Harden interactive service pipe

- Append a version 4 uuid to ovpn_pipe_name to make it less
  predictable
- Do not allow remote access to the pipe

This greatly reduces the possibility of a rogue process racing to
open the pipe before CreateFile() is called in the worker thread.

Reported-by: Marc Heuse <marc@srlabs.de>
Change-Id: Ie66a142751354e421d48b273784fc79bcb9f7208
Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1401
Message-Id: <20251124183839.24803-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg34654.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

src/openvpnserv/interactive.c

@@ -1955,11 +1955,30 @@ RunOpenvpn(LPVOID p)
         goto out;
     }
 
+    UUID pipe_uuid;
+    RPC_STATUS rpc_stat = UuidCreate(&pipe_uuid);
+    if (rpc_stat != RPC_S_OK)
+    {
+        ReturnError(pipe, rpc_stat, L"UuidCreate", 1, &exit_event);
+        goto out;
+    }
+
+    RPC_WSTR pipe_uuid_str = NULL;
+    rpc_stat = UuidToStringW(&pipe_uuid, &pipe_uuid_str);
+    if (rpc_stat != RPC_S_OK)
+    {
+        ReturnError(pipe, rpc_stat, L"UuidToString", 1, &exit_event);
+        goto out;
+    }
     openvpn_swprintf(ovpn_pipe_name, _countof(ovpn_pipe_name),
-                     TEXT("\\\\.\\pipe\\" PACKAGE "%ls\\service_%lu"), service_instance, GetCurrentThreadId());
+                     TEXT("\\\\.\\pipe\\" PACKAGE "%ls\\service_%lu_%ls"), service_instance,
+                     GetCurrentThreadId(), pipe_uuid_str);
+    RpcStringFree(&pipe_uuid_str);
+
     ovpn_pipe = CreateNamedPipe(ovpn_pipe_name,
                                 PIPE_ACCESS_DUPLEX | FILE_FLAG_FIRST_PIPE_INSTANCE | FILE_FLAG_OVERLAPPED,
-                                PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_WAIT, 1, 128, 128, 0, NULL);
+                                PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_WAIT | PIPE_REJECT_REMOTE_CLIENTS,
+                                1, 128, 128, 0, NULL);
     if (ovpn_pipe == INVALID_HANDLE_VALUE)
     {
         ReturnLastError(pipe, L"CreateNamedPipe");