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/CMakeLists.txt

@@ -114,6 +114,10 @@ if (NETTLE_FOUND)
                           ${NETTLE_LIBRARY_DIRS} ${GMP_LIBRARY_DIRS})
 endif()
 
+if (LIBSYSTEMD_FOUND)
+  target_link_libraries(rfb ${LIBSYSTEMD_LIBRARIES})
+endif()
+
 if(UNIX)
   libtool_create_control_file(rfb)
 endif()

common/rfb/ServerCore.cxx

@@ -99,3 +99,11 @@ rfb::BoolParameter rfb::Server::queryConnect
 ("QueryConnect",
  "Prompt the local user to accept or reject incoming connections.",
  false);
+#ifdef HAVE_LIBSYSTEMD
+rfb::BoolParameter rfb::Server::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.",
+  false);
+#endif

common/rfb/ServerCore.h

@@ -47,6 +47,9 @@ namespace rfb {
     static BoolParameter sendCutText;
     static BoolParameter acceptSetDesktopSize;
     static BoolParameter queryConnect;
+#ifdef HAVE_LIBSYSTEMD
+    static BoolParameter approveLoggedUserOnly;
+#endif
 
   };
 

common/rfb/VNCServerST.cxx

@@ -69,6 +69,10 @@
 #include <rfb/util.h>
 #include <rfb/ledStates.h>
 
+#ifdef HAVE_LIBSYSTEMD
+#  include <pwd.h>
+#  include <systemd/sd-login.h>
+#endif
 
 using namespace rfb;
 
@@ -661,6 +665,69 @@ void VNCServerST::handleTimeout(Timer* t)
   }
 }
 
+#ifdef HAVE_LIBSYSTEMD
+bool VNCServerST::checkUserLogged(const char* userName)
+{
+  bool ret = false;
+  bool noUserSession = true;
+  int res;
+  char **sessions;
+
+  res = sd_get_sessions(&sessions);
+  if (res < 0) {
+    slog.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;
+
+      res = sd_session_get_uid(sessions[i], &uid);
+      if (res < 0) {
+        slog.debug("logind: failed to determine user id of session");
+        break;
+      }
+
+      res = sd_session_get_class(sessions[i], &clazz);
+      if (res < 0) {
+        slog.debug("logind: failed to determine session class");
+        break;
+      }
+
+      if (uid != 0 && strcmp(clazz, "user") == 0) {
+        noUserSession = false;
+      }
+      free(clazz);
+
+      struct passwd *pw = getpwnam(userName);
+      if (!pw) {
+        slog.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 VNCServerST::queryConnection(VNCSConnectionST* client,
                                   const char* userName)
 {
@@ -680,6 +747,16 @@ void VNCServerST::queryConnection(VNCSConnectionST* client,
     return;
   }
 
+#ifdef HAVE_LIBSYSTEMD
+  // - Only owner of the session can be approved
+  if (rfb::Server::approveLoggedUserOnly &&
+      !checkUserLogged(userName)) {
+    approveConnection(client->getSock(), false,
+                      "The user is not owner of the running session");
+    return;
+  }
+#endif
+
   // - Are we configured to do queries?
   if (!rfb::Server::queryConnect &&
       !client->getSock()->requiresQuery()) {

common/rfb/VNCServerST.h

@@ -165,6 +165,13 @@ namespace rfb {
     // - Check how many of the clients are authenticated.
     int authClientCount();
 
+#ifdef HAVE_LIBSYSTEMD
+    // - 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 needRenderedCursor();
     void startFrameClock();
     void stopFrameClock();