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#!/usr/bin/python2
'''
Perform basic ELF security checks on a series of executables.
Exit status will be 0 if successful, and the program will be silent.
Otherwise the exit status will be 1 and it will log which executables failed which checks.
Needs `readelf` (for ELF) and `objdump` (for PE).
'''
from __future__ import division,print_function,unicode_literals
import subprocess
import sys
import os
READELF_CMD = os.getenv('READELF', '/usr/bin/readelf')
OBJDUMP_CMD = os.getenv('OBJDUMP', '/usr/bin/objdump')
NONFATAL = {'HIGH_ENTROPY_VA'} # checks which are non-fatal for now but only generate a warning
def check_ELF_PIE(executable):
'''
Check for position independent executable (PIE), allowing for address space randomization.
'''
p = subprocess.Popen([READELF_CMD, '-h', '-W', executable], stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
(stdout, stderr) = p.communicate()
if p.returncode:
raise IOError('Error opening file')
ok = False
for line in stdout.split(b'\n'):
line = line.split()
if len(line)>=2 and line[0] == b'Type:' and line[1] == b'DYN':
ok = True
return ok
def get_ELF_program_headers(executable):
'''Return type and flags for ELF program headers'''
p = subprocess.Popen([READELF_CMD, '-l', '-W', executable], stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
(stdout, stderr) = p.communicate()
if p.returncode:
raise IOError('Error opening file')
in_headers = False
count = 0
headers = []
for line in stdout.split(b'\n'):
if line.startswith(b'Program Headers:'):
in_headers = True
if line == b'':
in_headers = False
if in_headers:
if count == 1: # header line
ofs_typ = line.find(b'Type')
ofs_offset = line.find(b'Offset')
ofs_flags = line.find(b'Flg')
ofs_align = line.find(b'Align')
if ofs_typ == -1 or ofs_offset == -1 or ofs_flags == -1 or ofs_align == -1:
raise ValueError('Cannot parse elfread -lW output')
elif count > 1:
typ = line[ofs_typ:ofs_offset].rstrip()
flags = line[ofs_flags:ofs_align].rstrip()
headers.append((typ, flags))
count += 1
return headers
def check_ELF_NX(executable):
'''
Check that no sections are writable and executable (including the stack)
'''
have_wx = False
have_gnu_stack = False
for (typ, flags) in get_ELF_program_headers(executable):
if typ == b'GNU_STACK':
have_gnu_stack = True
if b'W' in flags and b'E' in flags: # section is both writable and executable
have_wx = True
return have_gnu_stack and not have_wx
def check_ELF_RELRO(executable):
'''
Check for read-only relocations.
GNU_RELRO program header must exist
Dynamic section must have BIND_NOW flag
'''
have_gnu_relro = False
for (typ, flags) in get_ELF_program_headers(executable):
# Note: not checking flags == 'R': here as linkers set the permission differently
# This does not affect security: the permission flags of the GNU_RELRO program header are ignored, the PT_LOAD header determines the effective permissions.
# However, the dynamic linker need to write to this area so these are RW.
# Glibc itself takes care of mprotecting this area R after relocations are finished.
# See also http://permalink.gmane.org/gmane.comp.gnu.binutils/71347
if typ == b'GNU_RELRO':
have_gnu_relro = True
have_bindnow = False
p = subprocess.Popen([READELF_CMD, '-d', '-W', executable], stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
(stdout, stderr) = p.communicate()
if p.returncode:
raise IOError('Error opening file')
for line in stdout.split(b'\n'):
tokens = line.split()
if len(tokens)>1 and tokens[1] == b'(BIND_NOW)' or (len(tokens)>2 and tokens[1] == b'(FLAGS)' and b'BIND_NOW' in tokens[2]):
have_bindnow = True
return have_gnu_relro and have_bindnow
def check_ELF_Canary(executable):
'''
Check for use of stack canary
'''
p = subprocess.Popen([READELF_CMD, '--dyn-syms', '-W', executable], stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
(stdout, stderr) = p.communicate()
if p.returncode:
raise IOError('Error opening file')
ok = False
for line in stdout.split(b'\n'):
if b'__stack_chk_fail' in line:
ok = True
return ok
def get_PE_dll_characteristics(executable):
'''
Get PE DllCharacteristics bits.
Returns a tuple (arch,bits) where arch is 'i386:x86-64' or 'i386'
and bits is the DllCharacteristics value.
'''
p = subprocess.Popen([OBJDUMP_CMD, '-x', executable], stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)
(stdout, stderr) = p.communicate()
if p.returncode:
raise IOError('Error opening file')
arch = ''
bits = 0
for line in stdout.split('\n'):
tokens = line.split()
if len(tokens)>=2 and tokens[0] == 'architecture:':
arch = tokens[1].rstrip(',')
if len(tokens)>=2 and tokens[0] == 'DllCharacteristics':
bits = int(tokens[1],16)
return (arch,bits)
IMAGE_DLL_CHARACTERISTICS_HIGH_ENTROPY_VA = 0x0020
IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE = 0x0040
IMAGE_DLL_CHARACTERISTICS_NX_COMPAT = 0x0100
def check_PE_DYNAMIC_BASE(executable):
'''PIE: DllCharacteristics bit 0x40 signifies dynamicbase (ASLR)'''
(arch,bits) = get_PE_dll_characteristics(executable)
reqbits = IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE
return (bits & reqbits) == reqbits
# On 64 bit, must support high-entropy 64-bit address space layout randomization in addition to DYNAMIC_BASE
# to have secure ASLR.
def check_PE_HIGH_ENTROPY_VA(executable):
'''PIE: DllCharacteristics bit 0x20 signifies high-entropy ASLR'''
(arch,bits) = get_PE_dll_characteristics(executable)
if arch == 'i386:x86-64':
reqbits = IMAGE_DLL_CHARACTERISTICS_HIGH_ENTROPY_VA
else: # Unnecessary on 32-bit
assert(arch == 'i386')
reqbits = 0
return (bits & reqbits) == reqbits
def check_PE_NX(executable):
'''NX: DllCharacteristics bit 0x100 signifies nxcompat (DEP)'''
(arch,bits) = get_PE_dll_characteristics(executable)
return (bits & IMAGE_DLL_CHARACTERISTICS_NX_COMPAT) == IMAGE_DLL_CHARACTERISTICS_NX_COMPAT
CHECKS = {
'ELF': [
('PIE', check_ELF_PIE),
('NX', check_ELF_NX),
('RELRO', check_ELF_RELRO),
('Canary', check_ELF_Canary)
],
'PE': [
('DYNAMIC_BASE', check_PE_DYNAMIC_BASE),
('HIGH_ENTROPY_VA', check_PE_HIGH_ENTROPY_VA),
('NX', check_PE_NX)
]
}
def identify_executable(executable):
with open(filename, 'rb') as f:
magic = f.read(4)
if magic.startswith(b'MZ'):
return 'PE'
elif magic.startswith(b'\x7fELF'):
return 'ELF'
return None
if __name__ == '__main__':
retval = 0
for filename in sys.argv[1:]:
try:
etype = identify_executable(filename)
if etype is None:
print('%s: unknown format' % filename)
retval = 1
continue
failed = []
warning = []
for (name, func) in CHECKS[etype]:
if not func(filename):
if name in NONFATAL:
warning.append(name)
else:
failed.append(name)
if failed:
print('%s: failed %s' % (filename, ' '.join(failed)))
retval = 1
if warning:
print('%s: warning %s' % (filename, ' '.join(warning)))
except IOError:
print('%s: cannot open' % filename)
retval = 1
exit(retval)
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