HFS Heap Overflow

By fuzzing the HFS btree parser, a heap overflow in the zone allocator was found. Mounting a clean, overflowed and payload images in a Heap Feng Shui way worked. The kernel heap overflow exploit copies 0x200 bytes from the vnimage.payload file to the kernel sysent, replacing a syscall to a write anywhere gadget. Some syscalls (first 0xA0 bytes and the last 6 bytes) are trashed in the operation because the HFS protocol needed to be respected. So these bytes are restored as fast as possible to get a stable exploit, then the write anywhere is used to copy the kernel exploit and jump to it. The kernel exploit just patches the kernel security features, as usual.

Credit

• pod2g for finding the vulnerability and writing the exploit
• i0n1c for his papers on Heap Feng Shui

References

• pod2g's blog
• i0n1c's Heap Feng Shui paper
• All about Heap Spraying

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