netutils/dropbear: add Dropbear SSH server port for NuttX

Integrated SSH daemon authenticating against FSUTILS_PASSWD, with an
ECDSA P-256 host key and an NSH session over a PTY per connection. Built
from the upstream Dropbear tarball (pinned commit) and patched for
NuttX, using Dropbear's bundled libtomcrypt for all crypto. setsid()
(apache/nuttx#19184) and link() now come from NuttX, not local stubs.

Signed-off-by: Felipe Moura <moura.fmo@gmail.com>

nshlib/CMakeLists.txt

@@ -101,6 +101,10 @@ if(CONFIG_NSH_LIBRARY)
     endif()
   endif()
 
+  if(CONFIG_NSH_DROPBEAR)
+    list(APPEND CSRCS nsh_dropbear.c)
+  endif()
+
   if(NOT CONFIG_NSH_DISABLESCRIPT)
     list(APPEND CSRCS nsh_test.c)
   endif()

nshlib/Kconfig

@@ -1201,6 +1201,28 @@ config NSH_DISABLE_TELNETSTART
 
 endmenu # Telnet Configuration
 
+menu "Dropbear Configuration"
+
+config NSH_DROPBEAR
+	bool "Start Dropbear SSH server"
+	default n
+	depends on NETUTILS_DROPBEAR
+	depends on NSH_BUILTIN_APPS
+	depends on !NSH_DISABLEBG
+	select NSH_LOGIN
+	---help---
+		If NSH_DROPBEAR is set to 'y', then NSH starts the Dropbear SSH
+		server automatically.
+
+config NSH_DISABLE_DROPBEARSTART
+	bool "Disable to start Dropbear"
+	default n
+	depends on NSH_DROPBEAR
+	---help---
+		Determines if NSH starts Dropbear automatically.
+
+endmenu # Dropbear Configuration
+
 config NSH_LOGIN
 	bool
 	default n

nshlib/Makefile

@@ -86,6 +86,10 @@ CSRCS += nsh_telnetlogin.c
 endif
 endif
 
+ifeq ($(CONFIG_NSH_DROPBEAR),y)
+CSRCS += nsh_dropbear.c
+endif
+
 ifneq ($(CONFIG_NSH_DISABLESCRIPT),y)
 CSRCS += nsh_test.c
 endif

nshlib/nsh.h

@@ -844,6 +844,10 @@ int nsh_login(FAR struct console_stdio_s *pstate);
 int nsh_telnetlogin(FAR struct console_stdio_s *pstate);
 #endif
 
+#if defined(CONFIG_NSH_DROPBEAR) && !defined(CONFIG_NSH_DISABLE_DROPBEARSTART)
+int nsh_dropbearstart(void);
+#endif
+
 /* Application interface */
 
 int nsh_command(FAR struct nsh_vtbl_s *vtbl, int argc, FAR char *argv[]);

nshlib/nsh_dropbear.c

@@ -0,0 +1,76 @@
+/****************************************************************************
+ * apps/nshlib/nsh_dropbear.c
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+
+#include <assert.h>
+#include <debug.h>
+
+#include "nsh.h"
+#include "nsh_console.h"
+
+#ifdef CONFIG_NSH_DROPBEAR
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: nsh_dropbearstart
+ *
+ * Description:
+ *   nsh_dropbearstart() starts the Dropbear SSH server.  This function
+ *   returns immediately after the daemon has been started.
+ *
+ * Returned Values:
+ *   Zero is returned if Dropbear was started.  A negated errno value will be
+ *   returned on failure.
+ *
+ ****************************************************************************/
+
+#ifndef CONFIG_NSH_DISABLE_DROPBEARSTART
+int nsh_dropbearstart(void)
+{
+  FAR struct console_stdio_s *pstate = nsh_newconsole(false);
+  char cmdline[] = CONFIG_NETUTILS_DROPBEAR_PROGNAME " &";
+  int ret;
+
+  DEBUGASSERT(pstate != NULL);
+
+  ninfo("Starting the Dropbear SSH server\n");
+
+  ret = nsh_parse(&pstate->cn_vtbl, cmdline);
+  if (ret < 0)
+    {
+      nerr("ERROR: Failed to start Dropbear: %d\n", ret);
+    }
+
+  nsh_release(&pstate->cn_vtbl);
+  return ret;
+}
+#endif
+
+#endif /* CONFIG_NSH_DROPBEAR */

nshlib/nsh_init.c

@@ -175,4 +175,16 @@ void nsh_initialize(void)
 
   nsh_telnetstart(AF_UNSPEC);
 #endif
+
+#if defined(CONFIG_NSH_DROPBEAR) && \
+    !defined(CONFIG_NSH_DISABLE_DROPBEARSTART) && \
+    !defined(CONFIG_NETINIT_NETLOCAL)
+  /* If Dropbear is selected as an SSH front-end, then start the daemon
+   * UNLESS network initialization is deferred via CONFIG_NETINIT_NETLOCAL.
+   * In that case, Dropbear must be started manually with the dropbear
+   * command after the network has been initialized.
+   */
+
+  nsh_dropbearstart();
+#endif
 }