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I'm working on i386:x86_64 and I'm writing a simple program to start a shell. Bellow is my assembly.

.section .data
.section .text
.globl _start

_start:
    xor %rax, %rax
    mov $70, %al
    xor %rbx, %rbx
    xor %rcx, %rcx
    int $0x80

    jmp ender

starter:
    pop %rbx
    xor %rax, %rax
    mov %al, 0x07(%rbx)
    mov %rbx, 0x08(%rbx)
    mov %rax, 0x0c(%rbx)
    mov $11, %al
    lea 0x08(%rbx), %rcx
    lea 0x0c(%rbx), %rdx
    int $0x80

ender:
    call starter
    .string "/bin/sh"

The problem is that I keep getting a segmentation fault. When I use objdump -D my_prog the output is this ...

Disassembly of section .text:

0000000000400078 <_start>:
  400078:   48 31 c0                xor    %rax,%rax
  40007b:   b0 46                   mov    $0x46,%al
  40007d:   48 31 db                xor    %rbx,%rbx
  400080:   48 31 c9                xor    %rcx,%rcx
  400083:   cd 80                   int    $0x80
  400085:   eb 1b                   jmp    4000a2 <ender>

0000000000400087 <starter>:
  400087:   5b                      pop    %rbx
  400088:   48 31 c0                xor    %rax,%rax
  40008b:   88 43 07                mov    %al,0x7(%rbx)
  40008e:   48 89 5b 08             mov    %rbx,0x8(%rbx)
  400092:   48 89 43 0c             mov    %rax,0xc(%rbx)
  400096:   b0 0b                   mov    $0xb,%al
  400098:   48 8d 4b 08             lea    0x8(%rbx),%rcx
  40009c:   48 8d 53 0c             lea    0xc(%rbx),%rdx
  4000a0:   cd 80                   int    $0x80

00000000004000a2 <ender>:
  4000a2:   e8 e0 ff ff ff          callq  400087 <starter>
  4000a7:   2f                      (bad)  
  4000a8:   62                      (bad)  
  4000a9:   69                      .byte 0x69
  4000aa:   6e                      outsb  %ds:(%rsi),(%dx)
  4000ab:   2f                      (bad)  
  4000ac:   73 68                   jae    400116 <ender+0x74>

I'm going to take guess and say that it is addresses that are marked (bad) which is causing the seg fault. I know this is happening because it wants mem access to mem not allocated to it. I'm not really to sure what I should do. I am running Linux.

Peter Cordes
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Dr.Knowitall
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  • Why did you put the string in a text section? – Ignacio Vazquez-Abrams Apr 18 '12 at 14:57
  • I was following a tutorial and I copied pasted the code. That could be a problem. – Dr.Knowitall Apr 18 '12 at 14:58
  • see also [What happens if you use the 32-bit int 0x80 Linux ABI in 64-bit code?](//stackoverflow.com/q/46087730) - use `syscall`. Plus you don't need jmp / call / pop in x86-64, you have RIP-relative addressing. [Avoiding 0xFF bytes in shellcode using CALL to read RIP?](//stackoverflow.com/q/55778839) is a simpler way position-independent way to get a nearby address into a register. (Slightly larger code-size, though.) – Peter Cordes Jun 20 '19 at 00:00
  • Another duplicate which works better for non-shellcode uses: [x86\_64 Assembly - Segfault when trying to edit a byte within an array in x64 assembly](https://stackoverflow.com/q/45005078) – Peter Cordes Oct 15 '21 at 22:11

1 Answers1

3

It's segfaulting because you are attempting to write data to a code (.text) area of memory. Executable code areas are almost always marked as read-only.

Here's your code with some additional comments. 

.section .data
.section .text
.globl _start

_start:
    xor %rax, %rax
    mov $70, %al
    xor %rbx, %rbx
    xor %rcx, %rcx
    ; call sys_setreuid(0,0)
    int $0x80

    jmp ender

starter:
    ; take the return address off the stack
    ; rbx will point to the /bin/sh string after the call instruction
    pop %rbx
    ; zero rax
    xor %rax, %rax
    ; save a zero byte to the end of the /bin/sh string (it's 7 characters long)...
    ; (it will segfault here because you're writing to a read-only area)
    mov %al, 0x07(%rbx)
    ; ...followed by a pointer to the string... 
    mov %rbx, 0x08(%rbx)
    ; ...followed by another zero value 
    mov %rax, 0x0c(%rbx)
    ; setup the parameters for a sys_execve call
    mov $11, %al
    lea 0x08(%rbx), %rcx
    lea 0x0c(%rbx), %rdx
    int $0x80

    ; what happens when int 0x80 returns? 
    ; you should do something here or starter will be called again

ender:
    call starter
    .string "/bin/sh"

There are other issues with the code. Consider:

mov %rbx, 0x08(%rbx)
mov %rax, 0x0c(%rbx)

%rbx is an 8 byte value, but the code only gives it 4 bytes worth of space (0x0c-0x08 = 4). If you want to get it working, you'll need to move the string into a .data area (with some additional space after it) and change the code to make it 64-bit friendly.

adelphus
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