Flagvent 2025 (Hard) FV25.08 Kellerspeicher aufm Haufen (GNU...

src/SUMMARY.md

@@ -843,6 +843,7 @@
   - [Use After Free](binary-exploitation/libc-heap/use-after-free/README.md)
     - [First Fit](binary-exploitation/libc-heap/use-after-free/first-fit.md)
   - [Double Free](binary-exploitation/libc-heap/double-free.md)
+  - [Gnu Obstack Function Pointer Hijack](binary-exploitation/libc-heap/gnu-obstack-function-pointer-hijack.md)
   - [Overwriting a freed chunk](binary-exploitation/libc-heap/overwriting-a-freed-chunk.md)
   - [Heap Overflow](binary-exploitation/libc-heap/heap-overflow.md)
   - [Unlink Attack](binary-exploitation/libc-heap/unlink-attack.md)

src/binary-exploitation/libc-heap/README.md

@@ -535,6 +535,10 @@ Study allocator-specific primitives derived from real-world bugs:
 virtualbox-slirp-nat-packet-heap-exploitation.md
 {{#endref}}
 
+{{#ref}}
+gnu-obstack-function-pointer-hijack.md
+{{#endref}}
+
 ## References
 
 - [https://azeria-labs.com/heap-exploitation-part-1-understanding-the-glibc-heap-implementation/](https://azeria-labs.com/heap-exploitation-part-1-understanding-the-glibc-heap-implementation/)

src/binary-exploitation/libc-heap/gnu-obstack-function-pointer-hijack.md

@@ -0,0 +1,67 @@
+# GNU obstack function-pointer hijack
+
+{{#include ../../banners/hacktricks-training.md}}
+
+## Overview
+
+GNU obstacks embed allocator state together with two indirect call targets:
+
+- `chunkfun` (offset `+0x38`) with signature `void *(*chunkfun)(void *, size_t)`
+- `freefun` (offset `+0x40`) with signature `void (*freefun)(void *, void *)`
+- `extra_arg` and a `use_extra_arg` flag select whether `_obstack_newchunk` calls `chunkfun(new_size)` or `chunkfun(extra_arg, new_size)`
+
+If an attacker can corrupt an application-owned `struct obstack *` or its fields, the next growth of the obstack (when `next_free == chunk_limit`) triggers an indirect call through `chunkfun`, enabling code execution primitives.
+
+## Primitive: size_t desync → 0-byte allocation → pointer OOB write
+
+A common bug pattern is using a **32-bit register** to compute `sizeof(ptr) * count` while storing the logical length in a 64-bit `size_t`.
+
+- Example: `elements = obstack_alloc(obs, sizeof(void *) * size);` is compiled as `SHL EAX,0x3` for `size << 3`.
+- With `size = 0x20000000` and `sizeof(void *) = 8`, the multiplication wraps to `0x0` in 32-bit, so the pointer array is **0 bytes**, but the recorded `size` remains `0x20000000`.
+- Subsequent `elements[curr++] = ptr;` writes perform **8-byte OOB pointer stores** into adjacent heap objects, giving a controlled cross-object overwrite primitive.
+
+## Leaking libc via `obstack.chunkfun`
+
+1. Place two heap objects adjacent (e.g., two stacks built with separate obstacks).
+2. Use the pointer-array OOB write from object A to overwrite object B’s `elements` pointer so that a `pop`/read from B dereferences an address inside object A’s obstack.
+3. Read `chunkfun` (`malloc` by default) at offset `0x38` to disclose a libc function pointer, then compute `libc_base = leak - malloc_offset` and derive other symbols (e.g., `system`, `"/bin/sh"`).
+
+## Hijacking `chunkfun` with a fake obstack
+
+Overwrite a victim’s stored `struct obstack *` to point at attacker-controlled data that mimics the obstack header. Minimal fields needed:
+
+- `next_free == chunk_limit` to force `_obstack_newchunk` on next push
+- `chunkfun = system_addr`
+- `extra_arg = binsh_addr`, `use_extra_arg = 1` to select the two-argument call form
+
+Then trigger an allocation on the victim obstack to execute `system("/bin/sh")` through the indirect call.
+
+Example fake obstack layout (glibc 2.42 offsets):
+
+```python
+fake  = b""
+fake += p64(0x1000)          # chunk_size
+fake += p64(heap_leak)       # chunk
+fake += p64(heap_leak)       # object_base
+fake += p64(heap_leak)       # next_free == chunk_limit
+fake += p64(heap_leak)       # chunk_limit
+fake += p64(0xF)             # alignment_mask
+fake += p64(0)               # temp
+fake += p64(system_addr)     # chunkfun
+fake += p64(0)               # freefun
+fake += p64(binsh_addr)      # extra_arg
+fake += p64(1)               # use_extra_arg flag set
+```
+
+## Attack recipe
+
+1. **Trigger size wrap** to create a 0-byte pointer array with a huge logical length.
+2. **Groom adjacency** so an OOB pointer store reaches a neighbor object containing an obstack pointer.
+3. **Leak libc** by redirecting a victim pointer to the neighbor obstack’s `chunkfun` and reading the function pointer.
+4. **Forge obstack** data with controlled `chunkfun`/`extra_arg` and force `_obstack_newchunk` to land in the forged header, yielding a function-pointer call of the attacker’s choice.
+
+## References
+
+- [Flagvent 2025 FV25.08 obstack exploit (0xdf)](https://0xdf.gitlab.io/flagvent2025/hard)
+
+{{#include ../../banners/hacktricks-training.md}}