[PATCH] tools: add a simple script to generate EFI variables

Heinrich Schuchardt
2020-11-30T16:38:53Z

On 11/30/20 4:16 PM, Paulo Alcantara wrote:
> This script generates EFI variables for U-Boot variable store format.

Hello Paulo,

thanks for you valuable contribution.

Wouldn't it make sense to allow overwriting and deleting variables too?

Best regards

Heinrich

>
> An example of generating secure boot variables
>
> $ openssl x509 -in foo.crt -outform DER -out foo.der
> $ efisiglist -a -c foo.der -o foo.esl
> $ efivar.py -i ubootefi.var add -n db -d foo.esl -t file
> $ efivar.py -i ubootefi.var add -n kek -d foo.esl -t file
> $ efivar.py -i ubootefi.var add -n pk -d foo.esl -t file
>
> Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
> ---
> tools/efivar.py | 141 ++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 141 insertions(+)
> create mode 100755 tools/efivar.py
>
> diff --git a/tools/efivar.py b/tools/efivar.py
> new file mode 100755
> index 000000000000..31e5508f08fd
> --- /dev/null
> +++ b/tools/efivar.py
> @@ -0,0 +1,141 @@
> +#!/usr/bin/env python3
> +## SPDX-License-Identifier: GPL-2.0-only
> +#
> +# Generate UEFI variables for U-Boot.
> +#
> +# (c) 2020 Paulo Alcantara <palcantara@suse.de>
> +#
> +
> +import os
> +import struct
> +import uuid
> +import time
> +import zlib
> +import argparse
> +import subprocess as sp
> +
> +# U-Boot variable store format (version 1)
> +UBOOT_EFI_VAR_FILE_MAGIC = 0x0161566966456255
> +
> +# UEFI variable attributes
> +EFI_VARIABLE_NON_VOLATILE = 0x1
> +EFI_VARIABLE_BOOTSERVICE_ACCESS = 0x2
> +EFI_VARIABLE_RUNTIME_ACCESS = 0x4
> +EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS = 0x20
> +NV_BS = EFI_VARIABLE_NON_VOLATILE | EFI_VARIABLE_BOOTSERVICE_ACCESS
> +NV_BS_RT = NV_BS | EFI_VARIABLE_RUNTIME_ACCESS
> +NV_BS_RT_AT = NV_BS_RT | EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS
> +
> +# UEFI variable GUIDs
> +EFI_GLOBAL_VARIABLE_GUID = '{8be4df61-93ca-11d2-aa0d-00e098032b8c}'
> +EFI_IMAGE_SECURITY_DATABASE_GUID = '{d719b2cb-3d3a-4596-a3bc-dad00e67656f}'
> +
> +class EfiStruct:
> + # struct efi_var_file
> + var_file_fmt = '<QQLL'
> + var_file_size = struct.calcsize(var_file_fmt)
> + # struct efi_var_entry
> + var_entry_fmt = '<LLQ16s'
> + var_entry_size = struct.calcsize(var_entry_fmt)
> +
> +class EfiVariableStore:
> + def __init__(self, infile):
> + self.infile = infile
> + self.efi = EfiStruct()
> + if os.path.exists(self.infile) and os.stat(self.infile).st_size > self.efi.var_file_size:
> + with open(self.infile, 'rb') as f:
> + # skip header since it will be recreated by save()
> + self.buf = f.read()[self.efi.var_file_size:]
> + else:
> + self.buf = bytearray()
> +
> + def _set_var(self, guid, name_data, size, attr, tsec):
> + ent = struct.pack(self.efi.var_entry_fmt,
> + size,
> + attr,
> + tsec,
> + uuid.UUID(guid).bytes_le)
> + ent += name_data
> + self.buf += ent
> +
> + def set_var(self, guid, name, data, size, attr):
> + tsec = int(time.time()) if attr & EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS else 0
> + nd = name.encode('utf_16_le') + b"\x00\x00" + data
> + # U-Boot variable format requires the name + data blob to be 8-byte aligned
> + pad = ((len(nd) + 7) & ~7) - len(nd)
> + nd += bytes([0] * pad)
> +
> + return self._set_var(guid, nd, size, attr, tsec)
> +
> + def save(self):
> + hdr = struct.pack(self.efi.var_file_fmt,
> + 0,
> + UBOOT_EFI_VAR_FILE_MAGIC,
> + len(self.buf) + self.efi.var_file_size,
> + zlib.crc32(self.buf) & 0xffffffff)
> +
> + with open(self.infile, 'wb') as f:
> + f.write(hdr)
> + f.write(self.buf)
> +
> +def parse_attrs(attr):
> + attrs = {
> + 'nv': EFI_VARIABLE_NON_VOLATILE,
> + 'bs': EFI_VARIABLE_BOOTSERVICE_ACCESS,
> + 'rt': EFI_VARIABLE_RUNTIME_ACCESS,
> + 'at': EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS,
> + }
> + v = 0
> + for i in attr.split(','):
> + v |= attrs[i.lower()]
> + return v
> +
> +def parse_data(val, vtype):
> + fmt = { 'u8': '<B', 'u16': '<H', 'u32': '<L', 'u64': '<Q' }
> + if vtype.lower() == 'file':
> + with open(val, 'rb') as f:
> + data = f.read()
> + return data, len(data)
> + if vtype.lower() == 'str':
> + data = val.encode('utf-8') + b'\x00'
> + return data, len(data)
> + i = fmt[vtype.lower()]
> + return struct.pack(i, int(val)), struct.calcsize(i)
> +
> +def cmd_add(args):
> + env = EfiVariableStore(args.infile)
> + data, size = parse_data(args.data, args.type)
> +
> + if args.name.lower() == 'pk':
> + env.set_var(guid=EFI_GLOBAL_VARIABLE_GUID, name='PK', data=data, size=size, attr=NV_BS_RT_AT)
> + elif args.name.lower() == 'kek':
> + env.set_var(guid=EFI_GLOBAL_VARIABLE_GUID, name='KEK', data=data, size=size, attr=NV_BS_RT_AT)
> + elif args.name.lower() == 'db':
> + env.set_var(guid=EFI_IMAGE_SECURITY_DATABASE_GUID, name='db', data=data, size=size, attr=NV_BS_RT_AT)
> + elif args.name.lower() == 'dbx':
> + env.set_var(guid=EFI_IMAGE_SECURITY_DATABASE_GUID, name='dbx', data=data, size=size, attr=NV_BS_RT_AT)
> + else:
> + guid = args.guid if args.guid else EFI_GLOBAL_VARIABLE_GUID
> + attr = parse_attrs(args.attr) if args.attr else NV_BS
> + env.set_var(guid=guid, name=args.name, data=data, size=size, attr=attr)
> +
> + env.save()
> +
> +def main():
> + ap = argparse.ArgumentParser(description='Generate U-Boot variable store')
> + ap.add_argument('--infile', '-i', required=True, help='file to save the UEFI variables')
> + subp = ap.add_subparsers(help="sub-command help")
> +
> + addp = subp.add_parser('add', help='add UEFI variable')
> + addp.add_argument('--name', '-n', required=True, help='variable name')
> + addp.add_argument('--attr', '-a', help='variable attributes (default: nv,bs)')
> + addp.add_argument('--guid', '-g', help="variable guid (default: %s)"%EFI_GLOBAL_VARIABLE_GUID)
> + addp.add_argument('--type', '-t', required=True, help='variable type (values: file|u8|u16|u32|u64|str)')
> + addp.add_argument('--data', '-d', required=True, help='variable data')
> + addp.set_defaults(func=cmd_add)
> +
> + args = ap.parse_args()
> + args.func(args)
> +
> +if __name__ == '__main__':
> + main()
>