, s_in, ref.udp.port);
+	ini = flow_initiate_sa(flow, ref.udp.pif, s_in, ref.udp.port);
+
+	if (!inany_is_unicast(&ini->eaddr) ||
+	    ini->eport == 0 || ini->oport == 0) {
+		/* In principle ini->oddr also must be unicast, but when we've
+		 * been initiated from a socket bound to 0.0.0.0 or ::, we don't
+		 * know our address, so we have to leave it unpopulated.
+		 */
+		flow_err(flow, "Invalid endpoint on UDP recvfrom()");
+		flow_alloc_cancel(flow);
+		return FLOW_SIDX_NONE;
+	}
+
 	return udp_flow_new(c, flow, ref.fd, now);
 }
 
@@ -233,6 +240,7 @@ flow_sidx_t udp_flow_from_tap(const struct ctx *c,
 			      in_port_t srcport, in_port_t dstport,
 			      const struct timespec *now)
 {
+	const struct flowside *ini;
 	struct udp_flow *uflow;
 	union flow *flow;
 	flow_sidx_t sidx;
@@ -256,7 +264,15 @@ flow_sidx_t udp_flow_from_tap(const struct ctx *c,
 		return FLOW_SIDX_NONE;
 	}
 
-	flow_initiate_af(flow, PIF_TAP, af, saddr, srcport, daddr, dstport);
+	ini = flow_initiate_af(flow, PIF_TAP, af, saddr, srcport,
+			       daddr, dstport);
+
+	if (!inany_is_unicast(&ini->eaddr) || ini->eport == 0 ||
+	    !inany_is_unicast(&ini->oaddr) || ini->oport == 0) {
+		flow_dbg(flow, "Invalid endpoint on UDP packet");
+		flow_alloc_cancel(flow);
+		return FLOW_SIDX_NONE;
+	}
 
 	return udp_flow_new(c, flow, -1, now);
 }