qrexec: validate socket fd before use

Check return value of connect_unix_socket*() before use to
avoid passing invalid file descriptors and triggering EBADF.

Fixes QubesOS/qubes-issues#10792

Author: therohityadav

daemon/qrexec-client.c

@@ -345,6 +345,9 @@ int main(int argc, char **argv)
                                    wait_connection_end, use_uuid) ? 0 : 137;
         } else {
             s = connect_unix_socket(domname);
+            if (s < 0) {
+                goto cleanup;
+            }
             if (!negotiate_connection_params(s,
                         src_domain_id,
                         just_exec ? MSG_JUST_EXEC : MSG_EXEC_CMDLINE,

daemon/qrexec-daemon-common.c

@@ -206,8 +206,13 @@ bool qrexec_execute_vm(const char *target, bool autostart, int remote_domain_id,
     }
 
     s = connect_unix_socket_by_id((unsigned)remote_domain_id);
-    rc = send_service_connect(s, request_id, data_domain, data_port);
-    close(s);
+    if (s < 0) {
+        rc = false;
+    } else {
+        rc = send_service_connect(s, request_id, data_domain, data_port);
+        close(s);
+    }
+
     if (wait_connection_end) {
         /* wait for EOF */
         struct pollfd fds[1] = {