Unshare the FD table when creating threads in init to avoid fd leaks

src/linux/init/WSLAInit.cpp

@@ -446,13 +446,25 @@ void HandleMessageImpl(wsl::shared::SocketChannel& Channel, const WSLA_FORK& Mes
     std::promise<pid_t> childPid;
 
     {
-        auto childLogic = [ListenSocket = std::move(ListenSocket), &SocketAddress, &Channel, &Message, &childPid]() mutable {
+        auto childLogic = [ListenSocketFd = ListenSocket.get(), &SocketAddress, &Channel, &Message, &childPid]() mutable {
+            wil::unique_fd ListenSocket;
+
             // Close parent channel
             if (Message.ForkType == WSLA_FORK::Process || Message.ForkType == WSLA_FORK::Pty)
             {
                 Channel.Close();
             }
 
+            if (Message.ForkType == WSLA_FORK::Thread)
+            {
+                // If this is a thread, detach from the process' fd table.
+                // This prevents other threads from creating child processes that could inherit fds that this thread could create.
+                // N.B. This needs to happen before childPid is signalled to ensure that ListenSocket() is not closed by the parent before getting duplicated in the child's fd table.
+                THROW_LAST_ERROR_IF(unshare(CLONE_FILES) < 0);
+            }
+
+            // ListenSocket should only be assigned after this thread is guaranteed to have its own fd table (either via unshare() or a child process).
+            ListenSocket.reset(ListenSocketFd);
             childPid.set_value(getpid());
 
             wil::unique_fd ProcessSocket{UtilAcceptVsock(ListenSocket.get(), SocketAddress, SESSION_LEADER_ACCEPT_TIMEOUT_MS)};