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https://github.com/mandiant/capa.git
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helpers: move ELF and IDA helpers out of script and into common module
This commit is contained in:
William Ballenthin
@@ -13,278 +13,20 @@ detect-elf-os
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Attempt to detect the underlying OS that the given ELF file targets.
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"""
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import sys
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import struct
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import logging
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import argparse
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import contextlib
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from enum import Enum
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from typing import BinaryIO
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import capa.helpers
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import capa.features.extractors.elf
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logger = logging.getLogger("capa.detect-elf-os")
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def align(v, alignment):
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remainder = v % alignment
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if remainder == 0:
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return v
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else:
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return v + remainder
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class IDAIO:
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"""
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An object that acts as a file-like object,
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using bytes from the current IDB workspace.
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"""
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def __init__(self):
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assert IDAIO.is_runtime_ida() == True
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super(IDAIO, self).__init__()
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import idc
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import ida_bytes
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import ida_loader
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self.offset = 0
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def seek(self, offset, whence=0):
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assert whence == 0
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self.offset = offset
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def read(self, size):
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ea = ida_loader.get_fileregion_ea(self.offset)
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if ea == idc.BADADDR:
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# best guess, such as if file is mapped at address 0x0.
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ea = self.offset
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logger.debug("reading 0x%x bytes at 0x%x (ea: 0x%x)", size, self.offset, ea)
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return ida_bytes.get_bytes(ea, size)
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def close(self):
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return
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@staticmethod
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def is_runtime_ida():
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try:
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import idc
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except ImportError:
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return False
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else:
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return True
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class CorruptElfFile(ValueError):
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pass
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class OS(str, Enum):
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HPUX = "HPUX"
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NETBSD = "NETBSD"
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LINUX = "LINUX"
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HURD = "HURD"
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_86OPEN = "86OPEN"
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SOLARIS = "SOLARIS"
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AIX = "AIX"
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IRIX = "IRIX"
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FREEBSD = "FREEBSD"
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TRU64 = "TRU64"
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MODESTO = "MODESTO"
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OPENBSD = "OPENBSD"
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OPENVMS = "OPENVMS"
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NSK = "NSK"
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AROS = "AROS"
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FENIXOS = "FENIXOS"
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CLOUD = "CLOUD"
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SORTFIX = "SORTFIX"
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ARM_AEABI = "ARM_AEABI"
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SYLLABLE = "SYLLABLE"
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NACL = "NACL"
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def detect_elf_os(f: BinaryIO) -> str:
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f.seek(0x0)
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file_header = f.read(0x40)
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# we'll set this to the detected OS
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# prefer the first heuristics,
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# but rather than short circuiting,
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# we'll still parse out the remainder, for debugging.
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ret = None
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if not file_header.startswith(b"\x7fELF"):
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raise CorruptElfFile("missing magic header")
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ei_class, ei_data = struct.unpack_from("BB", file_header, 4)
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logger.debug("ei_class: 0x%02x ei_data: 0x%02x", ei_class, ei_data)
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if ei_class == 1:
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bitness = 32
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elif ei_class == 2:
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bitness = 64
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else:
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raise CorruptElfFile("invalid ei_class: 0x%02x" % ei_class)
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if ei_data == 1:
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endian = "<"
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elif ei_data == 2:
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endian = ">"
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else:
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raise CorruptElfFile("not an ELF file: invalid ei_data: 0x%02x" % ei_data)
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if bitness == 32:
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(e_phoff,) = struct.unpack_from(endian + "I", file_header, 0x1C)
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e_phentsize, e_phnum = struct.unpack_from(endian + "HH", file_header, 0x2A)
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elif bitness == 64:
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(e_phoff,) = struct.unpack_from(endian + "Q", file_header, 0x20)
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e_phentsize, e_phnum = struct.unpack_from(endian + "HH", file_header, 0x36)
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else:
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raise NotImplemented
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logger.debug("e_phoff: 0x%02x e_phentsize: 0x%02x e_phnum: %d", e_phoff, e_phentsize, e_phnum)
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(ei_osabi,) = struct.unpack_from(endian + "B", file_header, 7)
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OSABI = {
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# via pyelftools: https://github.com/eliben/pyelftools/blob/0664de05ed2db3d39041e2d51d19622a8ef4fb0f/elftools/elf/enums.py#L35-L58
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# 0: "SYSV",
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1: OS.HPUX,
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2: OS.NETBSD,
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3: OS.LINUX,
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4: OS.HURD,
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5: OS._86OPEN,
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6: OS.SOLARIS,
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7: OS.AIX,
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8: OS.IRIX,
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9: OS.FREEBSD,
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10: OS.TRU64,
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11: OS.MODESTO,
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12: OS.OPENBSD,
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13: OS.OPENVMS,
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14: OS.NSK,
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15: OS.AROS,
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16: OS.FENIXOS,
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17: OS.CLOUD,
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# 53: "SORTFIX",
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# 64: "ARM_AEABI",
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# 97: "ARM",
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# 255: "STANDALONE",
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}
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logger.debug("ei_osabi: 0x%02x (%s)", ei_osabi, OSABI.get(ei_osabi, "unknown"))
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if ei_osabi in OSABI and ei_osabi != 0x0:
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# update only if not set
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# so we can get the debugging output of subsequent strategies
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ret = OSABI[ei_osabi] if not ret else ret
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f.seek(e_phoff)
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program_header_size = e_phnum * e_phentsize
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program_headers = f.read(program_header_size)
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if len(program_headers) != program_header_size:
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logger.warning("failed to read program headers")
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e_phnum = 0
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for i in range(e_phnum):
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offset = i * e_phentsize
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phent = program_headers[offset : offset + e_phentsize]
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PT_NOTE = 0x4
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(p_type,) = struct.unpack_from(endian + "I", phent, 0x0)
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logger.debug("p_type: 0x%04x", p_type)
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if p_type != PT_NOTE:
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continue
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if bitness == 32:
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p_offset, _, _, p_filesz = struct.unpack_from(endian + "IIII", phent, 0x4)
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elif bitness == 64:
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p_offset, _, _, p_filesz = struct.unpack_from(endian + "QQQQ", phent, 0x8)
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else:
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raise NotImplemented
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logger.debug("p_offset: 0x%02x p_filesz: 0x%04x", p_offset, p_filesz)
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f.seek(p_offset)
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note = f.read(p_filesz)
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if len(note) != p_filesz:
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logger.warning("failed to read note content")
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continue
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namesz, descsz, type_ = struct.unpack_from(endian + "III", note, 0x0)
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name_offset = 0xC
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desc_offset = name_offset + align(namesz, 0x4)
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logger.debug("namesz: 0x%02x descsz: 0x%02x type: 0x%04x", namesz, descsz, type_)
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name = note[name_offset : name_offset + namesz].partition(b"\x00")[0].decode("ascii")
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logger.debug("name: %s", name)
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if type_ != 1:
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continue
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if name == "GNU":
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if descsz < 16:
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continue
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desc = note[desc_offset : desc_offset + descsz]
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abi_tag, kmajor, kminor, kpatch = struct.unpack_from(endian + "IIII", desc, 0x0)
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# via readelf: https://github.com/bminor/binutils-gdb/blob/c0e94211e1ac05049a4ce7c192c9d14d1764eb3e/binutils/readelf.c#L19635-L19658
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# and here: https://github.com/bminor/binutils-gdb/blob/34c54daa337da9fadf87d2706d6a590ae1f88f4d/include/elf/common.h#L933-L939
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GNU_ABI_TAG = {
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0: OS.LINUX,
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1: OS.HURD,
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2: OS.SOLARIS,
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3: OS.FREEBSD,
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4: OS.NETBSD,
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5: OS.SYLLABLE,
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6: OS.NACL,
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}
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logger.debug("GNU_ABI_TAG: 0x%02x", abi_tag)
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if abi_tag in GNU_ABI_TAG:
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# update only if not set
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# so we can get the debugging output of subsequent strategies
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ret = GNU_ABI_TAG[abi_tag] if not ret else ret
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logger.debug("abi tag: %s earliest compatible kernel: %d.%d.%d", ret, kmajor, kminor, kpatch)
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elif name == "OpenBSD":
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logger.debug("note owner: %s", "OPENBSD")
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ret = OS.OPENBSD if not ret else ret
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elif name == "NetBSD":
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logger.debug("note owner: %s", "NETBSD")
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ret = OS.NETBSD if not ret else ret
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for i in range(e_phnum):
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offset = i * e_phentsize
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phent = program_headers[offset : offset + e_phentsize]
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PT_INTERP = 0x3
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(p_type,) = struct.unpack_from(endian + "I", phent, 0x0)
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if p_type != PT_INTERP:
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continue
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if bitness == 32:
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p_offset, _, _, p_filesz = struct.unpack_from(endian + "IIII", phent, 0x4)
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elif bitness == 64:
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p_offset, _, _, p_filesz = struct.unpack_from(endian + "QQQQ", phent, 0x8)
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else:
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raise NotImplemented
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f.seek(p_offset)
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interp = f.read(p_filesz)
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if len(interp) != p_filesz:
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logger.warning("failed to read interp content")
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continue
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linker = interp.partition(b"\x00")[0].decode("ascii")
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logger.debug("linker: %s", linker)
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if "ld-linux" in linker:
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# update only if not set
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# so we can get the debugging output of subsequent strategies
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ret = OS.LINUX if ret is None else ret
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return ret.value if ret is not None else "unknown"
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def main(argv=None):
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if IDAIO.is_runtime_ida():
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if capa.helpers.is_runtime_ida():
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from capa.ida.helpers import IDAIO
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f: BinaryIO = IDAIO()
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else:
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@@ -318,15 +60,15 @@ def main(argv=None):
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with contextlib.closing(f):
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try:
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print(detect_elf_os(f))
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print(capa.features.extractors.elf.detect_elf_os(f))
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return 0
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except CorruptElfFile as e:
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except capa.features.extractors.elf.CorruptElfFile as e:
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logger.error("corrupt ELF file: %s", str(e.args[0]))
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return -1
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if __name__ == "__main__":
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if IDAIO.is_runtime_ida():
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if capa.helpers.is_runtime_ida():
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main()
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else:
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sys.exit(main())
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