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edk2.py: Improve firmware volume (de)serialization #11

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merged 1 commit into from
Mar 6, 2024
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edk2.py: Improve firmware volume (de)serialization #11

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90 changes: 69 additions & 21 deletions pyuefivars/edk2.py
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Original file line number Diff line number Diff line change
Expand Up @@ -79,7 +79,6 @@ class EDK2UEFIVarStore(UEFIVarStore):
STATE_SETTLED = 0x3f
VARSTORE_STATUS = b'\x5a\xfe\x00\x00\x00\x00\x00\x00'
DEFAULT_LENGTH = 540672
HEADER_LENGTH = 0x48
EFI_FVB2_READ_DISABLED_CAP = 0x00000001
EFI_FVB2_READ_ENABLED_CAP = 0x00000002
EFI_FVB2_READ_STATUS = 0x00000004
Expand Down Expand Up @@ -129,6 +128,8 @@ class EDK2UEFIVarStore(UEFIVarStore):
EFI_FVB2_ALIGNMENT_512M = 0x001D0000
EFI_FVB2_ALIGNMENT_1G = 0x001E0000
EFI_FVB2_ALIGNMENT_2G = 0x001F0000
AAVMF_BLOCK_SIZE = 0x00040000
OVMF_BLOCK_SIZE = 0x00001000

def __init__(self, data):
super().__init__()
Expand All @@ -141,7 +142,7 @@ def __init__(self, data):
file.seek(0, os.SEEK_SET)
file = AWSVarStoreFile(file)

# Validate header
# Parse FV header
zerovector = file.read(0x10)
if zerovector != b'\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0':
raise Exception("Invalid Zero Vector: %s" % zerovector)
Expand All @@ -159,20 +160,43 @@ def __init__(self, data):
raise Exception('Invalid FVH signature: %s' % sig)

self.attrs = file.read32()

hlength = file.read16()
if hlength != self.HEADER_LENGTH:
raise Exception('Invalid FVH Header Length: 0x%x' % hlength)

csum_hdr = file.read16()
if (self.csum16(data[:self.HEADER_LENGTH]) != 0):
if (self.csum16(data[:hlength]) != 0):
raise Exception("Invalid header checksum: 0x%x" % csum_hdr)
file.read(3)

# Ensure there isn't an extension header we don't understand
ext_hdr_offset = file.read16()
if ext_hdr_offset != 0:
raise Exception('FVH with extension header not supported')

# Reserved field (should always be 0)
reserved = file.read8()
if reserved != 0:
raise Exception('Wrong value for FVH.Reserved: %s' % reserved)

# Read revision
rev = file.read8()
if rev != 0x2:
raise Exception('Invalid FVH Revision: 0x%x' % rev)

file.read(0x10)
# Read blockmap
self.blockmap = []
total_bytes = 0
while True:
block_cnt, block_bytes = file.read32(), file.read32()
if block_cnt == 0 and block_bytes == 0:
break
self.blockmap.append((block_cnt, block_bytes))
total_bytes += block_cnt * block_bytes
if self.length != total_bytes:
raise Exception('Invalid blockmap: %s' % self.blockmap)

# Verify header length (ext headers not supported so must match current pos)
if hlength != file.file.tell():
raise Exception('Invalid header length: %s' % hlength)

# Parse varstore header
vsguid = file.readguid()
if vsguid != self.GUID_VARSTORE:
raise Exception('Invalid Varstore GUID: %s' % vsguid)
Expand Down Expand Up @@ -266,44 +290,68 @@ def __bytes__(self) -> bytes:
self.EFI_FVB2_ALIGNMENT_16
if not hasattr(self, 'varsize'):
self.varsize = int(self.length / 2) - 8264
if not hasattr(self, 'blockmap'):
# What is a sensible default here? Examples:
# - AAVMF uses 256K blocks (virtual NOR)
# - OVMF uses 4K blocks (page size)
self.blockmap = [(self.length // self.OVMF_BLOCK_SIZE, self.OVMF_BLOCK_SIZE)]

# Assemble the flash file
raw = AWSVarStoreFile(tempfile.SpooledTemporaryFile())

# Write FV header
raw.write(b'\0' * 16)
raw.write(self.GUID_NVFS)
raw.write64(self.length)
raw.write(b'_FVH')
raw.write32(self.attrs)
raw.write16(self.HEADER_LENGTH)
# Leave room for header length
hlength_pos = raw.file.tell()
raw.write16(0)
# Skip checksum - need to fill in later
csum_pos = raw.file.tell()
raw.write16(0) # checksum - need to fill in later
raw.write16(0)
# No ext header offset + set reversed byte to 0
raw.write(b'\0' * 3)
raw.write8(0x2) # revision
raw.write(b'\x84\x00\x00\x00\x00\x10\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')
# Revision
raw.write8(0x2)
# Blockmap
for block_cnt, block_bytes in self.blockmap:
raw.write32(block_cnt)
raw.write32(block_bytes)
raw.write32(0)
raw.write32(0)
# Write header length
hlength = raw.file.tell()
raw.file.seek(hlength_pos, os.SEEK_SET)
raw.write16(hlength)
# Checksum
raw.file.seek(0, os.SEEK_SET)
fvh = raw.file.read(hlength)
raw.file.seek(csum_pos, os.SEEK_SET)
raw.write16((0x10000 - self.csum16(fvh) & 0xffff))
# Go to end of FV header
raw.file.seek(hlength, os.SEEK_SET)

# Write varstore header
raw.write(self.GUID_VARSTORE)
raw.write32(self.varsize)
raw.write(self.VARSTORE_STATUS)
header_end_pos = raw.file.tell()
header_end_pos # Unused, silence F841

# Write variables
self.write_var(raw, self.certdb.to_var(self.vars))

for var in self.vars:
self.write_var(raw, var)

# Make sure it all fits
if raw.file.tell() > self.length:
raise Exception("Can not fit variables into store")

# Expand to maximum file size
raw.file.seek(self.length - 1, os.SEEK_SET)
raw.write8(0)

# Calculate checksums
raw.file.seek(0, os.SEEK_SET)
full = raw.file.read()
raw.file.seek(csum_pos, os.SEEK_SET)
raw.write16((0x10000 - self.csum16(full[:self.HEADER_LENGTH])) & 0xffff)

raw.file.seek(0, os.SEEK_SET)
return raw.file.read()

Expand Down
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