Add option allowing to connect only the user owning the running session

Checks, whether the user who is trying to authenticate is already logged
into the running session in order to allow or reject the connection.
This is expected to be used with 'plain' security type in combination
with 'PlainUsers=*' option allowing everyone to connect to the session.

common/rfb/VNCServerST.cxx

@@ -680,13 +680,6 @@ void VNCServerST::queryConnection(VNCSConnectionST* client,
     return;
   }
 
-  // - Are we configured to do queries?
-  if (!rfb::Server::queryConnect &&
-      !client->getSock()->requiresQuery()) {
-    approveConnection(client->getSock(), true, nullptr);
-    return;
-  }
-
   // - Does the client have the right to bypass the query?
   if (client->accessCheck(AccessNoQuery))
   {

unix/xserver/hw/vnc/XserverDesktop.cc

@@ -52,6 +52,11 @@
 #include "XorgGlue.h"
 #include "vncInput.h"
 
+#if HAVE_SYSTEMD_DAEMON
+#  include <pwd.h>
+#  include <systemd/sd-login.h>
+#endif
+
 extern "C" {
 void vncSetGlueContext(int screenIndex);
 void vncPresentMscEvent(uint64_t id, uint64_t msc);
@@ -71,7 +76,15 @@ IntParameter queryConnectTimeout("QueryConnectTimeout",
                                  "Accept Connection dialog before "
                                  "rejecting the connection",
                                  10);
-
+#ifdef HAVE_SYSTEMD_DAEMON
+BoolParameter approveLoggedUserOnly
+("ApproveLoggedUserOnly",
+ "Approve only the user who is currently logged into the session."
+ "This is expected to be combined with 'plain' security type and with "
+ "'PlainUsers=*' option allowing everyone to connect to the session."
+ "Default is off.",
+  false);
+#endif
 
 XserverDesktop::XserverDesktop(int screenIndex_,
                                std::list<network::SocketListener*> listeners_,
@@ -165,11 +178,121 @@ void XserverDesktop::init(rfb::VNCServer* vs)
   // ready state
 }
 
+#ifdef HAVE_SYSTEMD_DAEMON
+bool XserverDesktop::checkUserLogged(const char* userName)
+{
+  bool ret = false;
+  bool noUserSession = true;
+  int res;
+  char **sessions;
+
+  res = sd_get_sessions(&sessions);
+  if (res < 0) {
+    vlog.debug("logind: failed to get sessions");
+    return false;
+  }
+
+  if (sessions != nullptr && sessions[0] != nullptr) {
+    for (int i = 0; sessions[i]; i++) {
+      uid_t uid;
+      char *clazz;
+      char *display;
+      char *type;
+
+      res = sd_session_get_type(sessions[i], &type);
+      if (res < 0) {
+        vlog.debug("logind: failed to determine session type");
+        break;
+      }
+
+      if (strcmp(type, "x11") != 0) {
+        free(type);
+        continue;
+      }
+      free(type);
+
+      res = sd_session_get_display(sessions[i], &display);
+      if (res < 0) {
+        vlog.debug("logind: failed to determine display of session");
+        break;
+      }
+
+      std::string serverDisplay = ":" + std::to_string(screenIndex);
+      std::string serverDisplayIPv4 = "127.0.0.1:" + std::to_string(screenIndex);
+      std::string serverDisplayIPv6 = "::1:" + std::to_string(screenIndex);
+      if ((strcmp(display, serverDisplay.c_str()) != 0) &&
+          (strcmp(display, serverDisplayIPv4.c_str()) != 0) &&
+          (strcmp(display, serverDisplayIPv6.c_str()) != 0)) {
+        free(display);
+        continue;
+      }
+      free(display);
+
+      res = sd_session_get_class(sessions[i], &clazz);
+      if (res < 0) {
+        vlog.debug("logind: failed to determine session class");
+        break;
+      }
+
+      res = sd_session_get_uid(sessions[i], &uid);
+      if (res < 0) {
+        vlog.debug("logind: failed to determine user id of session");
+        break;
+      }
+
+      if (uid != 0 && strcmp(clazz, "user") == 0) {
+        noUserSession = false;
+      }
+      free(clazz);
+
+      struct passwd *pw = getpwnam(userName);
+      if (!pw) {
+        vlog.debug("logind: user not found");
+        break;
+      }
+
+      if (uid == pw->pw_uid) {
+        ret = true;
+        break;
+      }
+    }
+  }
+
+  if (sessions) {
+    for (int i = 0; sessions[i]; i ++) {
+      free(sessions[i]);
+    }
+
+    free (sessions);
+  }
+
+  // If we didn't find a matching user, we can still allow the user
+  // to log in if there is no user session yet.
+  return !ret ? noUserSession : ret;
+}
+#endif
+
 void XserverDesktop::queryConnection(network::Socket* sock,
                                      const char* userName)
 {
   int count;
 
+#ifdef HAVE_SYSTEMD_DAEMON
+  // - Only owner of the session can be approved
+  if (approveLoggedUserOnly && !checkUserLogged(userName)) {
+    server->approveConnection(sock, false,
+                              "The user is not owner of the running session");
+    return;
+  }
+#endif
+
+  // - Are we configured to do queries?
+  if (!rfb::Server::queryConnect &&
+      !sock->requiresQuery()) {
+    server->approveConnection(sock, true, nullptr);
+    return;
+  }
+
   if (queryConnectTimer.isStarted()) {
     server->approveConnection(sock, false, "Another connection is currently being queried.");
     return;

unix/xserver/hw/vnc/XserverDesktop.h

@@ -108,6 +108,13 @@ class XserverDesktop : public rfb::SDesktop, public rfb::FullFramePixelBuffer,
   void grabRegion(const rfb::Region& r) override;
 
 protected:
+#ifdef HAVE_SYSTEMD_DAEMON
+  // - Check whether user is logged into a session
+  //   Returns true if user is already logged or there is no
+  //   user session at all.
+  bool checkUserLogged(const char* userName);
+#endif
+
   bool handleListenerEvent(int fd,
                            std::list<network::SocketListener*>* sockets,
                            rfb::VNCServer* sockserv);

unix/xserver/hw/vnc/Xvnc.man

@@ -204,6 +204,13 @@ to allow any user to authenticate using this security type. Specify \fB%u\fP
 to allow the user of the server process. Default is to deny all users.
 .
 .TP
+.B \-ApproveLoggedUserOnly
+Approve only the user who is currently logged into the session.
+This is expected to be combined with "Plain" security type and with
+"PlainUsers=*" option allowing everyone to connect to the session.
+Default is off.
+.
+.TP
 .B \-pam_service \fIname\fP, \-PAMService \fIname\fP
 PAM service name to use when authentication users using any of the "Plain"
 security types. Default is \fBvnc\fP.