[PATCH v2] tools: add a simple script to generate EFI variables

Heinrich Schuchardt
2020-12-02T10:50:12Z

On 12/1/20 11:58 PM, Paulo Alcantara wrote:
> This script generates EFI variables for U-Boot variable store format.
>
> A few examples:
>
> - Generating secure boot keys
>
> $ openssl x509 -in foo.crt -outform DER -out foo.der
> $ efisiglist -a -c foo.der -o foo.esl
> $ ./efivar.py -i ubootefi.var set -n db -d foo.esl -t file
> $ ./efivar.py -i ubootefi.var set -n kek -d foo.esl -t file
> $ ./efivar.py -i ubootefi.var set -n pk -d foo.esl -t file
>
> - Printing out variables
>
> $ ./efivar.py -i ubootefi.var set -n var1 -a nv,bs -d foo -t str
> $ ./efivar.py -i ubootefi.var set -n var2 -a nv,bs -d bar -t str
> $ ./efivar.py -i ubootefi.var print
> var1:
> 8be4df61-93ca-11d2-aa0d-00e098032b8c EFI_GLOBAL_VARIABLE_GUID
> NV|BS, DataSize = 0x3
> 0000000000: 66 6F 6F foo
> var2:
> 8be4df61-93ca-11d2-aa0d-00e098032b8c EFI_GLOBAL_VARIABLE_GUID
> NV|BS, DataSize = 0x3
> 0000000000: 62 61 72 bar
>
> - Removing variables
>
> $ ./efivar.py -i ubootefi.var set -n var1 -a nv,bs -d foo -t str
> $ ./efivar.py -i ubootefi.var print -n var1
> var1:
> 8be4df61-93ca-11d2-aa0d-00e098032b8c EFI_GLOBAL_VARIABLE_GUID
> NV|BS, DataSize = 0x3
> 0000000000: 66 6F 6F foo
> $ ./efivar.py -i ubootefi.var set -n var1
> $ ./efivar.py -i ubootefi.var print -n var1
> err: variable not found
>
> Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
> ---
> tools/efivar.py | 276 ++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 276 insertions(+)
> create mode 100755 tools/efivar.py
>
> diff --git a/tools/efivar.py b/tools/efivar.py
> new file mode 100755
> index 000000000000..ecd12319c43b
> --- /dev/null
> +++ b/tools/efivar.py
> @@ -0,0 +1,276 @@
> +#!/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
> +
> +# 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_AUTHENTICATED_WRITE_ACCESS = 0x10
> +EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS = 0x20
> +EFI_VARIABLE_READ_ONLY = 1 << 31
> +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'
> +
> +var_attrs = {
> + 'NV': EFI_VARIABLE_NON_VOLATILE,
> + 'BS': EFI_VARIABLE_BOOTSERVICE_ACCESS,
> + 'RT': EFI_VARIABLE_RUNTIME_ACCESS,
> + 'AT': EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS,
> + 'RO': EFI_VARIABLE_READ_ONLY,
> + 'AW': EFI_VARIABLE_AUTHENTICATED_WRITE_ACCESS,
> +}
> +
> +var_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 EfiVariable:
> + def __init__(self, size, attrs, time, guid, name, data):
> + self.size = size
> + self.attrs = attrs
> + self.time = time
> + self.guid = guid
> + self.name = name
> + self.data = data
> +
> +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:
> + buf = f.read()
> + self._check_header(buf)
> + self.ents = buf[self.efi.var_file_size:]
> + else:
> + self.ents = bytearray()
> +
> + def _check_header(self, buf):
> + hdr = struct.unpack_from(self.efi.var_file_fmt, buf, 0)
> + magic, crc32 = hdr[1], hdr[3]
> +
> + if magic != UBOOT_EFI_VAR_FILE_MAGIC:
> + print("err: invalid magic number: %s"%hex(magic))
> + exit(1)
> + if crc32 != zlib.crc32(buf[self.efi.var_file_size:]) & 0xffffffff:
> + print("err: invalid crc32: %s"%hex(crc32))
> + exit(1)
> +
> + def _get_var_name(self, buf):
> + name = ''
> + for i in range(0, len(buf) - 1, 2):
> + if not buf[i] and not buf[i+1]:
> + break
> + name += chr(buf[i])
> + return ''.join([chr(x) for x in name.encode('utf_16_le') if x]), i + 2
> +
> + def _next_var(self, offs=0):
> + size, attrs, time, guid = struct.unpack_from(self.efi.var_entry_fmt, self.ents, offs)
> + data_fmt = str(size)+"s"
> + offs += self.efi.var_entry_size
> + name, namelen = self._get_var_name(self.ents[offs:])
> + offs += namelen
> + data = struct.unpack_from(data_fmt, self.ents, offs)[0]
> + offs = (offs + len(data) + 7) & ~7
> + return EfiVariable(size, attrs, time, uuid.UUID(bytes_le=guid), name, data), offs
> +
> + def __iter__(self):
> + self.offs = 0
> + return self
> +
> + def __next__(self):
> + if self.offs < len(self.ents):
> + var, noffs = self._next_var(self.offs)
> + self.offs = noffs
> + return var
> + else:
> + raise StopIteration
> +
> + def __len__(self):
> + return len(self.ents)
> +
> + def _set_var(self, guid, name_data, size, attrs, tsec):
> + ent = struct.pack(self.efi.var_entry_fmt,
> + size,
> + attrs,
> + tsec,
> + uuid.UUID(guid).bytes_le)
> + ent += name_data
> + self.ents += ent
> +
> + def set_var(self, guid, name, data, size, attrs):
> + offs = 0
> + while offs < len(self.ents):
> + var, loffs = self._next_var(offs)
> + if var.name == name and str(var.guid) == guid:
> + if not data or not attrs:
> + self.ents = self.ents[:offs] + self.ents[loffs:]
> + return
> + if var.attrs != attrs:
> + print("err: invalid attributes")

Thanks for adding deletion and modification of variables.

print("attributes don't match")

would better convey the meaning here.

> + exit(1)
> + # make room for updating var
> + self.ents = self.ents[:offs] + self.ents[loffs:]
> + break
> + offs = loffs
> +
> + if not data or not attrs:
> + print("err: variable not found")
> + exit(1)
> +
> + tsec = int(time.time()) if attrs & 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, attrs, tsec)
> +
> + def save(self):
> + hdr = struct.pack(self.efi.var_file_fmt,
> + 0,
> + UBOOT_EFI_VAR_FILE_MAGIC,
> + len(self.ents) + self.efi.var_file_size,
> + zlib.crc32(self.ents) & 0xffffffff)
> +
> + with open(self.infile, 'wb') as f:
> + f.write(hdr)
> + f.write(self.ents)
> +
> +def parse_attrs(attrs):
> + v = 0
> + if attrs:
> + for i in attrs.split(','):
> + v |= var_attrs[i.upper()]
> + return v
> +
> +def parse_data(val, vtype):
> + if not val or not vtype:
> + return None, 0
> + 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')
> + return data, len(data)
> + i = fmt[vtype.lower()]
> + return struct.pack(i, int(val)), struct.calcsize(i)
> +
> +def cmd_set(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, attrs=NV_BS_RT_AT)
> + elif args.name.lower() == 'kek':
> + env.set_var(guid=EFI_GLOBAL_VARIABLE_GUID, name='KEK', data=data, size=size, attrs=NV_BS_RT_AT)
> + elif args.name.lower() == 'db':
> + env.set_var(guid=EFI_IMAGE_SECURITY_DATABASE_GUID, name='db', data=data, size=size, attrs=NV_BS_RT_AT)
> + elif args.name.lower() == 'dbx':
> + env.set_var(guid=EFI_IMAGE_SECURITY_DATABASE_GUID, name='dbx', data=data, size=size, attrs=NV_BS_RT_AT)
> + else:
> + guid = args.guid if args.guid else EFI_GLOBAL_VARIABLE_GUID
> + attrs = parse_attrs(args.attrs)
> + env.set_var(guid=guid, name=args.name, data=data, size=size, attrs=attrs)
> +
> + env.save()
> +
> +def print_var(var):
> + print(var.name+':')
> + print(" "+str(var.guid)+' '+''.join([x for x in var_guids if str(var.guid) == var_guids[x]]))
> + print(" "+'|'.join([x for x in var_attrs if var.attrs & var_attrs[x]])+", DataSize = %s"%hex(var.size))
> + hexdump(var.data)
> +
> +def cmd_print(args):
> + env = EfiVariableStore(args.infile)
> + if not args.name and not args.guid and not len(env):
> + return
> +
> + found = False
> + for var in env:
> + if not args.name:
> + if args.guid and args.guid != str(var.guid):
> + continue
> + print_var(var)
> + found = True
> + else:
> + if args.name != var.name or (args.guid and args.guid != str(var.guid)):
> + continue
> + print_var(var)
> + found = True
> +
> + if not found:
> + print("err: variable not found")
> + exit(1)
> +
> +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")
> +
> + printp = subp.add_parser('print', help='get/list UEFI variables')
> + printp.add_argument('--guid', '-g', help='variable guid')
> + printp.add_argument('--name', '-n', help='variable name')
> + printp.set_defaults(func=cmd_print)
> +
> + setp = subp.add_parser('set', help='set UEFI variable')
> + setp.add_argument('--name', '-n', required=True, help='variable name')
> + setp.add_argument('--attrs', '-a', help='variable attributes (values: nv,bs,rt,at,ro,aw)')

The online help should indicate how deletion works.

I find it problematic that if a user forgets to specify -a, the variable
is deleted inadvertently. PK, KEK, db, dbx cannot be deleted at all.

Please, use a separate sub-command for deletion and reintroduce the
default for the attributes as nv,bs,rt which seems more useful than nv,bs.

> + setp.add_argument('--guid', '-g', help="variable guid (default: %s)"%EFI_GLOBAL_VARIABLE_GUID)

%s/variable guid/vendor GUID/

> + setp.add_argument('--type', '-t', help='variable type (values: file|u8|u16|u32|u64|str)')
> + setp.add_argument('--data', '-d', help='variable data')

help='data or filename'

Best regards

Heinrich

> + setp.set_defaults(func=cmd_set)
> +
> + args = ap.parse_args()
> + args.func(args)
> +
> +def group(a, *ns):
> + for n in ns:
> + a = [a[i:i+n] for i in range(0, len(a), n)]
> + return a
> +
> +def join(a, *cs):
> + return [cs[0].join(join(t, *cs[1:])) for t in a] if cs else a
> +
> +def hexdump(data):
> + toHex = lambda c: '{:02X}'.format(c)
> + toChr = lambda c: chr(c) if 32 <= c < 127 else '.'
> + make = lambda f, *cs: join(group(list(map(f, data)), 8, 2), *cs)
> + hs = make(toHex, ' ', ' ')
> + cs = make(toChr, ' ', '')
> + for i, (h, c) in enumerate(zip(hs, cs)):
> + print (' {:010X}: {:48} {:16}'.format(i * 16, h, c))
> +
> +if __name__ == '__main__':
> + main()
>