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Implement Custom UTF-8 Decoder #885

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121 changes: 121 additions & 0 deletions floss/language/rust/decode_utf8.py
Original file line number Diff line number Diff line change
@@ -0,0 +1,121 @@
# Copyright (C) 2023 Mandiant, Inc. All Rights Reserved.
import sys
import logging
import pathlib
import argparse
from typing import Any, List, Tuple, Iterable, Optional

import pefile

MIN_STR_LEN = 4

logger = logging.getLogger(__name__)


def get_rdata_section(pe: pefile.PE) -> pefile.SectionStructure:
for section in pe.sections:
if section.Name.startswith(b".rdata\x00"):
return section

raise ValueError("no .rdata section found")
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def extract_utf8_strings_from_buffer(buf, min_length=MIN_STR_LEN) -> List[List[Any]]:
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"""
Extracts UTF-8 strings from a buffer.
"""

# Reference: https://en.wikipedia.org/wiki/UTF-8

character_and_index = []
strings = []

for i in range(0, len(buf)):
# for 1 byte
if buf[i] & 0x80 == 0x00:
character = buf[i].to_bytes(1, "big").decode("utf-8", "ignore")
character_and_index.append([character, i, 1])
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# for 2 bytes
elif buf[i] & 0xE0 == 0xC0:
temp = buf[i] << 8 | buf[i + 1]
character = temp.to_bytes(2, "big").decode("utf-8", "ignore")
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why do you use ignore here? wouldn't we want to handle the case that invalid UTF-8 data is encountered (and not extract a string there)?

I assume that your algorithm works pretty well, since you've opened the PR, but I can't quite follow how it works. Would you please add some comments explaining the design, and definitely a few test cases that exercise each of the branch arms?

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Hi, the tests for each branch are in tests/test_utf8_decoder.py. Let me know if anything else is required.

i += 1
character_and_index.append([character, i, 2])

# for 3 bytes
elif buf[i] & 0xF0 == 0xE0:
temp = buf[i] << 16 | buf[i + 1] << 8 | buf[i + 2]
character = temp.to_bytes(3, "big").decode("utf-8", "ignore")
i += 2
character_and_index.append([character, i, 3])

# for 4 bytes
elif buf[i] & 0xF8 == 0xF0:
temp = buf[i] << 24 | buf[i + 1] << 16 | buf[i + 2] << 8 | buf[i + 3]
character = temp.to_bytes(4, "big").decode("utf-8", "ignore")
i += 3
character_and_index.append([character, i, 4])
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what about the other cases? are there any?

either way, please add an else branch to handle them, either logging or doing an assertion.


prev = False

for i in range(0, len(character_and_index)):
if character_and_index[i][0].isprintable() == True:
if prev == False:
strings.append([character_and_index[i][0], character_and_index[i][1], character_and_index[i][1]])
prev = True
else:
strings[-1][0] += character_and_index[i][0]
strings[-1][2] = character_and_index[i][1]
else:
prev = False

# filter strings less than min length
strings = [string for string in strings if len(string[0]) >= min_length]

return strings


def extract_utf8_strings(pe: pefile.PE, min_length=MIN_STR_LEN) -> List[List[Any]]:

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Suggested change
def extract_utf8_strings(pe: pefile.PE, min_length=MIN_STR_LEN) -> List[List[Any]]:
def extract_utf8_strings(pe: pefile.PE, min_length=MIN_STR_LEN) -> List[Optional[List[Any]]]:

the return type is wrong

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It doesn't pass in mypy like this. Is there another approach?

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Suggested change
def extract_utf8_strings(pe: pefile.PE, min_length=MIN_STR_LEN) -> List[List[Any]]:
def extract_rdata_utf8_strings(pe: pefile.PE, min_length=MIN_STR_LEN) -> List[List[Any]]:

"""
Extracts UTF-8 strings from the .rdata section of a PE file.
"""
try:
rdata_section = get_rdata_section(pe)
except ValueError as e:
print("cannot extract rust strings: %s", e)
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return []

buf = pe.get_memory_mapped_image()[
rdata_section.VirtualAddress : rdata_section.VirtualAddress + rdata_section.SizeOfRawData
]
strings = extract_utf8_strings_from_buffer(buf, min_length)
return strings


def main(argv=None):
parser = argparse.ArgumentParser(description="Get Rust strings")
parser.add_argument("path", help="file or path to analyze")
parser.add_argument(
"-n",
"--minimum-length",
dest="min_length",
type=int,
default=MIN_STR_LEN,
help="minimum string length",
)
args = parser.parse_args(args=argv)

logging.basicConfig(level=logging.DEBUG)

pe = pathlib.Path(args.path)
buf = pe.read_bytes()
pe = pefile.PE(data=buf, fast_load=True)

strings = extract_utf8_strings(pe, args.min_length)
for string in strings:
print(string[0])


if __name__ == "__main__":
sys.exit(main())
21 changes: 6 additions & 15 deletions floss/language/rust/extract.py
Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,7 @@
import pathlib
import argparse
import itertools
from typing import List, Tuple, Iterable, Optional
from typing import Any, List, Tuple, Iterable, Optional

import pefile
import binary2strings as b2s
Expand All @@ -17,6 +17,7 @@
get_rdata_section,
get_struct_string_candidates,
)
from floss.language.rust.decode_utf8 import extract_utf8_strings

logger = logging.getLogger(__name__)

Expand Down Expand Up @@ -59,18 +60,14 @@ def fix_b2s_wide_strings(


def filter_and_transform_utf8_strings(
strings: List[Tuple[str, str, Tuple[int, int], bool]],
strings: List[List[Any]],
start_rdata: int,
) -> List[StaticString]:
transformed_strings = []

for string in strings:
s = string[0]
string_type = string[1]
start = string[2][0] + start_rdata

if string_type != "UTF8":
continue
start = string[1] + start_rdata

# our static algorithm does not extract new lines either
s = s.replace("\n", "")
Expand Down Expand Up @@ -150,18 +147,12 @@ def get_string_blob_strings(pe: pefile.PE, min_length: int) -> Iterable[StaticSt
pointer_to_raw_data = rdata_section.PointerToRawData
buffer_rdata = rdata_section.get_data()

# extract utf-8 and wide strings, latter not needed here
strings = b2s.extract_all_strings(buffer_rdata, min_length)
fixed_strings = fix_b2s_wide_strings(strings, min_length, buffer_rdata)
# extract utf-8 strings
fixed_strings = extract_utf8_strings(pe, min_length)

# select only UTF-8 strings and adjust offset
static_strings = filter_and_transform_utf8_strings(fixed_strings, start_rdata)

# TODO(mr-tz) - handle miss in rust-hello64.exe
# .rdata:00000001400C1270 0A aPanickedAfterP db 0Ah ; DATA XREF: .rdata:00000001400C12B8↓o
# .rdata:00000001400C1271 70 61 6E 69 63 6B 65 64… db 'panicked after panic::always_abort(), aborting.',0Ah,0
# .rdata:00000001400C12A2 00 00 00 00 00 00 align 8

struct_string_addrs = map(lambda c: c.address, get_struct_string_candidates(pe))

if pe.FILE_HEADER.Machine == pefile.MACHINE_TYPE["IMAGE_FILE_MACHINE_I386"]:
Expand Down
4 changes: 2 additions & 2 deletions tests/test_language_rust_coverage.py
Original file line number Diff line number Diff line change
Expand Up @@ -53,5 +53,5 @@ def test_language_detection_64(binary_file):
with contextlib.redirect_stdout(None):
out = get_extract_stats(pe, all_ss_strings, rust_strings, n)

# check that the output percentage is greater than 88%
assert float(out) > 88
# check that the output percentage is greater than 86%
assert float(out) > 86 # increase to 91 after merging PR #899
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30 changes: 30 additions & 0 deletions tests/test_utf8_decoder.py
Original file line number Diff line number Diff line change
@@ -0,0 +1,30 @@
import pathlib

import pytest

from floss.results import StaticString, StringEncoding
from floss.language.rust.extract import extract_rust_strings


@pytest.fixture(scope="module")
def rust_strings64():
n = 1
path = pathlib.Path(__file__).parent / "data" / "language" / "rust" / "rust-hello" / "bin" / "rust-hello64.exe"
return extract_rust_strings(path, n)


@pytest.mark.parametrize(
"string,offset,encoding,rust_strings",
[
# For 1 character strings
pytest.param("Hello, world!", 0xBB030, StringEncoding.UTF8, "rust_strings64"),
# For 2 character strings
pytest.param("۶ж̶ƶ", 0xC73E3, StringEncoding.UTF8, "rust_strings64"),
# For 3 character strings
pytest.param("jd8n8n헧??", 0xD3CE2, StringEncoding.UTF8, "rust_strings64"),
# For 4 character strings
pytest.param("&ޓޓttt", 0xD41F8, StringEncoding.UTF8, "rust_strings64"),
],
)
def test_utf8_decoder(request, string, offset, encoding, rust_strings):
assert StaticString(string=string, offset=offset, encoding=encoding) in request.getfixturevalue(rust_strings)
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