Tools: added signing tools for secure boot

Tools/scripts/build_bootloaders.py

@@ -10,11 +10,17 @@ import subprocess
 import sys
 import fnmatch
 
-board_pattern = '*'
+# get command line arguments
+from argparse import ArgumentParser
+parser = ArgumentParser(description='make_secure_bl')
+parser.add_argument("--signing-key", type=str, default=None, help="signing key for secure bootloader")
+parser.add_argument("pattern", type=str, default='*', help="board wildcard pattern")
+args = parser.parse_args()
 
-# allow argument for pattern of boards to build
-if len(sys.argv)>1:
-    board_pattern = sys.argv[1]
+if args.signing_key is not None and os.path.basename(args.signing_key).lower().find("private") != -1:
+    # prevent the easy mistake of using private key
+    print("You must use the public key in the bootloader")
+    sys.exit(1)
 
 os.environ['PYTHONUNBUFFERED'] = '1'
 
@@ -39,7 +45,12 @@ def run_program(cmd_list):
     return True
 
 def build_board(board):
-    if not run_program(["./waf", "configure", "--board", board, "--bootloader", "--no-submodule-update", "--Werror"]):
+    configure_args = "--board %s --bootloader --no-submodule-update --Werror" % board
+    configure_args = configure_args.split()
+    if args.signing_key is not None:
+        print("Building secure bootloader")
+        configure_args.append("--signed-fw")
+    if not run_program(["./waf", "configure"] + configure_args):
         return False
     if not run_program(["./waf", "clean"]):
         return False
@@ -48,13 +59,19 @@ def build_board(board):
     return True
 
 for board in get_board_list():
-    if not fnmatch.fnmatch(board, board_pattern):
+    if not fnmatch.fnmatch(board, args.pattern):
         continue
     print("Building for %s" % board)
     if not build_board(board):
         failed_boards.add(board)
         continue
-    shutil.copy('build/%s/bin/AP_Bootloader.bin' % board, 'Tools/bootloaders/%s_bl.bin' % board)
+    bl_file = 'Tools/bootloaders/%s_bl.bin' % board
+    shutil.copy('build/%s/bin/AP_Bootloader.bin' % board, bl_file)
+    if args.signing_key is not None:
+        print("Signing bootloader with %s" % args.signing_key)
+        if not run_program(["./Tools/scripts/signing/make_secure_bl.py", bl_file, args.signing_key]):
+            print("Failed to sign bootloader for %s" % board)
+            sys.exit(1)
     if not run_program([sys.executable, "Tools/scripts/bin2hex.py", "--offset", "0x08000000", 'Tools/bootloaders/%s_bl.bin' % board, 'Tools/bootloaders/%s_bl.hex' % board]):
         failed_boards.add(board)
         continue

Tools/scripts/signing/README.md

@@ -0,0 +1,120 @@
+# Secure Boot Support
+
+To assist with vendors needing high levels of tamper resistance with
+RemoteID, you can optionally use secure boot with ArduPilot. This
+involves installing a bootloader with up to 10 public keys included
+and signing the ArduPilot vehicle firmware with one secret key. The
+bootloader will refuse to boot the firmware if the signature on the
+firmware doesn't match any of the public keys in the bootloader.
+
+## Generating Keys
+
+To generate a public/private key pair, run the following command:
+
+```
+  python3 -m pip install pymonocypher
+  Tools/scripts/signing/generate_keys.py NAME
+```
+
+That will create two files:
+ - NAME_private_key.dat
+ - NAME_public_key.dat
+
+NAME can be any string, but would usually be your vendor name. It is
+only used for the local filenames.
+
+The generated private key should be kept in a secure location. The
+public key will be used to create a secure bootloader that will only
+accept firmwares signed with one of the public keys in the bootloader.
+
+## Building secure bootloader
+
+To build a secure bootloader run this command:
+
+```
+ Tools/scripts/build_bootloaders.py BOARDNAME --signing-key=NAME_public.key
+```
+
+That will update the bootloader in Tools/bootloaders/BOARDNAME_bl.bin
+to enable secure boot with the specified public key. Next time you
+build a firmware for this board then that bootloader will be included
+in ROMFS.
+
+Note that this will include the 3 ArduPilot signing keys by default as
+well as your key. This is done so that your users can update to a
+standard ArduPilot firmware release and also prevents issues with
+vendors who can no longer provide firmware updates to users. If you
+have a very good reason for not including the ArduPilot signing keys
+then you can pass the option --omit-ardupilot-keys to the
+make_secure_bl.py script.
+
+## Building Signed Firmware
+
+To build a signed firmware run this command (example is for a copter build):
+
+```
+ ./waf configure --board BOARDNAME --signed-fw
+ ./waf copter
+ ./Tools/scripts/signing/make_secure_fw.py build/BOARDNAME/bin/arducopter.apj NAME_private_key.dat
+```
+
+The final step signs the apj firmware with your private key. You can
+then load that secure firmware as usual with your ground station, for
+example using load custom firmware in MissionPlanner or
+Tools/scripts/uploader.py on Linux.
+
+## Flashing the secure bootloader
+
+There are two methods of getting the secure bootloader onto the
+board. The simplest is to follow the above steps and then follow the
+usual method of updating the bootloader, which involves sending a
+MAVLink command to ask the firmware to flash the embedded bootloader
+from ROMFS. The firmware generated using the above steps will have
+your secure bootloader included in ROMFS, so when the users asks for
+the bootloader to update it will flash the secure bootloader.
+
+The second method is to put the board into DFU mode. If your hwdef.dat
+and hwdef-bl.dat include the ENABLE_DFU_BOOT options and your board is
+based on a STM32H7 then your ground station should be able to put the
+board into DFU mode. You can then flash the bootloader bin file to
+address 0x08000000 using any DFU capable client.
+
+Note that the flight controller will refuse a switch to DFU mode if it
+is running a secure bootloader already.
+
+## How to tell you are using secure boot
+
+When using a secure bootloader the USB ID presented by the bootloader
+will have a "Secure" string added. For example, you would see this in
+"dmesg" in Linux:
+
+```
+  Product: BOARDNAME-Secure-BL-v10
+```
+
+On Windows you can look at the device properties in device manager
+when the bootloader is running and look for the "Bus reported device
+description". It will have the above "Secure" string. Note that this
+string only appears when in the bootloader. To ensure the board stays
+in the bootloader for long enough to see this string just flash a
+normal unsigned firmware. With a secure bootloader and an unsigned
+firmware the board will stay in the bootloader forever as it will be
+failing the secure boot checks.
+
+## Reverting to normal boot
+
+If you have installed secure boot on a board then to revert to normal
+boot you would need to flash a new bootloader that does not have
+secure boot enabled. To do that you should replace
+Tools/bootloaders/BOARDNAME_bl.bin with the normal bootloader for your
+board then build and sign a firmware as above. Then ask the flight
+controller to flash the updated bootloader using the GCS interface and
+you will then be running a normal bootloader.
+
+## Supported Boards
+
+Secure boot is only supported on boards with at least 32k of flash
+space for the bootloader. This includes all boards based on the
+STM32H7 and STM32F7. You can use secure boot on older other boards if
+you change the hwdef.dat and hwdef-bl.dat to add more space for the
+bootloader.

Tools/scripts/signing/generate_keys.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python3
+'''
+generate a public/private key pair using Monocypher
+'''
+
+import sys
+import base64
+
+try:
+    import monocypher
+except ImportError:
+    print("Please install monocypher with: python3 -m pip install pymonocypher")
+    sys.exit(1)
+
+
+if len(sys.argv) != 2:
+    print("Usage: generate_keys.py BASENAME")
+    sys.exit(1)
+
+bname = sys.argv[1]
+
+def encode_key(ktype, key):
+    return ktype + "_KEYV1:" + base64.b64encode(key).decode('utf-8')
+
+private_key = monocypher.generate_key()
+public_key = monocypher.compute_signing_public_key(private_key)
+
+public_fname = "%s_public_key.dat" % bname
+private_fname = "%s_private_key.dat" % bname
+
+open(private_fname, "w").write(encode_key("PRIVATE", private_key))
+print("Generated %s" % private_fname)
+
+open(public_fname, "w").write(encode_key("PUBLIC", public_key))
+print("Generated %s" % public_fname)

Tools/scripts/signing/make_secure_bl.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python3
+'''
+add a set of up to 10 public keys to an ArduPilot bootloader bin file
+'''
+
+import sys
+import os
+import base64
+
+try:
+    import monocypher
+except ImportError:
+    print("Please install monocypher with: python3 -m pip install pymonocypher")
+    sys.exit(1)
+
+# get command line arguments
+from argparse import ArgumentParser
+parser = ArgumentParser(description='make_secure_bl')
+parser.add_argument("--omit-ardupilot-keys", action='store_true', default=False, help="omit ArduPilot signing keys")
+parser.add_argument("bootloader", type=str, default=None, help="bootloader")
+parser.add_argument("keys", type=str, nargs='+', help="keys")
+args = parser.parse_args()
+    
+descriptor = b'\x4e\xcf\x4e\xa5\xa6\xb6\xf7\x29'
+max_keys = 10
+key_len = 32
+
+if len(args.keys) <= 0:
+    print("At least one key file required")
+    sys.exit(1)
+
+img = open(args.bootloader, 'rb').read()
+
+offset = img.find(descriptor)
+if offset == -1:
+    print("Failed to find descriptor")
+    sys.exit(1)
+
+offset += 8
+desc = b''
+desc_len = 0
+
+keys = args.keys[:]
+
+if not args.omit_ardupilot_keys:
+    print("Adding ArduPilot keys")
+    signing_dir = os.path.dirname(os.path.realpath(__file__))
+    keydir = os.path.join(signing_dir,"ArduPilotKeys")
+    for root, dirs, files in os.walk(keydir):
+        for f in files:
+            if f.endswith(".dat"):
+                keys.append(os.path.relpath(os.path.join(keydir, f)))
+
+if len(keys) > max_keys:
+    print("Too many key files %u, max is %u" % (len(keys), max_keys))
+    sys.exit(1)
+
+def decode_key(ktype, key):
+    ktype += "_KEYV1:"
+    if not key.startswith(ktype):
+        print("Invalid key type")
+        sys.exit(1)
+    return base64.b64decode(key[len(ktype):])
+
+for kfile in keys:
+    key = decode_key("PUBLIC", open(kfile, "r").read())
+    print("Applying Public Key %s" % kfile)
+    if len(key) != key_len:
+        print("Bad key length %u in %s" % (len(key), kfile))
+        sys.exit(1)
+    desc += key
+    desc_len += key_len
+img = img[:offset] + desc + img[offset+desc_len:]
+open(sys.argv[1], 'wb').write(img)

Tools/scripts/signing/make_secure_fw.py

@@ -0,0 +1,91 @@
+#!/usr/bin/env python3
+'''
+sign an ArduPilot APJ firmware with a private key
+'''
+
+import sys
+import struct
+import json, base64, zlib
+
+try:
+    import monocypher
+except ImportError:
+    print("Please install monocypher with: python3 -m pip install pymonocypher")
+    sys.exit(1)
+
+key_len = 32
+sig_len = 64
+sig_version = 30437
+descriptor = b'\x41\xa3\xe5\xf2\x65\x69\x92\x07'
+
+if len(sys.argv) < 3:
+    print("Usage: make_secure_fw.py APJ_FILE PRIVATE_KEYFILE")
+    sys.exit(1)
+
+def to_unsigned(i):
+    '''convert a possibly signed integer to unsigned'''
+    if i < 0:
+        i += 2**32
+    return i
+
+apj_file = sys.argv[1]
+key_file = sys.argv[2]
+
+# open apj file
+apj = open(apj_file, 'r').read()
+
+# decode json in apj
+d = json.loads(apj)
+
+# get image data
+img = zlib.decompress(base64.b64decode(d['image']))
+img_len = len(img)
+
+def decode_key(ktype, key):
+    ktype += "_KEYV1:"
+    if not key.startswith(ktype):
+        print("Invalid key type")
+        sys.exit(1)
+    return base64.b64decode(key[len(ktype):])
+
+key = decode_key("PRIVATE", open(key_file, 'r').read())
+if len(key) != key_len:
+    print("Bad key length %u" % len(key))
+    sys.exit(1)
+
+offset = img.find(descriptor)
+if offset == -1:
+    print("No APP_DESCRIPTOR found")
+    sys.exit(1)
+
+offset += 8
+desc_len = 92
+
+flash1 = img[:offset]
+flash2 = img[offset+desc_len:]
+flash12 = flash1 + flash2
+
+signature = monocypher.signature_sign(key, flash12)
+if len(signature) != sig_len:
+    print("Bad signature length %u should be %u" % (len(signature), sig_len))
+    sys.exit(1)
+
+# pack signature in 4 bytes length, 8 byte signature version and 64 byte
+# signature. We have a signature version to allow for changes to signature
+# system in the future
+desc = struct.pack("<IQ64s", sig_len+8, sig_version, signature)
+img = img[:(offset + 16)] + desc + img[(offset + desc_len):]
+
+if len(img) != img_len:
+    print("Error: Image length changed")
+    sys.exit(1)
+
+print("Applying signature")
+
+d["image"] = base64.b64encode(zlib.compress(img,9)).decode('utf-8')
+d["signed_firmware"] = True
+
+f = open(sys.argv[1], "w")
+f.write(json.dumps(d, indent=4))
+f.close()
+print("Wrote %s" % apj_file)