Allow TLS pre-master secrets to be recorded

This allows for RDP sessions to be easily decrypted within Wireshark

common/os_calls.c

@@ -2311,6 +2311,30 @@ g_file_seek(int fd, int offset)
 #endif
 }
 
+/*****************************************************************************/
+/* move file pointer to end of file, plus an offset */
+int
+g_file_seek_end(int fd, int offset)
+{
+#if defined(_WIN32)
+    int rv;
+
+    rv = (int)SetFilePointer((HANDLE)fd, offset, 0, FILE_END);
+
+    if (rv == (int)INVALID_SET_FILE_POINTER)
+    {
+        return -1;
+    }
+    else
+    {
+        return rv;
+    }
+
+#else
+    return (int)lseek(fd, offset, SEEK_END);
+#endif
+}
+
 /*****************************************************************************/
 /* do a write lock on a file */
 /* return boolean */

common/os_calls.h

@@ -242,6 +242,7 @@ int      g_file_is_open(int fd);
 int      g_file_read(int fd, char *ptr, int len);
 int      g_file_write(int fd, const char *ptr, int len);
 int      g_file_seek(int fd, int offset);
+int      g_file_seek_end(int fd, int offset);
 int      g_file_lock(int fd, int start, int len);
 int
 g_file_map(int fd, int aread, int awrite, size_t length, void **addr);

common/ssl_calls.c

@@ -53,6 +53,11 @@ static EVP_CIPHER *g_cipher_des_ede3_cbc; /* DES3 CBC cipher */
 static EVP_MAC *g_mac_hmac; /* HMAC MAC */
 #endif
 
+#if OPENSSL_VERSION_NUMBER >= 0x10101000L
+#define HAS_KEYLOG_CALLBACK /* SSL_CTX_set_keylog_callback() is available */
+static char *g_keylog_filename = NULL;
+#endif
+
 /* definition of ssl_tls */
 struct ssl_tls
 {
@@ -173,13 +178,96 @@ ssl_finish(void)
     /* De-allocate any allocated globals
      * For OpenSSL 3, these can all safely be passed a NULL pointer */
     EVP_MD_free(g_md_md5);
+    g_md_md5 = NULL;
     EVP_MD_free(g_md_sha1);
+    g_md_sha1 = NULL;
     EVP_CIPHER_free(g_cipher_des_ede3_cbc);
+    g_cipher_des_ede3_cbc = NULL;
     EVP_MAC_free(g_mac_hmac);
+    g_mac_hmac = NULL;
+#endif
+
+#ifdef HAS_KEYLOG_CALLBACK
+    free(g_keylog_filename);
+    g_keylog_filename = NULL;
 #endif
     return 0;
 }
 
+/*****************************************************************************/
+int
+ssl_set_pre_master_secret_logfile(const char *filename)
+{
+    int rv = 0;
+#ifdef HAS_KEYLOG_CALLBACK
+    int fd = -1;
+
+    /* Remove any existing setting */
+    free(g_keylog_filename);
+    g_keylog_filename = NULL;
+
+    if (filename == NULL || filename[0] == '\0')
+    {
+        /* all done */
+    }
+    else if (filename[0] != '/')
+    {
+        LOG(LOG_LEVEL_ERROR, "TLS pre-master log file must start with '/'");
+    }
+    else if ((fd = g_file_open_rw(filename)) < 0)
+    {
+        /* Can't open file for writing. We'll log a single error here
+         * rather than lots of errors in the callback */
+        LOG(LOG_LEVEL_ERROR, "Can't write TLS pre-master secrets to %s [%s]",
+            filename, g_get_strerror());
+    }
+    else if (g_file_close(fd) != 0)
+    {
+        /* Ignore this one */
+    }
+    else if ((g_keylog_filename = g_strdup(filename)) == NULL)
+    {
+        LOG(LOG_LEVEL_ERROR, "Out of memory setting TLS pre-master log");
+    }
+    else
+    {
+        rv = 1;
+    }
+#else
+    LOG(LOG_LEVEL_WARNING,
+        "This system is unable to log TLS pre-master secrets");
+#endif
+    return rv;
+}
+
+/*****************************************************************************/
+/* Log the pre-master secret for debugging purposes */
+#ifdef HAS_KEYLOG_CALLBACK
+static void
+log_pre_master_secret(const SSL *ssl, const char *line)
+{
+    if (g_keylog_filename != NULL && g_keylog_filename[0] == '/')
+    {
+        int fd = g_file_open_rw(g_keylog_filename);
+        if (fd < 0)
+        {
+            LOG(LOG_LEVEL_ERROR, "Can't write pre-master secret to %s [ %s]",
+                g_keylog_filename, g_get_strerror());
+        }
+        else
+        {
+            g_chmod_hex(g_keylog_filename, 0x640); // Must be group readable
+            g_file_seek_end(fd, 0);
+            g_file_write(fd, line, strlen(line));
+            g_file_write(fd, "\n", 1);
+            (void)g_file_close(fd);
+        }
+    }
+}
+#endif // HAS_KEYLOG_CALLBACK
+
+/*****************************************************************************/
+
 /* rc4 stuff
  *
  * For OpenSSL 3.0, the rc4 encryption algorithm is only provided by the
@@ -1167,6 +1255,10 @@ ssl_tls_accept(struct ssl_tls *self, long ssl_protocols,
     }
 #endif
 
+#ifdef HAS_KEYLOG_CALLBACK
+    SSL_CTX_set_keylog_callback(self->ctx, log_pre_master_secret);
+#endif // HAS_KEYLOG_CALLBACK
+
     self->ssl = SSL_new(self->ctx);
 
     if (self->ssl == NULL)

common/ssl_calls.h

@@ -30,6 +30,14 @@ int
 ssl_init(void);
 int
 ssl_finish(void);
+/**
+ * Sets a log file for recording TLS pre-master secrets
+ *
+ * @param filename Filename to log secrets in
+ * @return != 0 if the log was successfully set
+ */
+int
+ssl_set_pre_master_secret_logfile(const char *filename);
 void *
 ssl_rc4_info_create(void);
 void

docs/man/xrdp.ini.5.in

@@ -73,6 +73,19 @@ If not specified, defaults to \fB@sysconfdir@/@sysconfsubdir@/cert.pem\fP, \fB@s
 
 This parameter is effective only if \fBsecurity_layer\fP is set to \fBtls\fP or \fBnegotiate\fP.
 
+.TP
+\fBtls_pms_log_file\fR=\fI<path-to-log-file>\fR
+Logs TLS pre-master secrets to the specified file. This allows packet capture
+tools (e.g. Wireshark) to decrypt captured PDUs.
+
+The file must be writeable by xrdp and readable by the packet capture tool.
+A good way to achive this is to create a temporary directory with
+permissions 2750 owned by the user running xrdp, and in the group used by
+the packet sniffer.
+
+SETTING THIS OPTION IS A SECURITY RISK. ONLY SET THIS OPTION FOR DEBUGGING
+COMMUNICATIONS BETWEEN XRDP AND A CLIENT.
+
 .TP
 \fBchannel_code\fP=\fI[true|false]\fP
 If set to \fB0\fR, \fBfalse\fR or \fBno\fR this option disables all channels \fBxrdp\fR(8).

libxrdp/xrdp_rdp.c

@@ -277,6 +277,14 @@ xrdp_rdp_read_config(const char *xrdp_ini, struct xrdp_client_info *client_info)
                     client_info->key_file, g_get_strerror());
             }
         }
+        else if (g_strcasecmp(item, "tls_pms_log_file") == 0)
+        {
+            if (ssl_set_pre_master_secret_logfile(value))
+            {
+                LOG(LOG_LEVEL_WARNING, "TLS pre-master secrets will be logged. "
+                    "This is a security risk.");
+            }
+        }
         else if (g_strcasecmp(item, "domain_user_separator") == 0
                  && g_strlen(value) > 0)
         {

xrdp/xrdp.ini.in

@@ -66,6 +66,9 @@ crypt_level=high
 certificate=
 key_file=
 
+; [Debug] Log file for TLS pre-master secrets - see xrdp.ini(5)
+#tls_pms_log_file=/tmp/xrdp-pms/premaster.log
+
 ; set SSL protocols
 ; can be comma separated list of 'SSLv3', 'TLSv1', 'TLSv1.1', 'TLSv1.2', 'TLSv1.3'
 ssl_protocols=TLSv1.2, TLSv1.3