pwnable.kr-passcode
程序员文章站
2022-05-15 14:22:08
...
做了好长时间的passcode终于写出来了。我真的是......太弱了。
不过这题也是收获最大的一题了。
首先看看题目。
打开passcode.c看看里面有什么
#include <stdio.h>
#include <stdlib.h>
void login(){
int passcode1;
int passcode2;
printf("enter passcode1 : ");
scanf("%d", passcode1);
fflush(stdin);
// ha! mommy told me that 32bit is vulnerable to bruteforcing :)
printf("enter passcode2 : ");
scanf("%d", passcode2);
printf("checking...\n");
if(passcode1==338150 && passcode2==13371337){
printf("Login OK!\n");
system("/bin/cat flag");
}
else{
printf("Login Failed!\n");
exit(0);
}
}
void welcome(){
char name[100];
printf("enter you name : ");
scanf("%100s", name);
printf("Welcome %s!\n", name);
}
int main(){
printf("Toddler's Secure Login System 1.0 beta.\n");
welcome();
login();
// something after login...
printf("Now I can safely trust you that you have credential :)\n");
return 0;
}
很明显，一眼就看到了scanf没有加‘&’，可是这会带来什么问题呢？？
这就牵扯到一个GOT覆写技术（具体的请百度，因为我自己也没看懂）。
简单来说，就是当scanf没有'&'时，导致读入数据的时候，scanf会把这个变量中的值当成存储地址来存放数据，即它会以为passcode1所代表的是一个地址。
现在我们看一下passcode的反汇编代码。
objdump -d passcode
passcode: file format elf32-i386
Disassembly of section .init:
080483e0 <_init>:
80483e0: 53 push %ebx
80483e1: 83 ec 08 sub $0x8,%esp
80483e4: e8 00 00 00 00 call 80483e9 <_init+0x9>
80483e9: 5b pop %ebx
80483ea: 81 c3 0b 1c 00 00 add $0x1c0b,%ebx
80483f0: 8b 83 fc ff ff ff mov -0x4(%ebx),%eax
80483f6: 85 c0 test %eax,%eax
80483f8: 74 05 je 80483ff <_init+0x1f>
80483fa: e8 71 00 00 00 call 8048470 <aaa@qq.com>
80483ff: e8 3c 01 00 00 call 8048540 <frame_dummy>
8048404: e8 17 03 00 00 call 8048720 <__do_global_ctors_aux>
8048409: 83 c4 08 add $0x8,%esp
804840c: 5b pop %ebx
804840d: c3 ret
Disassembly of section .plt:
08048410 <aaa@qq.com>:
8048410: ff 35 f8 9f 04 08 pushl 0x8049ff8
8048416: ff 25 fc 9f 04 08 jmp *0x8049ffc
804841c: 00 00 add %al,(%eax)
...
08048420 <aaa@qq.com>:
8048420: ff 25 00 a0 04 08 jmp *0x804a000
8048426: 68 00 00 00 00 push $0x0
804842b: e9 e0 ff ff ff jmp 8048410 <_init+0x30>
08048430 <aaa@qq.com>:
8048430: ff 25 04 a0 04 08 jmp *0x804a004
8048436: 68 08 00 00 00 push $0x8
804843b: e9 d0 ff ff ff jmp 8048410 <_init+0x30>
08048440 <aaa@qq.com>:
8048440: ff 25 08 a0 04 08 jmp *0x804a008
8048446: 68 10 00 00 00 push $0x10
804844b: e9 c0 ff ff ff jmp 8048410 <_init+0x30>
08048450 <aaa@qq.com>:
8048450: ff 25 0c a0 04 08 jmp *0x804a00c
8048456: 68 18 00 00 00 push $0x18
804845b: e9 b0 ff ff ff jmp 8048410 <_init+0x30>
08048460 <aaa@qq.com>:
8048460: ff 25 10 a0 04 08 jmp *0x804a010
8048466: 68 20 00 00 00 push $0x20
804846b: e9 a0 ff ff ff jmp 8048410 <_init+0x30>
08048470 <aaa@qq.com>:
8048470: ff 25 14 a0 04 08 jmp *0x804a014
8048476: 68 28 00 00 00 push $0x28
804847b: e9 90 ff ff ff jmp 8048410 <_init+0x30>
08048480 <aaa@qq.com>:
8048480: ff 25 18 a0 04 08 jmp *0x804a018
8048486: 68 30 00 00 00 push $0x30
804848b: e9 80 ff ff ff jmp 8048410 <_init+0x30>
08048490 <aaa@qq.com>:
8048490: ff 25 1c a0 04 08 jmp *0x804a01c
8048496: 68 38 00 00 00 push $0x38
804849b: e9 70 ff ff ff jmp 8048410 <_init+0x30>
080484a0 <aaa@qq.com>:
80484a0: ff 25 20 a0 04 08 jmp *0x804a020
80484a6: 68 40 00 00 00 push $0x40
80484ab: e9 60 ff ff ff jmp 8048410 <_init+0x30>
Disassembly of section .text:
080484b0 <_start>:
80484b0: 31 ed xor %ebp,%ebp
80484b2: 5e pop %esi
80484b3: 89 e1 mov %esp,%ecx
80484b5: 83 e4 f0 and $0xfffffff0,%esp
80484b8: 50 push %eax
80484b9: 54 push %esp
80484ba: 52 push %edx
80484bb: 68 10 87 04 08 push $0x8048710
80484c0: 68 a0 86 04 08 push $0x80486a0
80484c5: 51 push %ecx
80484c6: 56 push %esi
80484c7: 68 65 86 04 08 push $0x8048665
80484cc: e8 bf ff ff ff call 8048490 <aaa@qq.com>
80484d1: f4 hlt
80484d2: 90 nop
80484d3: 90 nop
80484d4: 90 nop
80484d5: 90 nop
80484d6: 90 nop
80484d7: 90 nop
80484d8: 90 nop
80484d9: 90 nop
80484da: 90 nop
80484db: 90 nop
80484dc: 90 nop
80484dd: 90 nop
80484de: 90 nop
80484df: 90 nop
080484e0 <__do_global_dtors_aux>:
80484e0: 55 push %ebp
80484e1: 89 e5 mov %esp,%ebp
80484e3: 53 push %ebx
80484e4: 83 ec 04 sub $0x4,%esp
80484e7: 80 3d 30 a0 04 08 00 cmpb $0x0,0x804a030
80484ee: 75 3f jne 804852f <__do_global_dtors_aux+0x4f>
80484f0: a1 34 a0 04 08 mov 0x804a034,%eax
80484f5: bb 20 9f 04 08 mov $0x8049f20,%ebx
80484fa: 81 eb 1c 9f 04 08 sub $0x8049f1c,%ebx
8048500: c1 fb 02 sar $0x2,%ebx
8048503: 83 eb 01 sub $0x1,%ebx
8048506: 39 d8 cmp %ebx,%eax
8048508: 73 1e jae 8048528 <__do_global_dtors_aux+0x48>
804850a: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
8048510: 83 c0 01 add $0x1,%eax
8048513: a3 34 a0 04 08 mov %eax,0x804a034
8048518: ff 14 85 1c 9f 04 08 call *0x8049f1c(,%eax,4)
804851f: a1 34 a0 04 08 mov 0x804a034,%eax
8048524: 39 d8 cmp %ebx,%eax
8048526: 72 e8 jb 8048510 <__do_global_dtors_aux+0x30>
8048528: c6 05 30 a0 04 08 01 movb $0x1,0x804a030
804852f: 83 c4 04 add $0x4,%esp
8048532: 5b pop %ebx
8048533: 5d pop %ebp
8048534: c3 ret
8048535: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi
8048539: 8d bc 27 00 00 00 00 lea 0x0(%edi,%eiz,1),%edi
08048540 <frame_dummy>:
8048540: 55 push %ebp
8048541: 89 e5 mov %esp,%ebp
8048543: 83 ec 18 sub $0x18,%esp
8048546: a1 24 9f 04 08 mov 0x8049f24,%eax
804854b: 85 c0 test %eax,%eax
804854d: 74 12 je 8048561 <frame_dummy+0x21>
804854f: b8 00 00 00 00 mov $0x0,%eax
8048554: 85 c0 test %eax,%eax
8048556: 74 09 je 8048561 <frame_dummy+0x21>
8048558: c7 04 24 24 9f 04 08 movl $0x8049f24,(%esp)
804855f: ff d0 call *%eax
8048561: c9 leave
8048562: c3 ret
8048563: 90 nop
08048564 <login>:
8048564: 55 push %ebp
8048565: 89 e5 mov %esp,%ebp
8048567: 83 ec 28 sub $0x28,%esp //分配栈空间
804856a: b8 70 87 04 08 mov $0x8048770,%eax
804856f: 89 04 24 mov %eax,(%esp)
8048572: e8 a9 fe ff ff call 8048420 <aaa@qq.com>
8048577: b8 83 87 04 08 mov $0x8048783,%eax
804857c: 8b 55 f0 mov -0x10(%ebp),%edx //passcode1
804857f: 89 54 24 04 mov %edx,0x4(%esp)
8048583: 89 04 24 mov %eax,(%esp)
8048586: e8 15 ff ff ff call 80484a0 <aaa@qq.com>
804858b: a1 2c a0 04 08 mov 0x804a02c,%eax
8048590: 89 04 24 mov %eax,(%esp)
8048593: e8 98 fe ff ff call 8048430 <aaa@qq.com>
8048598: b8 86 87 04 08 mov $0x8048786,%eax
804859d: 89 04 24 mov %eax,(%esp)
80485a0: e8 7b fe ff ff call 8048420 <aaa@qq.com>
80485a5: b8 83 87 04 08 mov $0x8048783,%eax
80485aa: 8b 55 f4 mov -0xc(%ebp),%edx //passcode2
80485ad: 89 54 24 04 mov %edx,0x4(%esp)
80485b1: 89 04 24 mov %eax,(%esp)
80485b4: e8 e7 fe ff ff call 80484a0 <aaa@qq.com>
80485b9: c7 04 24 99 87 04 08 movl $0x8048799,(%esp)
80485c0: e8 8b fe ff ff call 8048450 <aaa@qq.com>
80485c5: 81 7d f0 e6 28 05 00 cmpl $0x528e6,-0x10(%ebp) //第一个比较
80485cc: 75 23 jne 80485f1 <login+0x8d>
80485ce: 81 7d f4 c9 07 cc 00 cmpl $0xcc07c9,-0xc(%ebp) //第二个比较
80485d5: 75 1a jne 80485f1 <login+0x8d>
80485d7: c7 04 24 a5 87 04 08 movl $0x80487a5,(%esp)
80485de: e8 6d fe ff ff call 8048450 <aaa@qq.com>
80485e3: c7 04 24 af 87 04 08 movl $0x80487af,(%esp)
80485ea: e8 71 fe ff ff call 8048460 <aaa@qq.com> //调用系统
80485ef: c9 leave
80485f0: c3 ret
80485f1: c7 04 24 bd 87 04 08 movl $0x80487bd,(%esp)
80485f8: e8 53 fe ff ff call 8048450 <aaa@qq.com>
80485fd: c7 04 24 00 00 00 00 movl $0x0,(%esp)
8048604: e8 77 fe ff ff call 8048480 <aaa@qq.com>
08048609 <welcome>:
8048609: 55 push %ebp
804860a: 89 e5 mov %esp,%ebp
804860c: 81 ec 88 00 00 00 sub $0x88,%esp
8048612: 65 a1 14 00 00 00 mov %gs:0x14,%eax
8048618: 89 45 f4 mov %eax,-0xc(%ebp)
804861b: 31 c0 xor %eax,%eax
804861d: b8 cb 87 04 08 mov $0x80487cb,%eax
8048622: 89 04 24 mov %eax,(%esp)
8048625: e8 f6 fd ff ff call 8048420 <aaa@qq.com>
804862a: b8 dd 87 04 08 mov $0x80487dd,%eax
804862f: 8d 55 90 lea -0x70(%ebp),%edx //name数组的建立
8048632: 89 54 24 04 mov %edx,0x4(%esp)
8048636: 89 04 24 mov %eax,(%esp)
8048639: e8 62 fe ff ff call 80484a0 <aaa@qq.com>
804863e: b8 e3 87 04 08 mov $0x80487e3,%eax
8048643: 8d 55 90 lea -0x70(%ebp),%edx
8048646: 89 54 24 04 mov %edx,0x4(%esp)
804864a: 89 04 24 mov %eax,(%esp)
804864d: e8 ce fd ff ff call 8048420 <aaa@qq.com>
8048652: 8b 45 f4 mov -0xc(%ebp),%eax
8048655: 65 33 05 14 00 00 00 xor %gs:0x14,%eax
804865c: 74 05 je 8048663 <welcome+0x5a>
804865e: e8 dd fd ff ff call 8048440 <aaa@qq.com>
8048663: c9 leave
8048664: c3 ret
08048665 <main>:
8048665: 55 push %ebp
8048666: 89 e5 mov %esp,%ebp
8048668: 83 e4 f0 and $0xfffffff0,%esp
804866b: 83 ec 10 sub $0x10,%esp
804866e: c7 04 24 f0 87 04 08 movl $0x80487f0,(%esp)
8048675: e8 d6 fd ff ff call 8048450 <aaa@qq.com>
804867a: e8 8a ff ff ff call 8048609 <welcome>
804867f: e8 e0 fe ff ff call 8048564 <login> //连续调用导致栈底相同
8048684: c7 04 24 18 88 04 08 movl $0x8048818,(%esp)
804868b: e8 c0 fd ff ff call 8048450 <aaa@qq.com>
8048690: b8 00 00 00 00 mov $0x0,%eax
8048695: c9 leave
8048696: c3 ret
8048697: 90 nop
8048698: 90 nop
8048699: 90 nop
804869a: 90 nop
804869b: 90 nop
804869c: 90 nop
804869d: 90 nop
804869e: 90 nop
804869f: 90 nop
080486a0 <__libc_csu_init>:
80486a0: 55 push %ebp
80486a1: 57 push %edi
80486a2: 56 push %esi
80486a3: 53 push %ebx
80486a4: e8 69 00 00 00 call 8048712 <__i686.get_pc_thunk.bx>
80486a9: 81 c3 4b 19 00 00 add $0x194b,%ebx
80486af: 83 ec 1c sub $0x1c,%esp
80486b2: 8b 6c 24 30 mov 0x30(%esp),%ebp
80486b6: 8d bb 20 ff ff ff lea -0xe0(%ebx),%edi
80486bc: e8 1f fd ff ff call 80483e0 <_init>
80486c1: 8d 83 20 ff ff ff lea -0xe0(%ebx),%eax
80486c7: 29 c7 sub %eax,%edi
80486c9: c1 ff 02 sar $0x2,%edi
80486cc: 85 ff test %edi,%edi
80486ce: 74 29 je 80486f9 <__libc_csu_init+0x59>
80486d0: 31 f6 xor %esi,%esi
80486d2: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
80486d8: 8b 44 24 38 mov 0x38(%esp),%eax
80486dc: 89 2c 24 mov %ebp,(%esp)
80486df: 89 44 24 08 mov %eax,0x8(%esp)
80486e3: 8b 44 24 34 mov 0x34(%esp),%eax
80486e7: 89 44 24 04 mov %eax,0x4(%esp)
80486eb: ff 94 b3 20 ff ff ff call *-0xe0(%ebx,%esi,4)
80486f2: 83 c6 01 add $0x1,%esi
80486f5: 39 fe cmp %edi,%esi
80486f7: 75 df jne 80486d8 <__libc_csu_init+0x38>
80486f9: 83 c4 1c add $0x1c,%esp
80486fc: 5b pop %ebx
80486fd: 5e pop %esi
80486fe: 5f pop %edi
80486ff: 5d pop %ebp
8048700: c3 ret
8048701: eb 0d jmp 8048710 <__libc_csu_fini>
8048703: 90 nop
8048704: 90 nop
8048705: 90 nop
8048706: 90 nop
8048707: 90 nop
8048708: 90 nop
8048709: 90 nop
804870a: 90 nop
804870b: 90 nop
804870c: 90 nop
804870d: 90 nop
804870e: 90 nop
804870f: 90 nop
08048710 <__libc_csu_fini>:
8048710: f3 c3 repz ret
08048712 <__i686.get_pc_thunk.bx>:
8048712: 8b 1c 24 mov (%esp),%ebx
8048715: c3 ret
8048716: 90 nop
8048717: 90 nop
8048718: 90 nop
8048719: 90 nop
804871a: 90 nop
804871b: 90 nop
804871c: 90 nop
804871d: 90 nop
804871e: 90 nop
804871f: 90 nop
08048720 <__do_global_ctors_aux>:
8048720: 55 push %ebp
8048721: 89 e5 mov %esp,%ebp
8048723: 53 push %ebx
8048724: 83 ec 04 sub $0x4,%esp
8048727: a1 14 9f 04 08 mov 0x8049f14,%eax
804872c: 83 f8 ff cmp $0xffffffff,%eax
804872f: 74 13 je 8048744 <__do_global_ctors_aux+0x24>
8048731: bb 14 9f 04 08 mov $0x8049f14,%ebx
8048736: 66 90 xchg %ax,%ax
8048738: 83 eb 04 sub $0x4,%ebx
804873b: ff d0 call *%eax
804873d: 8b 03 mov (%ebx),%eax
804873f: 83 f8 ff cmp $0xffffffff,%eax
8048742: 75 f4 jne 8048738 <__do_global_ctors_aux+0x18>
8048744: 83 c4 04 add $0x4,%esp
8048747: 5b pop %ebx
8048748: 5d pop %ebp
8048749: c3 ret
804874a: 90 nop
804874b: 90 nop
Disassembly of section .fini:
0804874c <_fini>:
804874c: 53 push %ebx
804874d: 83 ec 08 sub $0x8,%esp
8048750: e8 00 00 00 00 call 8048755 <_fini+0x9>
8048755: 5b pop %ebx
8048756: 81 c3 9f 18 00 00 add $0x189f,%ebx
804875c: e8 7f fd ff ff call 80484e0 <__do_global_dtors_aux>
8048761: 83 c4 08 add $0x8,%esp
8048764: 5b pop %ebx
8048765: c3 ret
我们会发现，welcome和login是连续调用的，看了一些师傅的WP之后我知道连续调用函数会导致他们的栈底相同，即他们的ebp相同，我们可以算出name[100]的后4个字节其实是passcode1的，正好可以用到我们的GOT表覆写技术。
objdump -R ./passcode命令，可以看到其入口点，即GOT表
DYNAMIC RELOCATION RECORDS
OFFSET TYPE VALUE
08049ff0 R_386_GLOB_DAT __gmon_start__
0804a02c R_386_COPY aaa@qq.com@GLIBC_2.0
0804a000 R_386_JUMP_SLOT aaa@qq.com_2.0
0804a004 R_386_JUMP_SLOT aaa@qq.com_2.0
0804a008 R_386_JUMP_SLOT aaa@qq.com_2.4
0804a00c R_386_JUMP_SLOT aaa@qq.com_2.0
0804a010 R_386_JUMP_SLOT aaa@qq.com_2.0
0804a014 R_386_JUMP_SLOT __gmon_start__
0804a018 R_386_JUMP_SLOT aaa@qq.com_2.0
0804a01c R_386_JUMP_SLOT aaa@qq.com_2.0
0804a020 R_386_JUMP_SLOT aaa@qq.com_2.7
好！现在我们先列出来几个我们一会要用到的东西。
要覆写的函数地址（我用的fflush）：0x0804a004
system地址：0x80485ea
因为system地址一会是要通过scanf输入的，所以我们把它转化为int， 即134514147
现在开始！！！
flag get√
以上有一些说不清的地方，在这里补充一下：
1.关于name最后四位是passcode1的问题。我们可以看一下长度，通过汇编代码看到name变量的起始地址为ebp-0x70, passcode变量起始地址为ebp-0x10,相差0x60=96，恰巧还有4个字节用于覆盖passcode的值。
2.必须覆写fflush函数吗？并不是的，当然，我们也可以把它改成printf的GOT表的地址，这样当我们调用printf函数时，就会调用system。
3.再解释一下，GOT表就是一个函数指针数组(具体搜索百度)，我们看到程序在我们输入之后会调用fflush函数，所以我们可以将passcode1的值改为fflush的地址，然后接下来会通过scanf将上面的system的地址写进去，改变整个程序的执行过程，当程序调用fflush函数的时候，由于它的地址已经被改变了，所以会跳到system的地方去。也就是说，我们将system的地址写在了被改为fflush地址上面去。当程序调用fflush函数的时候，就会执行system。
4.附上一张简单的GOT表示意图
结束！收工吃饭！
上一篇： 阿里云 OSS 对象存储简单使用【随笔】
下一篇： pwnable.kr-coin1 WP
推荐阅读