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87 lines
2.9 KiB
Markdown
87 lines
2.9 KiB
Markdown
# unlink
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{{#include ../../../banners/hacktricks-training.md}}
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### Code
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```c
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// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c
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/* Take a chunk off a bin list. */
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static void
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unlink_chunk (mstate av, mchunkptr p)
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{
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if (chunksize (p) != prev_size (next_chunk (p)))
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malloc_printerr ("corrupted size vs. prev_size");
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mchunkptr fd = p->fd;
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mchunkptr bk = p->bk;
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if (__builtin_expect (fd->bk != p || bk->fd != p, 0))
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malloc_printerr ("corrupted double-linked list");
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fd->bk = bk;
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bk->fd = fd;
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if (!in_smallbin_range (chunksize_nomask (p)) && p->fd_nextsize != NULL)
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{
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if (p->fd_nextsize->bk_nextsize != p
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|| p->bk_nextsize->fd_nextsize != p)
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malloc_printerr ("corrupted double-linked list (not small)");
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// Added: If the FD is not in the nextsize list
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if (fd->fd_nextsize == NULL)
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{
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if (p->fd_nextsize == p)
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fd->fd_nextsize = fd->bk_nextsize = fd;
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else
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// Link the nexsize list in when removing the new chunk
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{
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fd->fd_nextsize = p->fd_nextsize;
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fd->bk_nextsize = p->bk_nextsize;
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p->fd_nextsize->bk_nextsize = fd;
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p->bk_nextsize->fd_nextsize = fd;
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}
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}
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else
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{
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p->fd_nextsize->bk_nextsize = p->bk_nextsize;
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p->bk_nextsize->fd_nextsize = p->fd_nextsize;
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}
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}
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}
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```
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### Graphical Explanation
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Check this great graphical explanation of the unlink process:
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<figure><img src="../../../images/image (3) (1) (1) (1) (1) (1).png" alt=""><figcaption><p><a href="https://ctf-wiki.mahaloz.re/pwn/linux/glibc-heap/implementation/figure/unlink_smallbin_intro.png">https://ctf-wiki.mahaloz.re/pwn/linux/glibc-heap/implementation/figure/unlink_smallbin_intro.png</a></p></figcaption></figure>
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### Security Checks
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- Check if the indicated size of the chunk is the same as the prev_size indicated in the next chunk
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- Check also that `P->fd->bk == P` and `P->bk->fw == P`
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- If the chunk is not small, check that `P->fd_nextsize->bk_nextsize == P` and `P->bk_nextsize->fd_nextsize == P`
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### Leaks
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An unlinked chunk is not cleaning the allocated addreses, so having access to rad it, it's possible to leak some interesting addresses:
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Libc Leaks:
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- If P is located in the head of the doubly linked list, `bk` will be pointing to `malloc_state` in libc
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- If P is located at the end of the doubly linked list, `fd` will be pointing to `malloc_state` in libc
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- When the doubly linked list contains only one free chunk, P is in the doubly linked list, and both `fd` and `bk` can leak the address inside `malloc_state`.
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Heap leaks:
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- If P is located in the head of the doubly linked list, `fd` will be pointing to an available chunk in the heap
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- If P is located at the end of the doubly linked list, `bk` will be pointing to an available chunk in the heap
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- If P is in the doubly linked list, both `fd` and `bk` will be pointing to an available chunk in the heap
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{{#include ../../../banners/hacktricks-training.md}}
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