qrexec: add option to wait for VM-VM connection termination

Normally when qrexec-client setup VM-VM connection it exits
immediatelly. But it may be useful to wait for the connection to
terminate - for example to cleanup DispVM.

qrexec-daemon (the one that allocated vchan port) do receive such
notification, so expose such option to qrexec-client.

QubesOS/qubes-issues#2253

qrexec/qrexec-client.c

@@ -537,11 +537,12 @@ static void select_loop(libvchan_t *vchan)
 static void usage(char *name)
 {
     fprintf(stderr,
-            "usage: %s [-w timeout] [-t] [-T] -d domain_name ["
+            "usage: %s [-w timeout] [-W] [-t] [-T] -d domain_name ["
             "-l local_prog|"
             "-c request_id,src_domain_name,src_domain_id|"
             "-e] remote_cmdline\n"
             "-e means exit after sending cmd,\n"
+            "-W waits for connection end even in case of VM-VM (-c)\n"
             "-t enables replacing ESC character with '_' in command output, -T is the same for stderr\n"
             "-c: connect to existing process (response to trigger service call)\n"
             "-w timeout: override default connection timeout of 5s (set 0 for no timeout)\n",
@@ -641,6 +642,7 @@ int main(int argc, char **argv)
     int msg_type;
     int s;
     int just_exec = 0;
+    int wait_connection_end = 0;
     int connect_existing = 0;
     char *local_cmdline = NULL;
     char *remote_cmdline = NULL;
@@ -649,7 +651,7 @@ int main(int argc, char **argv)
     int src_domain_id = 0; /* if not -c given, the process is run in dom0 */
     int connection_timeout = 5;
     struct service_params svc_params;
-    while ((opt = getopt(argc, argv, "d:l:ec:tTw:")) != -1) {
+    while ((opt = getopt(argc, argv, "d:l:ec:tTw:W")) != -1) {
         switch (opt) {
             case 'd':
                 domname = strdup(optarg);
@@ -674,6 +676,9 @@ int main(int argc, char **argv)
             case 'w':
                 connection_timeout = atoi(optarg);
                 break;
+            case 'W':
+                wait_connection_end = 1;
+                break;
             default:
                 usage(argv[0]);
         }
@@ -749,6 +754,11 @@ int main(int argc, char **argv)
                 strlen(remote_cmdline) + 1,
                 &data_domain,
                 &data_port);
+        if (wait_connection_end && connect_existing)
+            /* save socket fd, 's' will be reused for the other qrexec-daemon
+             * connection */
+            wait_connection_end = s;
+        else
             close(s);
         setenv("QREXEC_REMOTE_DOMAIN", domname, 1);
         prepare_local_fds(local_cmdline);
@@ -756,6 +766,13 @@ int main(int argc, char **argv)
             s = connect_unix_socket(src_domain_name);
             send_service_connect(s, request_id, data_domain, data_port);
             close(s);
+            if (wait_connection_end) {
+                /* wait for EOF */
+                fd_set read_fd;
+                FD_ZERO(&read_fd);
+                FD_SET(wait_connection_end, &read_fd);
+                select(wait_connection_end+1, &read_fd, NULL, NULL, 0);
+            }
         } else {
             data_vchan = libvchan_server_init(data_domain, data_port,
                     VCHAN_BUFFER_SIZE, VCHAN_BUFFER_SIZE);

qrexec/qrexec-daemon.c

@@ -71,6 +71,12 @@ int policy_pending_max = -1;
  * either VCHAN_PORT_* or remote domain id for used port */
 int used_vchan_ports[MAX_CLIENTS];
 
+/* notify client (close its connection) when connection initiated by it was
+ * terminated - used by qrexec-policy to cleanup (disposable) VM; indexed with
+ * vchan port number relative to VCHAN_BASE_DATA_PORT; stores fd of given
+ * client or -1 if none requested */
+int vchan_port_notify_client[MAX_CLIENTS];
+
 int max_client_fd = -1;		// current max fd of all clients; so that we need not to scan all the "clients" table
 int qrexec_daemon_unix_socket_fd;	// /var/run/qubes/qrexec.xid descriptor
 const char *default_user = "user";
@@ -312,6 +318,7 @@ void init(int xid)
         clients[i].state = CLIENT_INVALID;
         policy_pending[i].pid = 0;
         used_vchan_ports[i] = VCHAN_PORT_UNUSED;
+        vchan_port_notify_client[i] = VCHAN_PORT_UNUSED;
     }
 
     /* When running as root, make the socket accessible; perms on /var/run/qubes still apply */
@@ -358,14 +365,6 @@ static int allocate_vchan_port(int new_state)
     return -1;
 }
 
-static void release_vchan_port(int port, int expected_remote_id)
-{
-    /* release only if was reserved for connection to given domain */
-    if (used_vchan_ports[port-VCHAN_BASE_DATA_PORT] == expected_remote_id) {
-        used_vchan_ports[port-VCHAN_BASE_DATA_PORT] = VCHAN_PORT_UNUSED;
-    }
-}
-
 static void handle_new_client()
 {
     int fd = do_accept(qrexec_daemon_unix_socket_fd);
@@ -387,8 +386,25 @@ static void handle_new_client()
 
 static void terminate_client(int fd)
 {
+    int port;
     clients[fd].state = CLIENT_INVALID;
     close(fd);
+    /* if client requested vchan connection end notify, cancel it */
+    for (port = 0; port < MAX_CLIENTS; port++) {
+        if (vchan_port_notify_client[port] == fd)
+            vchan_port_notify_client[port] = VCHAN_PORT_UNUSED;
+    }
+}
+
+static void release_vchan_port(int port, int expected_remote_id)
+{
+    /* release only if was reserved for connection to given domain */
+    if (used_vchan_ports[port-VCHAN_BASE_DATA_PORT] == expected_remote_id) {
+        used_vchan_ports[port-VCHAN_BASE_DATA_PORT] = VCHAN_PORT_UNUSED;
+        /* notify client if requested - it will clear notification request */
+        if (vchan_port_notify_client[port-VCHAN_BASE_DATA_PORT] != VCHAN_PORT_UNUSED)
+            terminate_client(vchan_port_notify_client[port-VCHAN_BASE_DATA_PORT]);
+    }
 }
 
 static int handle_cmdline_body_from_client(int fd, struct msg_header *hdr)
@@ -433,6 +449,8 @@ static int handle_cmdline_body_from_client(int fd, struct msg_header *hdr)
             terminate_client(fd);
             return 0;
         }
+        /* notify the client when this connection got terminated */
+        vchan_port_notify_client[params.connect_port-VCHAN_BASE_DATA_PORT] = fd;
         client_params.connect_port = params.connect_port;
         client_params.connect_domain = remote_domain_id;
         hdr->len = sizeof(client_params);