leg

Daddy told me I should study arm.
But I prefer to study my leg!

Download : http://pwnable.kr/bin/leg.c
Download : http://pwnable.kr/bin/leg.asm

ssh leg@pwnable.kr -p2222 (pw:guest)

 

leg.c如下：

#include <stdio.h>
#include <fcntl.h>
int key1(){
    asm("mov r3, pc\n");
}
int key2(){
    asm(
    "push    {r6}\n"
    "add    r6, pc, $1\n"
    "bx    r6\n"
    ".code   16\n"
    "mov    r3, pc\n"
    "add    r3, $0x4\n"
    "push    {r3}\n"
    "pop    {pc}\n"
    ".code    32\n"
    "pop    {r6}\n"
    );
}
int key3(){
    asm("mov r3, lr\n");
}
int main(){
    int key=0;
    printf("Daddy has very strong arm! : ");
    scanf("%d", &key);
    if( (key1()+key2()+key3()) == key ){
        printf("Congratz!\n");
        int fd = open("flag", O_RDONLY);
        char buf[100];
        int r = read(fd, buf, 100);
        write(0, buf, r);
    }
    else{
        printf("I have strong leg :P\n");
    }
    return 0;
}

leg.asm如下：

(gdb) disass main
Dump of assembler code for function main:
   0x00008d3c <+0>:    push    {r4, r11, lr}
   0x00008d40 <+4>:    add    r11, sp, #8
   0x00008d44 <+8>:    sub    sp, sp, #12
   0x00008d48 <+12>:    mov    r3, #0
   0x00008d4c <+16>:    str    r3, [r11, #-16]
   0x00008d50 <+20>:    ldr    r0, [pc, #104]    ; 0x8dc0 <main+132>
   0x00008d54 <+24>:    bl    0xfb6c <printf>
   0x00008d58 <+28>:    sub    r3, r11, #16
   0x00008d5c <+32>:    ldr    r0, [pc, #96]    ; 0x8dc4 <main+136>
   0x00008d60 <+36>:    mov    r1, r3
   0x00008d64 <+40>:    bl    0xfbd8 <__isoc99_scanf>
   0x00008d68 <+44>:    bl    0x8cd4 <key1>
   0x00008d6c <+48>:    mov    r4, r0
   0x00008d70 <+52>:    bl    0x8cf0 <key2>
   0x00008d74 <+56>:    mov    r3, r0
   0x00008d78 <+60>:    add    r4, r4, r3
   0x00008d7c <+64>:    bl    0x8d20 <key3>
   0x00008d80 <+68>:    mov    r3, r0
   0x00008d84 <+72>:    add    r2, r4, r3
   0x00008d88 <+76>:    ldr    r3, [r11, #-16]
   0x00008d8c <+80>:    cmp    r2, r3
   0x00008d90 <+84>:    bne    0x8da8 <main+108>
   0x00008d94 <+88>:    ldr    r0, [pc, #44]    ; 0x8dc8 <main+140>
   0x00008d98 <+92>:    bl    0x1050c <puts>
   0x00008d9c <+96>:    ldr    r0, [pc, #40]    ; 0x8dcc <main+144>
   0x00008da0 <+100>:    bl    0xf89c <system>
   0x00008da4 <+104>:    b    0x8db0 <main+116>
   0x00008da8 <+108>:    ldr    r0, [pc, #32]    ; 0x8dd0 <main+148>
   0x00008dac <+112>:    bl    0x1050c <puts>
   0x00008db0 <+116>:    mov    r3, #0
   0x00008db4 <+120>:    mov    r0, r3
   0x00008db8 <+124>:    sub    sp, r11, #8
   0x00008dbc <+128>:    pop    {r4, r11, pc}
   0x00008dc0 <+132>:    andeq    r10, r6, r12, lsl #9
   0x00008dc4 <+136>:    andeq    r10, r6, r12, lsr #9
   0x00008dc8 <+140>:            ; <UNDEFINED> instruction: 0x0006a4b0
   0x00008dcc <+144>:            ; <UNDEFINED> instruction: 0x0006a4bc
   0x00008dd0 <+148>:    andeq    r10, r6, r4, asr #9
End of assembler dump.
(gdb) disass key1
Dump of assembler code for function key1:
   0x00008cd4 <+0>:    push    {r11}        ; (str r11, [sp, #-4]!)
   0x00008cd8 <+4>:    add    r11, sp, #0
   0x00008cdc <+8>:    mov    r3, pc
   0x00008ce0 <+12>:    mov    r0, r3
   0x00008ce4 <+16>:    sub    sp, r11, #0
   0x00008ce8 <+20>:    pop    {r11}        ; (ldr r11, [sp], #4)
   0x00008cec <+24>:    bx    lr
End of assembler dump.
(gdb) disass key2
Dump of assembler code for function key2:
   0x00008cf0 <+0>:    push    {r11}        ; (str r11, [sp, #-4]!)
   0x00008cf4 <+4>:    add    r11, sp, #0
   0x00008cf8 <+8>:    push    {r6}        ; (str r6, [sp, #-4]!)
   0x00008cfc <+12>:    add    r6, pc, #1
   0x00008d00 <+16>:    bx    r6
   0x00008d04 <+20>:    mov    r3, pc
   0x00008d06 <+22>:    adds    r3, #4
   0x00008d08 <+24>:    push    {r3}
   0x00008d0a <+26>:    pop    {pc}
   0x00008d0c <+28>:    pop    {r6}        ; (ldr r6, [sp], #4)
   0x00008d10 <+32>:    mov    r0, r3
   0x00008d14 <+36>:    sub    sp, r11, #0
   0x00008d18 <+40>:    pop    {r11}        ; (ldr r11, [sp], #4)
   0x00008d1c <+44>:    bx    lr
End of assembler dump.
(gdb) disass key3
Dump of assembler code for function key3:
   0x00008d20 <+0>:    push    {r11}        ; (str r11, [sp, #-4]!)
   0x00008d24 <+4>:    add    r11, sp, #0
   0x00008d28 <+8>:    mov    r3, lr
   0x00008d2c <+12>:    mov    r0, r3
   0x00008d30 <+16>:    sub    sp, r11, #0
   0x00008d34 <+20>:    pop    {r11}        ; (ldr r11, [sp], #4)
   0x00008d38 <+24>:    bx    lr
End of assembler dump.
(gdb) 

arm架构，只要输入为key1()+key2()+key3()就能得到flag

先看key1

由key1+12可知返回值r0是r3传递过去的

由key1+8可知r3是pc传递过去的

arm架构的pc和x86不一样，它指向的是当前语句之后两条的语句，因此pc传递过去的是0x8ce4，所以key1()就是0x8ce4

再看key2

由key2+32可知返回值r0是r3传递过去的

由key2+22可知r3被加上了4

由key2+20可知r3也是pc传递过去的

那么pc传递过去的是0x8d08，r3加上4之后传递给r0的是0x8d0c，所以key2()就是0x8d0c

再看key3

由key3+12可知返回值r0是r3传递过去的

由key3+8可知r3是lr传递过去的

lr是当函数发生bl调用时，指向当前语句的下一条语句

因此由main+64可知lr传递过去的是0x8d80，所以key3()就是0x8d80

因此最后的key就是0x8ce4+0x8d0c+0x8d80=108400