meta-flow: Don't allow negative port numbers.

If a negative number is supplied, the parsing code used to convert it
into a signed one.  We ran into an incident where a third-party script
was attempting to get the OpenFlow port number for an interface, but got
-1 from the database, since the number had not yet been assigned.  This
was converted to 65535, which maps to OFPP_NONE and all flows with
ingress port OFPP_NONE were modified.  This commit disallows negative
port numbers to help prevent broken integration scripts from disturbing
the flow table.

Issue #14036

Signed-off-by: Justin Pettit <jpettit@nicira.com>

lib/meta-flow.c

@@ -2087,7 +2087,10 @@ mf_from_ofp_port_string(const struct mf_field *mf, const char *s,
     uint16_t port;
 
     assert(mf->n_bytes == sizeof(ovs_be16));
-    if (ofputil_port_from_string(s, &port)) {
+    if (*s == '-') {
+        return xasprintf("%s: negative values not supported for %s",
+                         s, mf->name);
+    } else if (ofputil_port_from_string(s, &port)) {
         *valuep = htons(port);
         *maskp = htons(UINT16_MAX);
         return NULL;