#!/usr/bin/ruby

#    This file is part of Metasm, the Ruby assembly manipulation suite
#    Copyright (C) 2006-2009 Yoann GUILLOT
#
#    Licence is LGPL, see LICENCE in the top-level directory

#
# this exemple illustrates the use of the PTrace class to hijack a syscall in a running process
# the next syscall made is patched to run the syscall with the arguments of our choice, then
# run the original intended syscall
# Works on linux/x86
#


require 'metasm'

class SyscallHooker < Metasm::PTrace
	CTX = ['EBX', 'ECX', 'EDX', 'ESI', 'EDI', 'EAX', 'ESP', 'EBP', 'EIP', 'ORIG_EAX']

	def inject(sysnr, *args)
		sysnr = SYSCALLNR[sysnr] || sysnr

		syscall
		puts '[*] waiting syscall'
		Process.waitpid(@pid)

		savedctx = CTX.inject({}) { |ctx, reg| ctx.update reg => peekusr(REGS_I386[reg]) }

		eip = (savedctx['EIP'] - 2) & 0xffffffff
		fu = readmem(eip, 2)
		if fu == "\xcd\x80"
			mode = :int80
		elsif fu == "\xeb\xf3" and readmem(eip-14, 7).unpack('H*').first == "51525589e50f34"	# aoenthuasn
			mode = :sysenter
		else
			puts 'unhandled syscall convention, aborting, code = ' + readmem(eip-4, 8).unpack('H*').first
			cont
			return self
		end

		if args.length > 5
			puts 'too may arguments, unsupported, aborting'
		else
			puts "[*] hooking #{SYSCALLNR.index(savedctx['ORIG_EAX'])}"

			# stack pointer to store buffers to
			esp_ptr = savedctx['ESP']
			write_string = lambda { |s|
				esp_ptr -= s.length
				esp_ptr &= 0xffff_fff0
				writemem(esp_ptr, s)
				[esp_ptr].pack('L').unpack('l').first
			}
			set_arg = lambda { |a|
				case a
				when String; write_string[a + 0.chr]
				when Array; write_string[a.map { |aa| set_arg[aa] }.pack('L*')]
				else a
				end
			}
			args.zip(CTX).map { |arg, reg|
				# set syscall args, put buffers on the stack as needed
				pokeusr(REGS_I386[reg], set_arg[arg])
			}
			# patch syscall number
			pokeusr(REGS_I386['ORIG_EAX'], sysnr)


			# run hooked syscall
			syscall
			Process.waitpid(@pid)
			retval = peekusr(REGS_I386['EAX'])
			puts "[*] retval: #{'%X' % retval}#{" (Errno::#{ERRNO.index(-retval)})" if retval < 0}"

			if SYSCALLNR.index(sysnr) == 'execve' and retval >= 0
				cont
				return self
			end

			# restore eax & eip to run the orig syscall
			savedctx['EIP'] -= 2
			savedctx['EAX'] = savedctx['ORIG_EAX']
			savedctx.each { |reg, val| pokeusr(REGS_I386[reg], val) }
		end

		self
	end
end

if $0 == __FILE__
	SyscallHooker.new(ARGV.shift.to_i).inject('write', 2, "testic\n", 7).detach
end