sitnl: set FD_CLOEXEC on socket to prevent abuse

Since OpenVPN spawns various child processes, it is important
that sockets are closed after calling exec.

The sitnl socket didn't have the right flag set, resulting
in it surviving in, for example, connect/disconnect scripts
and giving the latter a chance to abuse the socket.

Ensure this doesn't happen by setting FD_CLOEXEC on
this socket right after creation.

Reported-by: Joshua Rogers <contact@joshua.hu>
Found-by: ZeroPath (https://zeropath.com/)
Change-Id: I54845bf4dd17d06cfc3b402f188795f74f4b1d3e
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1314
Message-Id: <20251028162843.18189-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg33952.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit b9b5470)

Author: ordex

CMakeLists.txt

@@ -711,6 +711,7 @@ if (BUILD_TESTING)
             src/openvpn/crypto_mbedtls.c
             src/openvpn/crypto_openssl.c
             src/openvpn/crypto.c
+            src/openvpn/fdmisc.c
             src/openvpn/otime.c
             src/openvpn/packet_id.c
             )

src/openvpn/networking_sitnl.c

@@ -28,6 +28,7 @@
 
 #include "dco.h"
 #include "errlevel.h"
+#include "fdmisc.h"
 #include "buffer.h"
 #include "misc.h"
 #include "networking.h"
@@ -166,6 +167,9 @@ sitnl_socket(void)
         return fd;
     }
 
+    /* set close on exec to avoid child processes access the socket */
+    set_cloexec(fd);
+
     if (setsockopt(fd, SOL_SOCKET, SO_SNDBUF, &sndbuf, sizeof(sndbuf)) < 0)
     {
         msg(M_WARN | M_ERRNO, "%s: SO_SNDBUF", __func__);

tests/unit_tests/openvpn/Makefile.am

@@ -130,6 +130,7 @@ networking_testdriver_SOURCES = test_networking.c mock_msg.c \
 	$(top_srcdir)/src/openvpn/crypto.c \
 	$(top_srcdir)/src/openvpn/crypto_mbedtls.c \
 	$(top_srcdir)/src/openvpn/crypto_openssl.c \
+	$(top_srcdir)/src/openvpn/fdmisc.c \
 	$(top_srcdir)/src/openvpn/otime.c \
 	$(top_srcdir)/src/openvpn/packet_id.c \
 	$(top_srcdir)/src/openvpn/platform.c